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  • What Is Heat?

Lesson What Is Heat?

Grade Level: 6 (5-7)

(three 60-minute class periods)

Lesson Dependency: None

Subject Areas: Physical Science

NGSS Performance Expectations:

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  • Keep It Hot!

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Engineering connection, learning objectives, worksheets and attachments, more curriculum like this, pre-req knowledge, introduction/motivation, associated activities, lesson closure, vocabulary/definitions, additional multimedia support, user comments & tips.

Engineers make a world of difference

Understanding heat transfer is essential knowledge for the engineering of mechanical, chemical and biological systems. Design of internal combustion engines, air conditioning and heating systems, chemical and biological reactors and even clothing technology requires an understanding of heat transfer. Design of insulating materials for homes, buildings and even beverage containers also requires an understanding of heat transfer.

After this lesson, students should be able to:

  • Explain that heat is the flow of energy from hot materials to cold materials.
  • Describe that molecules in a material begin to vibrate (or move) more quickly when the material is heated.
  • Identify conduction as heat transfer within and between solids.
  • Identify convection as heat transfer involving gases or liquids.
  • Identify radiation as heat transfer carried by little packets of energy that can travel through almost any material—even empty space.
  • List examples of each type of heat transfer.

Educational Standards Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards. All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN) , a project of D2L (www.achievementstandards.org). In the ASN, standards are hierarchically structured: first by source; e.g. , by state; within source by type; e.g. , science or mathematics; within type by subtype, then by grade, etc .

Ngss: next generation science standards - science, international technology and engineering educators association - technology.

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State Standards

California - science.

A familiarity with basic concepts about energy and its different forms, as well as a basic understanding of temperature.

Raise your hand if you ever put on a jacket? Or turned on a heater? Or melted an ice cube in your hand? (Expect every student to raise their hand.)

You probably appreciate heat on a cold day. But today, and over the next couple of days, we are going to talk about how scientists and engineers think about heat.

Lesson Background and Concepts for Teachers

Demonstration Materials: A few simple and powerful demonstrations are suggested for this lesson. A thermal energy demonstration requires two transparent containers that are capable of holding hot water, plus hot water, ice water and a few drops of food coloring. The conduction demonstration requires one candle, matches three small nails/thumb tacks, an oven mitt, and a hacksaw blade or metal rod (not stainless steel). An additional quick conduction demonstration requires five to 10 inflated balloons. Demo preparation and presentation instructions are provided on the slides and notes of slides 4 and 14.

The Additional Background Material section (below) provides a very detailed discussion about heat. While this material is generally above the sixth-grade level, it presents key background information for the teacher so they are able to answer advanced student questions.

Use the 21-slide What Is Heat? Presentation , a Microsoft PowerPoint® file, to directly deliver the lesson content, using the guidance provided below; alternatively, use the presentation to inform other teaching methods. Note that each slide includes background and discussion information in the notes sections that is not provided below and is unavailable in the PDF version. In addition, the slides are animated, so clicking brings up the next text or component on the slide.

( Slide 1 ) What is heat? Do the images on this slide give you any hints? Heat is energy that has something to do with temperature and is an important concept used by engineers to design many of the products we use every day.

( Slide 2 ) Open a discussion about what will happen to the temperature of the beverage in each case (hot chocolate, iced tea) when left unattended for 30 minutes. Why do some things get warmer while other things get colder when they are left out? Given time, both eventually become room temperature. The hot drink releases energy; the cold drink absorbs energy.

( Slide 3 ) Remind students about energy and some of its different forms. Expect them to recall that moving objects have kinetic energy. Show the animation to help visualize the relationship between temperature and kinetic energy: https://commons.wikimedia.org/wiki/File:Translational_motion.gif .

( Slide 4 ) Conduct a class demonstration to show temperature and kinetic energy using food coloring : Prepare separate transparent cups of hot and cold water (ice water is best; remove the ice for the demo). Into each cup, place a drop of food coloring and direct students to observe what happens. Expect them to notice that the food coloring in the hot water spreads out more quickly than that in the cold water. It is helpful to repeat this experiment after explaining the mechanism. Alternative: If conducting this demo is not possible, show a 2:52-minute video, "Moving Water Molecules"  (link also provided in the Additional Multimedia Support section).

( Slide 5 ) Talk about what students observed in the demo. The faster jiggling hot water dispersed the dye more quickly. Then show the animation of Brownian Motion at https://commons.wikimedia.org/wiki/File:Brownian_motion_large.gif . We can think of the small dots as water molecules, and the yellow dot as a much larger dye molecule being bounced around by the water molecules' thermal jiggling. This was discovered by Scottish botanist Robert Brown, who used a microscope to look at pollen samples in water. He could not see the water molecules, but noticed that pollen in hotter water jiggled around more than in colder water. The phenomenon was named in his honor: Brownian Motion.

( Slide 6 ) Make the point that thermal energy is in everything—even if it is something we consider cold.

( Slide 7 ) Explain the definition of heat as flowing thermal energy and clarify the direction of heat flow—from the hotter object to the cooler object. Energy transfers always occur from higher to lower states of energy.

( Slides 8-13 ) Use the provided images of a hot cup of coffee, an ice cream cone and a tea kettle on a burner as examples to talk about the direction of heat flow. Have students draw arrows to show the direction of heat flow; circulate around the room to verify their understanding. Make sure students realize that 1) heat is a form of energy that is transferred by a difference in temperature; a difference in temperature is needed for heat to flow, 2) heat always flows from hot to cold, or more precisely, heat flows from higher temperature to lower temperature, and 3) the units of heat are Joules, just like kinetic energy. The three different types of heat transfer (the movement of thermal energy) are conduction, convection and radiation. The "thought experiments" on slide 13 using the examples of hot soup and snowballs give students practice in using correct terminology and full sentences to explain how heat flows. Make sure students are able to realize that no heat transfer occurs between objects of the same temperature.

( Slide 14 ) Introduce the first type of heat transfer, conduction, which is heat transfer within or between solid objects. With our hands, we experience heat transfer by conduction any time we touch something that is hotter or colder than our skin.

At this point, present a conduction demonstration that you have prepared in advance . Before the activity, use drops of candle wax to "glue" two or three small nails or thumb tacks to a hacksaw blade or metal rod. Space the nails about 1 inch apart, with the first one located one to two inches from the end of the blade/rod. Hold the other end of the blade/rod with an oven mitt or nail it to a block of wood. Heat the end of the rod with a candle flame. As heat conducts down, the wax holding the nails melts and drops the nails, one by one, in sequence. This shows students the heat traveling down the rod.

Then conduct another class demonstration on heat conduction . Give each of five to 10 student volunteers an inflated balloon and have them hold them together, touching, in a line. Start to jiggle one end of the line and observe how this jiggling travels down the line of balloons.

( Slides 15-19 ) Introduce and go over the other two ways heat can move from one object to another: convection and radiation. Each slide starts with a discussion and examples and then gives a definition that can be used for building students' vocabulary.

( Slide 20 ) Introduce the concept of insulation, which is important in heat transfer and necessary background to understand the associated activity Keep It Hot! . Besides the oven mitt and pop can cozy, other examples of insulation include the walls and roof of houses, multi-pane windows, beverage thermos, insulation around car engines to keep passengers cool, inside a jet engine, material on the outside of the space shuttle, plastic casing on wires, a sweater or jacket, and refrigerator and oven walls.

( Slide 21 ) Wrap up with a brief review of key terms: heat, conduction, convection, radiation, insulation, and that heat flows from hot (or higher temperature) to cold (or lower temperature).

Additional Background Material

Heat in Engineering: Heat is the flow of thermal energy that arises from temperature differences. Whenever two things of different temperatures are near one another, thermal energy flows. This flowing energy is called heat. The fans heard whirring in computers are designed to remove heat generated by the electronics. Without these fans, computers would melt or create fires. On a winter morning, we put on coats to stay warm. Heat and how it flows within and between objects is something we experience every day and a fundamental engineering concern.

Thermal Energy and Heat: Every object in the universe has thermal energy stored within it. Thermal energy is the energy embodied in the vibrations, rotations and translations of atoms and molecules. This motion is extremely fast, significantly faster than indicated in the animations typically shown, and significantly faster than bulk translation (such as the flow of water molecules in a river). Expect the presence of energy in a system of jiggling, bouncing, molecules to be very obvious to students who already understand the concept of kinetic energy; indeed, the underlying physical mechanism is similar.

The energy contained in thermal "jiggling" is a function of many factors such as the mass of the particles and the speed of their motion. However, for a given material, faster molecular movement means more thermal energy is present.

Thermal energy is almost impossible to confine to a location. Rather, it can be causally observed every day. A cup of tea left on the counter cools off. Touching a hot pot lid burns one's hand. Objects that are in thermal contact tend towards thermal equilibrium, that is, they exchange thermal energy until both objects have the same temperature. When thermal energy moves around, the flowing thermal energy is called heat. This is somewhat confused by the engineering terminology of "heat transfer" (the study of just how that heat is moved around), which is somewhat redundant since the word "heat" already conveys the motion of thermal energy. In this document, "heat," "heat flow" and "heat transfer" all mean the flow of thermal energy.

One common example of thermal equilibrium is a cup of hot tea. Thermal energy in hot tea will flow (as heat) into the air because the tea temperature is higher than the air temperature. Heat leaving the tea causes the tea's temperature to decrease. Heat going into the air causes the air's temperature to increase. This process continues until the temperature of the tea and air is exactly the same, that is, until thermal equilibrium has been reached and no more impetus exists for thermal energy to move as heat. This is discussed further in the presentation using the analogy of a skier on a hill.

The mechanism of heat flow can be understood by remembering thermal "jiggling." Imagine placing a room temperature pot on a hot stove. Initially, the pot is 25 °C while the cooking element might be 600 °C. We know that heat is flowing from the element to the pot, because the pot's temperature increases. If we had a sufficiently powerful microscope, we could observe the atoms in the element and the pot. The lower temperature pot atoms would be jiggling around much more slowly than the atoms in the element. Since the two are touching, eventually a vigorously jiggling element atom collides with a slower jiggling pot atom. Just as a fast-moving cue ball collides with an eight ball and transfers some of its kinetic energy, the element transfers its thermal energy to the pot through countless such collisions.

The following is a very subtle point. The slowly jiggling pot atoms in the previous example might collide with the swiftly jiggling element atoms and transfer some kinetic energy FROM THE POT TO THE ELEMENT. This is quite the opposite from the established direction of heat transfer, that is, from high temperature to low temperature (or "hot to cold" in the easier-to-repeat shorthand phrase). Although this "opposite" mechanism may occur in isolated interactions, averaging the flow of heat over billions and billions of collisions always results in the "hot to cold" direction with which we are all familiar. Thermal equilibrium is reached when these collisions (again on average) involve the same amount of energy flowing into and out of the pot. At this point, both items are at the same temperature, and heat ceases to flow. Along these lines, "cold" is not a substance that flows. What happens when holding an icy soda can is NOT "cold flowing into my hand." The person holding the can experiences the sensation of a cold hand because the thermal energy in the hand has flowed, as heat, into the lower temperature soda can and given enough time, the two reach thermal equilibrium.

Types of Heat Transfer: Heat flows from objects of higher temperature to objects of lower temperature, and occurs in three forms, referred to by engineers as heat transfer: conduction, convection and radiation.

Conduction is heat flow in or between solid objects. If one touched the top edge of the pot in a previously described example, they would be burned. It is well known that heat flows from the bottom of a pot and into the upper edge, lid and handle. The mechanism of this heat flow is just as described in the pot and element example. Atoms in the bottom of the pot are jiggled by the hotter element atoms. The "front line" pot atoms then collide with their neighbors and then the next neighbors, eventually transferring thermal energy all through the pot.

A cast iron pan, left on the stove long enough, requires an oven mitt to handle. Heat flows from the element, into the pan, up the edge and along the handle. A pan with a wooden or plastic handle does not suffer from this problem because those materials have much lower thermal conductivity (the materials property that describes how well something conducts thermal energy) than the iron pot handle. Insulators such as wool, wood and Styrofoam have low thermal conductivity and are useful for slowing the flow of heat. Materials with high thermal conductivity such as copper, aluminum and glass are used to help heat move more quickly. As evidenced in the choice of materials used for electrical conductors and insulators, most materials with high electrical conductivity also have high thermal conductivity.

Convection is the flow of heat in gases or liquids; both are called "fluids" by engineers. A hair drier provides an excellent example of convection. Just as in the stove element, a piece of metal inside a hair drier is heated with electricity. Imagine if no fans were included inside hair driers. The air molecules near the hot elements atoms would be collided with, and heat would flow into them. In the case of the solid pot, the pot atoms are prevented from large movements because the pot is a solid. The pot atoms might jiggle and vibrate, but cannot go flying off across the room (unless heated to a very high temperature indeed). In the hair drier, the gaseous air molecules are much freer to move. They do this naturally in a process called free convection, which can be described by the familiar mechanism of "hot air rises." The rising hot air allows fresh cold air molecules to come into contact with the hot element atoms. Forced convection is what occurs in the hair drier—a fan blows high-speed air molecules over the hot element. In both cases of convection, the jiggling air molecules continue their jiggling when pushed away from the element. Depending on how fast the new air molecules are pushed past the element, convection can move heat over much larger distances, and much more quickly than conduction. The best remedy for a burned finger is to put it under flowing tap water. The subtleties of forced vs. free convection are beyond the scope of a sixth-grade class. The presentation simply refers to all heat transfer in liquids and gases as convection, with examples of the simpler fan-driven forced convection provided.

Radiation is the flow of heat carried by little packets of energy called photons. Radiation can transfer heat between two objects even in empty space, which is how the energy from the Sun gets to Earth. Although radiation does not need air to travel, it can travel through gases, liquids and even some solids. The cause of radiation is fairly complex. When a charged particle is accelerated, it emits a bit of radiation called a photon. Everything in the universe emits radiation because thermal energy causes electrons to accelerate and emit radiation (everything in the universe has some thermal energy). The amount of radiation an object emits is proportional to its temperature to the fourth power, so radiation is the dominant form of heat transfer only at fairly high temperatures. Just as before, the mechanism of heat flow through radiation can be imagined with the billiard ball collision example (although this is not as accurate an explanation of the underlying physics with radiation, it suffices). A photon from a high-temperature object strikes an atom in a lower-temperature object, causing it to jiggle more, raising the cooler object's temperature. Just as with the aside in the original pot/element discussion, some subtlety exists. Since all objects (even -400 °F comets) emit some radiation, an ice cube next to a red hot piece of iron is transferring energy from itself to the iron through radiation. But, for every one photon from the ice cube that strikes an iron atom, many thousands of photons transfer heat from the iron to the ice. So, on average, heat flows from hot to cold.

All three forms of heat flow occur at the same time, though some typically dominate, which permits engineers to ignore the others. Blowing a large fan over a 100 °C piece of metal involves almost entirely convection, but a little conduction (into the ground say) and a little radiation (heating the walls of the room) does occur.

Watch this activity on YouTube

(After the associated activity) We have discussed that heat is simply the flow of thermal energy that always goes from ________ to ________. (Expect everyone to chant out loud "from hot to cold.") We also know the three ways that heat can be transferred, which are _____________. (Answer: Conduction, convection and radiation.) Now, putting it all together and using what we understand about insulators, write and explain one way you can stay cool in the summertime and one way you can keep warm in the wintertime.

conduction: Heat transfer within or between solid objects.

convection: Heat transfer into or out of fluids.

heat: Thermal energy that flows due to a difference in temperature. Heat flows from hot to cold.

heat transfer: A method by which heat flows (conduction, convection, radiation).

insulation: A material that slows down heat transfer.

radiation: Heat transfer due to packets of energy called photons that can travel through many substances, even empty space.

temperature: the measure of the average speed of all particles.

thermal energy: the total energy of all particles in an object.

Pre-Lesson Assessment

Class Discussion & Assignment: To get students thinking about heat, lead a discussion and present a few everyday examples of heat, such as hot beverages, grabbing hot pans or touching ice cubes. Ask students to write a few sentences about how temperature and energy might be related. Also have each student draw an example of an everyday hot object. Provide a list of some examples: hot cocoa, a coal from a fire and a pan right out of the oven. Then ask students to draw a cold object near the hot one. This might be an ice cube, a can of soda from the refrigerator or cold air. Then ask students to draw arrows in their pictures that show what direction the energy flows (from the hot to the cold object, regardless of orientation).

Post-Introduction Assessment

Drawing Arrows: Use slide 8 of the What Is Heat? Presentation as an example and then have each student work individually during slides 9-11 to identify the direction of heat transfer by drawing arrows and writing a sentence. Circulate the room to verify and/or correct their understanding of the concepts.

Lesson Summary Assessment

Post-Quiz: After the lesson, and before starting the associated activity, administer the 10-question What Is Heat? Post-Quiz . Review students' answers to assess their comprehension of the thermal energy concepts.

Written Examples: As part of the Lesson Closure after completing the associated activity, assign students to write and explain one way they can stay cool in the summertime and one way they can keep warm in the wintertime. Require that they use scientific terminology as part of their explanations.

As an alternative to the thermal energy class demo, show this 2:52-minute video, "Moving Water Molecules" as a good illustration of the same demonstration: https://www.youtube.com/watch?v=CXY02tcgiBY .

transfer of heat physics ppt

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preview of 'Heat Transfer: No Magic About It' Lesson

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preview of 'Heat Transfer' Lesson

Other Related Information

Browse the NGSS Engineering-aligned Physics Curriculum hub for additional Physics and Physical Science curriculum featuring Engineering.

Contributors

Supporting program, acknowledgements.

The contents of this digital library curriculum were developed by the Renewable Energy Systems Opportunity for Unified Research Collaboration and Education (RESOURCE) project in the College of Engineering under National Science Foundation GK-12 grant no. DGE 0948021. However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.

Last modified: October 31, 2021

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Heat Transfer – Conduction, Convection, Radiation

Types of Heat Transfer

Heat transfer occurs when thermal energy moves from one place to another. Atoms and molecules inherently have kinetic and thermal energy, so all matter participates in heat transfer. There are three main types of heat transfer, plus other processes that move energy from high temperature to low temperature.

What Is Heat Transfer?

Heat transfer is the movement of heat due to a temperature difference between a system and its surroundings. The energy transfer is always from higher temperature to lower temperature, due to the second law of thermodynamics . The units of heat transfer are the joule (J), calorie (cal), and kilocalorie (kcal). The unit for the rate of heat transfer is the kilowatt (KW).

The Three Types of Heat Transfer With Examples

The three types of heat transfer differ according to the nature of the medium that transmits heat:

  • Conduction requires contact.
  • Convection requires fluid flow.
  • Radiation does not require any medium.
  • Conduction is heat transfer directly between neighboring atoms or molecules. Usually, it is heat transfer through a solid. For example, the metal handle of a pan on a stove becomes hot due to convection. Touching the hot pan conducts heat to your hand.
  • Convection is heat transfer via the movement of a fluid, such as air or water. Heating water on a stove is a good example. The water at the top of the pot becomes hot because water near the heat source rises. Another example is the movement of air around a campfire. Hot air rises, transferring heat upward. Meanwhile, the partial vacuum left by this movement draws in cool outside air that feeds the fire with fresh oxygen.
  • Radiation is the emission of electromagnetic radiation. While it occurs through a medium, it does not require one. For example, it’s warm outside on a sunny day because solar radiation crosses space and heats the atmosphere. The burner element of a stove also emits radiation. However, some heat from a burner comes from conduction between the hot element and a metal pan. Most real-life processes involve multiple forms of heat transfer.

Conduction requires that molecules touch each other, making it a slower process than convection or radiation. Atoms and molecules with a lot of energy have more kinetic energy and engage in more collisions with other matter. They are “hot.” When hot matter interacts with cold matter, some energy gets transferred during the collision. This drives conduction. Forms of matter that readily conduct heat are called thermal conductors .

Examples of Conduction

Conduction is a common process in everyday life. For example:

  • Holding an ice cube immediately makes your hands feel cold. Meanwhile, the heat transferred from your skin to the ice melts it into liquid water.
  • Walking barefoot on a hot road or sunny beach burns your feet because the solid material transmits heat into your foot.
  • Iron clothes transfers heat from the iron to the fabric.
  • The handle of a coffee cup filled with hot coffee becomes warm or even hot via conduction through the mug material.

Conduction Equation

One equation for conduction calculates heat transfer per unit of time from thermal conductivity, area, thickness of the material, and the temperature difference between two regions:

Q = [K ∙ A ∙ (T hot – T cold )] / d

  • Q is heat transfer per unit time
  • K is the coefficient of thermal conductivity of the substance
  • A is the area of heat transfer
  • T hot  is the temperature of the hot region
  • T cold  is the temperature of the cold region
  • d is the thickness of the body

Convection is the movement of fluid molecules from higher temperature to lower temperature regions. Changing the temperature of a fluid affects its density, producing convection currents. If the volume of a fluid increases, than its density decreases and it becomes buoyant.

Examples of Convection

Convection is a familiar process on Earth, primarily involving air or water. However, it applies to other fluids, such as refrigeration gases and magma. Examples of convection include:

  • Boiling water undergoes convection as less dense hot molecules rise through higher density cooler molecules.
  • Hot air rises and cooler air sinks and replaces it.
  • Convection drives global circulation in the oceans between the equators and poles.
  • A convection oven circulates hot air and cooks more evenly than one that only uses heating elements or a gas flame.

Convection Equation

The equation for the rate of convection relates area and the difference between the fluid temperature and surface temperature:

Q = h c  ∙ A ∙ (T s  – T f )

  • Q is the heat transfer per unit time
  • h c  is the coefficient of convective heat transfer
  • T s  is the surface temperature
  • T f  is the fluid temperature

Radiation is the release of electromagnetic energy. Another name for thermal radiation is radiant heat. Unlike conduction or convection, radiation requires no medium for heat transfer. So, radiation occurs both within a medium (solid, liquid, gas) or through a vacuum.

Examples of Radiation

There are many examples of radiation:

  • A microwave oven emits microwave radiation, which increases the thermal energy in food
  • The Sun emits light (including ultraviolet radiation) and heat
  • Uranium-238 emits alpha radiation as it decays into thorium-234

Radiation Equation

The Stephan-Boltzmann law describes relationship between the power and temperature of thermal radiation:

P = e ∙ σ ∙ A· (Tr – Tc) 4

  • P is the net power of radiation
  • A is the area of radiation
  • Tr is the radiator temperature
  • Tc is the surrounding temperature
  • e is emissivity
  • σ is Stefan’s constant (σ = 5.67 × 10 -8 Wm -2 K -4 )

More Heat Transfer – Chemical Bonds and Phase Transitions

While conduction, convection, and radiation are the three modes of heat transfer, other processes absorb and release heat. For example, atoms release energy when chemical bonds break and absorb energy in order to form bonds. Releasing energy is an exergonic process, while absorbing energy is an endergonic process. Sometimes the energy is light or sound, but most of the time it’s heat, making these processes exothermic and endothermic .

Phase transitions between the states of matter also involve the absorption or release of energy. A great example of this is evaporative cooling, where the phase transition from a liquid into a vapor absorbs thermal energy from the environment.

  • Faghri, Amir; Zhang, Yuwen; Howell, John (2010). Advanced Heat and Mass Transfer . Columbia, MO: Global Digital Press. ISBN 978-0-9842760-0-4.
  • Geankoplis, Christie John (2003). Transport Processes and Separation Principles (4th ed.). Prentice Hall. ISBN 0-13-101367-X.
  • Peng, Z.; Doroodchi, E.; Moghtaderi, B. (2020). “Heat transfer modelling in Discrete Element Method (DEM)-based simulations of thermal processes: Theory and model development”. Progress in Energy and Combustion Science . 79: 100847. doi: 10.1016/j.pecs.2020.100847
  • Welty, James R.; Wicks, Charles E.; Wilson, Robert Elliott (1976). Fundamentals of Momentum, Heat, and Mass Transfer (2nd ed.). New York: Wiley. ISBN 978-0-471-93354-0.

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transfer of heat physics ppt

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If you have been following along since the beginning of this lesson, then you have been developing a progressively sophisticated understanding of temperature and heat. You should be developing a model of matter as consisting of particles which vibrate (wiggle about a fixed position), translate (move from one location to another) and even rotate (revolve about an imaginary axis). These motions give the particles kinetic energy. Temperature is a measure of the average amount of kinetic energy possessed by the particles in a sample of matter. The more the particles vibrate, translate and rotate, the greater the temperature of the object. You have hopefully adopted an understanding of heat as a flow of energy from a higher temperature object to a lower temperature object. It is the temperature difference between the two neighboring objects that causes this heat transfer. The heat transfer continues until the two objects have reached thermal equilibrium and are at the same temperature. The discussion of heat transfer has been structured around some everyday examples such as the cooling of a hot mug of coffee and the warming of a cold can of pop. Finally, we have explored a thought experiment in which a metal can containing hot water is placed within a Styrofoam cup containing cold water. Heat is transferred from the hot water to the cold water until both samples have the same temperature.

  • What is happening at the particle level when energy is being transferred between two objects?
  • Why is thermal equilibrium always established when two objects transfer heat?
  • How does heat transfer work within the bulk of an object?
  • Is there more than one method of heat transfer? If so, then how are they similar and different than one another?

Conduction - A Particle View

Let's begin our discussion by returning to our thought experiment in which a metal can containing hot water was placed within a Styrofoam cup containing cold water. Heat is transferred from the hot water to the cold water until both samples have the same temperature. In this instance, the transfer of heat from the hot water through the metal can to the cold water is sometimes referred to as conduction . Conductive heat flow involves the transfer of heat from one location to another in the absence of any material flow. There is nothing physical or material moving from the hot water to the cold water. Only energy is transferred from the hot water to the cold water. Other than the loss of energy, there is nothing else escaping from the hot water. And other than the gain of energy, there is nothing else entering the cold water. How does this happen? What is the mechanism that makes conductive heat flow possible?

The container walls represent the perimeters of a sample of matter. Just as the perimeter of your property (as in real estate property) is the furthest extension of the property, so the perimeter of an object is the furthest extension of the particles within a sample of matter. At the perimeter, the little bangers are colliding with particles of another substance - the particles of the container or even the surrounding air. Even the wigglers that are fixed in a position along the perimeter are doing some banging. Being at the perimeter, their wiggling results in collisions with the particles that are next to them; these are the particles of the container or of the surrounding air.

The mechanism in which heat is transferred from one object to another object through particle collisions is known as conduction. In conduction, there is no net transfer of physical stuff between the objects. Nothing material moves across the boundary. The changes in temperature are wholly explained as the result of the gains and losses of kinetic energy during collisions.

Conduction Through The Bulk of an Object

We have discussed how heat transfers from one object to another through conduction. But how does it transfer through the bulk of an object? For instance, suppose we pull a ceramic coffee mug out of the cupboard and place it on the countertop. The mug is at room temperature - maybe at 26°C. Then suppose we fill the ceramic coffee mug with hot coffee at a temperature of 80°C. The mug quickly warms up. Energy first flows into the particles at the boundary between the hot coffee and the ceramic mug. But then it flows through the bulk of the ceramic to all parts of the ceramic mug. How does heat conduction occur in the ceramic itself?

The mechanism of heat transfer through the bulk of the ceramic mug is described in a similar manner as it before. The ceramic mug consists of a collection of orderly arranged wigglers. These are particles that wiggle about a fixed position. As the ceramic particles at the boundary between the hot coffee and the mug warm up, they attain a kinetic energy that is much higher than their neighbors. As they wiggle more vigorously, they bang into their neighbors and increase their vibrational kinetic energy. These particles in turn begin to wiggle more vigorously and their collisions with their neighbors increase their vibrational kinetic energy. The process of energy transfer by means of the little bangers continues from the particles at the inside of the mug (in contact with the coffee particles) to the outside of the mug (in contact with the surrounding air). Soon the entire coffee mug is warm and your hand feels it.

This mechanism of conduction by particle-to-particle interaction is very common in ceramic materials such as a coffee mug. Does it work the same in metal objects? For instance, you likely have noticed the high temperatures attained by the metal handle of a skillet when placed upon a stovetop. The burners on the stove transfer heat to the metal skillet. If the handle of the skillet is metallic, it too attains a high temperature, certainly high enough to cause a bad burn. The transfer of heat from the skillet to the skillet handle occurs by conduction. But in metals, the conduction mechanism is slightly more complicated. In a manner similar to electrical conductivity, thermal conductivity in metals occurs by the movement of free electrons . Outer shell electrons of metal atoms are shared among atoms and are free to move throughout the bulk of the metal. These electrons carry the energy from the skillet to the skillet handle. The details of this mechanism of thermal conduction in metals are considerably more complex than the discussion given here. The main point to grasp is that heat transfer through metals occurs without any movement of atoms from the skillet to the skillet handle. This qualifies the heat transfer as being categorized as thermal conduction.

Heat Transfer by Convection

Convection is the main method of heat transfer in fluids such as water and air. It is often said that heat rises in these situations. The more appropriate explanation is to say that heated fluid rises . For instance, as the heated air rises from the heater on a floor, it carries more energetic particles with it. As the more energetic particles of the heated air mix with the cooler air near the ceiling, the average kinetic energy of the air near the top of the room increases. This increase in the average kinetic energy corresponds to an increase in temperature. The net result of the rising hot fluid is the transfer of heat from one location to another location. The convection method of heat transfer always involves the transfer of heat by the movement of matter. This is not to be confused with the caloric theory discussed earlier in this lesson. In caloric theory, heat was the fluid and the fluid that moved was the heat. Our model of convection considers heat to be energy transfer that is simply the result of the movement of more energetic particles.

The two examples of convection discussed here - heating water in a pot and heating air in a room - are examples of natural convection . The driving force of the circulation of fluid is natural - differences in density between two locations as the result of fluid being heated at some source. (Some sources introduce the concept of buoyant forces to explain why the heated fluids rise. We will not pursue such explanations here.) Natural convection is common in nature. The earth's oceans and atmosphere are heated by natural convection. In contrast to natural convection, forced convection involves fluid being forced from one location to another by fans, pumps and other devices. Many home heating systems involve force air heating. Air is heated at a furnace and blown by fans through ductwork and released into rooms at vent locations. This is an example of forced convection. The movement of the fluid from the hot location (near the furnace) to the cool location (the rooms throughout the house) is driven or forced by a fan. Some ovens are forced convection ovens; they have fans that blow heated air from a heat source into the oven. Some fireplaces enhance the heating ability of the fire by blowing heated air from the fireplace unit into the adjacent room. This is another example of forced convection.

Heat Transfer by Radiation

A final method of heat transfer involves radiation. Radiation is the transfer of heat by means of electromagnetic waves . To radiate means to send out or spread from a central location. Whether it is light, sound, waves, rays, flower petals, wheel spokes or pain, if something radiates then it protrudes or spreads outward from an origin. The transfer of heat by radiation involves the carrying of energy from an origin to the space surrounding it. The energy is carried by electromagnetic waves and does not involve the movement or the interaction of matter. Thermal radiation can occur through matter or through a region of space that is void of matter (i.e., a vacuum). In fact, the heat received on Earth from the sun is the result of electromagnetic waves traveling through the void of space between the Earth and the sun.

All objects radiate energy in the form of electromagnetic waves. The rate at which this energy is released is proportional to the Kelvin temperature ( T ) raised to the fourth power.

Radiation rate = k•T 4

Our discussion on this page has pertained to the various methods of heat transfer. Conduction, convection and radiation have been described and illustrated. The macroscopic has been explained in terms of the particulate - an ongoing goal of this chapter of The Physics Classroom Tutorial. The last topic to be discussed in Lesson 1 is more quantitative in nature. On the next page , we will investigate the mathematics associated with the rate of heat transfer.

Check Your Understanding

1. Consider Object A which has a temperature of 65°C and Object B which has a temperature of 15°C. The two objects are placed next to each other and the little bangers begin colliding. Will any of the collisions result in the transfer of energy from Object B to Object A? Explain.

Answer: Most certainly yes.

The average kinetic energy of the particles in Object A is greater than the average kinetic energy of the particles in Object B. But there is a range of speeds and thus of kinetic energy in both objects. As such, there will be some highly energetic particles in Object B and some very non-energetic particles in Object A. When this combination of particles encounter a collision, there will a transfer of energy across the boundary from Object B (the colder object) to Object A (the hotter object). This is just one collision. Since majority of collisions result from the more energetic particles of Object A with less energetic particles of collision B, there will be a net kinetic energy transfer from Object A to Object B.

2. Suppose that Object A and Object B (from the previous problem) have reached a thermal equilibrium. Do the particles of the two objects still collide with each other? If so, do any of the collisions result in the transfer of energy between the two objects? Explain.

The collisions will still take place because the particles are still moving. Just because the temperatures are the same doesn't mean the collisions will stop. The fact that the temperature is identical means that the average kinetic energy of all the particles is the same for both objects. As such, there will be just as much energy transferred from Object B to Object A as there is energy transferred in the opposite direction. When the effect of these collisions is averaged , there is no net energy transfer. This explains why the temperature of the two objects remains the same. Thermal equilibrium persists.
  • What Does Heat Do?

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Heat TRANSFER MECHANISMS SHORT 4e Chap01 lecture (1)

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H eat transfer is a basic science that deals with the rate of transfer of thermal energy. This introductory text is intended for use in a first course in heat transfer for undergraduate engineering students, and as a reference book for practicing engineers. The objectives of this text are • To cover the basic principles of heat transfer. • To present a wealth of real-world engineering applications to give students a feel for engineering practice. • To develop an intuitive understanding of the subject matter by emphasizing the physics and physical arguments. Students are assumed to have completed their basic physics and calculus sequence. The completion of first courses in thermodynamics, fluid mechanics, and differential equations prior to taking heat transfer is desirable. The relevant concepts from these topics are introduced and reviewed as needed. In engineering practice, an understanding of the mechanisms of heat transfer is becoming increasingly important since heat transfer plays a crucial role in the design of vehicles, power plants, refrigerators, electronic devices, buildings , and bridges, among other things. Even a chef needs to have an intuitive understanding of the heat transfer mechanism in order to cook the food "right" by adjusting the rate of heat transfer. We may not be aware of it, but we already use the principles of heat transfer when seeking thermal comfort. We insulate our bodies by putting on heavy coats in winter, and we minimize heat gain by radiation by staying in shady places in summer. We speed up the cooling of hot food by blowing on it and keep warm in cold weather by cuddling up and thus minimizing the exposed surface area. That is, we already use heat transfer whether we realize it or not.

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1.5: Heat Transfer, Specific Heat, and Calorimetry

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Learning Objectives

By the end of this section, you will be able to:

  • Explain phenomena involving heat as a form of energy transfer
  • Solve problems involving heat transfer

We have seen in previous chapters that energy is one of the fundamental concepts of physics. Heat is a type of energy transfer that is caused by a temperature difference, and it can change the temperature of an object. As we learned earlier in this chapter, heat transfer is the movement of energy from one place or material to another as a result of a difference in temperature. Heat transfer is fundamental to such everyday activities as home heating and cooking, as well as many industrial processes. It also forms a basis for the topics in the remainder of this chapter.

We also introduce the concept of internal energy, which can be increased or decreased by heat transfer. We discuss another way to change the internal energy of a system, namely doing work on it. Thus, we are beginning the study of the relationship of heat and work, which is the basis of engines and refrigerators and the central topic (and origin of the name) of thermodynamics.

Internal Energy and Heat

A thermal system has internal energy (also called thermal energy ) , which is the sum of the mechanical energies of its molecules. A system’s internal energy is proportional to its temperature. As we saw earlier in this chapter, if two objects at different temperatures are brought into contact with each other, energy is transferred from the hotter to the colder object until the bodies reach thermal equilibrium (that is, they are at the same temperature). No work is done by either object because no force acts through a distance (as we discussed in Work and Kinetic Energy ). These observations reveal that heat is energy transferred spontaneously due to a temperature difference. Figure \(\PageIndex{1}\) shows an example of heat transfer.

Figure a shows a soda can at temperature T1 and an ice cube, some distance away at temperature T2. T1 is greater than T2. Figure b shows the can and cube in contact with each other. Both are at temperature T prime.

The meaning of “heat” in physics is different from its ordinary meaning. For example, in conversation, we may say “the heat was unbearable,” but in physics, we would say that the temperature was high. Heat is a form of energy flow, whereas temperature is not. Incidentally, humans are sensitive to heat flow rather than to temperature.

Since heat is a form of energy, its SI unit is the joule (J). Another common unit of energy often used for heat is the calorie (cal), defined as the energy needed to change the temperature of 1.00 g of water by \(1.00^oC\)—specifically, between \(14.5^oC\) and \(15.5^oC\) since there is a slight temperature dependence. Also commonly used is the kilocalorie (kcal), which is the energy needed to change the temperature of 1.00 kg of water by \(1.00^oC\). Since mass is most often specified in kilograms, the kilocalorie is convenient. Confusingly, food calories (sometimes called “big calories,” abbreviated Cal) are actually kilocalories, a fact not easily determined from package labeling.

Mechanical Equivalent of Heat

It is also possible to change the temperature of a substance by doing work, which transfers energy into or out of a system. This realization helped establish that heat is a form of energy. James Prescott Joule (1818–1889) performed many experiments to establish the mechanical equivalent of heat — the work needed to produce the same effects as heat transfer . In the units used for these two quantities, the value for this equivalence is

\[1.000 \, kcal = 4186 \, J.\] We consider this equation to represent the conversion between two units of energy. (Other numbers that you may see refer to calories defined for temperature ranges other than \(14.5^oC\) to \(15.5^oC\).)

Figure \(\PageIndex{2}\) shows one of Joule’s most famous experimental setups for demonstrating that work and heat can produce the same effects and measuring the mechanical equivalent of heat. It helped establish the principle of conservation of energy. Gravitational potential energy ( U ) was converted into kinetic energy ( K ), and then randomized by viscosity and turbulence into increased average kinetic energy of atoms and molecules in the system, producing a temperature increase. Joule’s contributions to thermodynamics were so significant that the SI unit of energy was named after him.

An insulated cylindrical container is filled with known volume water. A vertical rod is immersed in it. This has paddles which would stir the water if the rod were rotated. The top portion of the rod is outside the water. A string is tied around it, both ends of which go over pulleys and support weights on either side. A lever at the top is used to rotate the rod. A thermometer is kept in the water. The distance from the cente of the weight and the pully to the base of the container is labeled measured height of descent.

Increasing internal energy by heat transfer gives the same result as increasing it by doing work. Therefore, although a system has a well-defined internal energy, we cannot say that it has a certain “heat content” or “work content.” A well-defined quantity that depends only on the current state of the system, rather than on the history of that system, is known as a state variable . Temperature and internal energy are state variables. To sum up this paragraph, heat and work are not state variables .

Incidentally, increasing the internal energy of a system does not necessarily increase its temperature. As we’ll see in the next section, the temperature does not change when a substance changes from one phase to another. An example is the melting of ice, which can be accomplished by adding heat or by doing frictional work, as when an ice cube is rubbed against a rough surface.

Temperature Change and Heat Capacity

We have noted that heat transfer often causes temperature change. Experiments show that with no phase change and no work done on or by the system, the transferred heat is typically directly proportional to the change in temperature and to the mass of the system, to a good approximation. (Below we show how to handle situations where the approximation is not valid.) The constant of proportionality depends on the substance and its phase, which may be gas, liquid, or solid. We omit discussion of the fourth phase, plasma, because although it is the most common phase in the universe, it is rare and short-lived on Earth.

We can understand the experimental facts by noting that the transferred heat is the change in the internal energy, which is the total energy of the molecules. Under typical conditions, the total kinetic energy of the molecules \(K_{total}\) is a constant fraction of the internal energy (for reasons and with exceptions that we’ll see in the next chapter). The average kinetic energy of a molecule \(K_{ave}\) is proportional to the absolute temperature. Therefore, the change in internal energy of a system is typically proportional to the change in temperature and to the number of molecules, N . Mathematically, \(\Delta U \propto \Delta K_{total} = NK_{ave} \propto N\Delta T\). The dependence on the substance results in large part from the different masses of atoms and molecules. We are considering its heat capacity in terms of its mass, but as we will see in the next chapter, in some cases, heat capacities per molecule are similar for different substances. The dependence on substance and phase also results from differences in the potential energy associated with interactions between atoms and molecules.

Heat Transfer and Temperature Change

A practical approximation for the relationship between heat transfer and temperature change is:

\[Q = mc\Delta T,\]

where \(Q\) is the symbol for heat transfer (“quantity of heat”), m is the mass of the substance, and \(\Delta T\) is the change in temperature. The symbol c stands for the specific heat (also called “ specific heat capacity ”) and depends on the material and phase. The specific heat is numerically equal to the amount of heat necessary to change the temperature of \(1.00 \, kg\) of mass by \(1.00^oC\). The SI unit for specific heat is \(J/(kg \times K)\) or \(J/(kg \times ^oC)\). (Recall that the temperature change \(\Delta T\) is the same in units of kelvin and degrees Celsius.)

Values of specific heat must generally be measured, because there is no simple way to calculate them precisely. Table \(\PageIndex{1}\) lists representative values of specific heat for various substances. We see from this table that the specific heat of water is five times that of glass and 10 times that of iron, which means that it takes five times as much heat to raise the temperature of water a given amount as for glass, and 10 times as much as for iron. In fact, water has one of the largest specific heats of any material, which is important for sustaining life on Earth.

The specific heats of gases depend on what is maintained constant during the heating—typically either the volume or the pressure. In the table, the first specific heat value for each gas is measured at constant volume, and the second (in parentheses) is measured at constant pressure. We will return to this topic in the chapter on the kinetic theory of gases.

In general, specific heat also depends on temperature. Thus, a precise definition of c for a substance must be given in terms of an infinitesimal change in temperature. To do this, we note that \(c = \frac{1}{m} \frac{\Delta Q}{\Delta T}\) and replace \(\Delta\) with d:

\[c = \dfrac{1}{m} \dfrac{dQ}{dT}.\]

Except for gases, the temperature and volume dependence of the specific heat of most substances is weak at normal temperatures. Therefore, we will generally take specific heats to be constant at the values given in the table.

Example \(\PageIndex{1}\): Calculating the Required Heat

A 0.500-kg aluminum pan on a stove and 0.250 L of water in it are heated from \(20.0^oC\) to \(80.0^oC\). (a) How much heat is required? What percentage of the heat is used to raise the temperature of (b) the pan and (c) the water?

We can assume that the pan and the water are always at the same temperature. When you put the pan on the stove, the temperature of the water and that of the pan are increased by the same amount. We use the equation for the heat transfer for the given temperature change and mass of water and aluminum. The specific heat values for water and aluminum are given in Table \(\PageIndex{1}\).

  • Calculate the temperature difference: \[\Delta t = T_f - T_i = 60.0^oC.\]
  • Calculate the mass of water. Because the density of water is \(1000 \, kg/m^3\), 1 L of water has a mass of 1 kg, and the mass of 0.250 L of water is \(m_w = 0.250 \, kg.\)
  • Calculate the heat transferred to the water. Use the specific heat of water in Table \(\PageIndex{1}\): \[Q_w = m_wc_w\Delta T = (0.250 \, kg)(4186 \, J/kg ^oC)(60.0 ^oC) = 62.8 \, kJ.\]
  • Calculate the heat transferred to the aluminum. Use the specific heat for aluminum in Table \(\PageIndex{1}\): \[Q_{A1} = m_{A1}c_{A1}\Delta T = (0.500 \, kg)(900 \, J/kg^oC)(60.0^oC) = 27.0 \, kJ.\]
  • Find the total transferred heat: \[Q_{Total} = Q_W + Q_{A1} = 89.8 \, kJ.\]

Significance

In this example, the heat transferred to the container is a significant fraction of the total transferred heat. Although the mass of the pan is twice that of the water, the specific heat of water is over four times that of aluminum. Therefore, it takes a bit more than twice as much heat to achieve the given temperature change for the water as for the aluminum pan.

Example \(\PageIndex{2}\) illustrates a temperature rise caused by doing work. (The result is the same as if the same amount of energy had been added with a blowtorch instead of mechanically.)

Calculating the Temperature Increase from the Work Done on a Substance.

Truck brakes used to control speed on a downhill run do work, converting gravitational potential energy into increased internal energy (higher temperature) of the brake material (Figure \(\PageIndex{3}\)). This conversion prevents the gravitational potential energy from being converted into kinetic energy of the truck. Since the mass of the truck is much greater than that of the brake material absorbing the energy, the temperature increase may occur too fast for sufficient heat to transfer from the brakes to the environment; in other words, the brakes may overheat.

Figure shows a truck on a road. There is smoke near its tires.

Calculate the temperature increase of 10 kg of brake material with an average specific heat of \(800 \, J/kg \cdot ^C\) if the material retains 10% of the energy from a 10,000-kg truck descending 75.0 m (in vertical displacement) at a constant speed.

We calculate the gravitational potential energy ( Mgh ) that the entire truck loses in its descent, equate it to the increase in the brakes’ internal energy, and then find the temperature increase produced in the brake material alone.

First we calculate the change in gravitational potential energy as the truck goes downhill:

\[Mgh = (10,000 \, kg)(9.80 \, m/s^2)(75.0 \, m) = 7.35 \times 10^6 \, J. \nonumber\]

Because the kinetic energy of the truck does not change, conservation of energy tells us the lost potential energy is dissipated, and we assume that 10% of it is transferred to internal energy of the brakes, so take \(Q = Mgh/10\). Then we calculate the temperature change from the heat transferred, using

\[\Delta T = \dfrac{7.35 \times 10^5 \, J}{(10 \, kg)(800 \, J/kg^oC)} = 92^oC. \nonumber\]

If the truck had been traveling for some time, then just before the descent, the brake temperature would probably be higher than the ambient temperature. The temperature increase in the descent would likely raise the temperature of the brake material very high, so this technique is not practical. Instead, the truck would use the technique of engine braking. A different idea underlies the recent technology of hybrid and electric cars, where mechanical energy (kinetic and gravitational potential energy) is converted by the brakes into electrical energy in the battery, a process called regenerative braking.

In a common kind of problem, objects at different temperatures are placed in contact with each other but isolated from everything else, and they are allowed to come into equilibrium. A container that prevents heat transfer in or out is called a calorimeter , and the use of a calorimeter to make measurements (typically of heat or specific heat capacity) is called calorimetry .

We will use the term “calorimetry problem” to refer to any problem in which the objects concerned are thermally isolated from their surroundings. An important idea in solving calorimetry problems is that during a heat transfer between objects isolated from their surroundings, the heat gained by the colder object must equal the heat lost by the hotter object, due to conservation of energy:

\[Q_{cold} + Q_{hot} = 0.\]

We express this idea by writing that the sum of the heats equals zero because the heat gained is usually considered positive; the heat lost, negative.

Calculating the Final Temperature in Calorimetry

Suppose you pour 0.250 kg of \(20.0^oC\) water (about a cup) into a 0.500-kg aluminum pan off the stove with a temperature of \(150^oC\). Assume no heat transfer takes place to anything else: The pan is placed on an insulated pad, and heat transfer to the air is neglected in the short time needed to reach equilibrium. Thus, this is a calorimetry problem, even though no isolating container is specified. Also assume that a negligible amount of water boils off. What is the temperature when the water and pan reach thermal equilibrium?

Originally, the pan and water are not in thermal equilibrium: The pan is at a higher temperature than the water. Heat transfer restores thermal equilibrium once the water and pan are in contact; it stops once thermal equilibrium between the pan and the water is achieved. The heat lost by the pan is equal to the heat gained by the water—that is the basic principle of calorimetry.

  • Use the equation for heat transfer \(Q = mc\Delta T\) to express the heat lost by the aluminum pan in terms of the mass of the pan, the specific heat of aluminum, the initial temperature of the pan, and the final temperature: \[Q_{hot} = m_{A1}c_{A1}(T_f - 150^oC). \nonumber\]
  • Express the heat gained by the water in terms of the mass of the water, the specific heat of water, the initial temperature of the water, and the final temperature: \[Q_{cold} = m_wc_w(T_f - 20.0^oC). \nonumber\]
  • Note that \(Q_{hot} <0\) and \(Q_{cold} > 0 \) and that as stated above, they must sum to zero: \[Q_{cold} + Q_{hot} = 0\]\[Q_{cold} = -Q_{hot}\]\[m_wc_w(T_f - 20.0 ^C) = -m_{A1}c_{A1} (T_f - 150^oC). \nonumber\]
  • This a linear equation for the unknown final temperature, \(T_f\). Solving for \(T_f\), \[T_f = \dfrac{m_{A1}c_{A1}(150^oC) + m_wc_w(20.0^oC)}{m_{A1}c_{A1} + m_wc_w}, \nonumber\] and insert the numerical values: \[T_f = \dfrac{(0.500 \, kg)(900 \, J/kg^oC)(150^oC) + (0.250 \, kg)(4186 \, J/kg^oC)(20.0^oC)}{(0.500 \, kg)(900 \, J/kg^oC) + (0.250 \, kg)(4186 \, J/kg^oC)} = 59.1 \, ^oC. \nonumber\]

Significance Why is the final temperature so much closer to \(20.0^oC\) than to \(150^oC\)? The reason is that water has a greater specific heat than most common substances and thus undergoes a smaller temperature change for a given heat transfer. A large body of water, such as a lake, requires a large amount of heat to increase its temperature appreciably. This explains why the temperature of a lake stays relatively constant during the day even when the temperature change of the air is large. However, the water temperature does change over longer times (e.g., summer to winter).

Exercise \(\PageIndex{3}\)

If 25 kJ is necessary to raise the temperature of a rock from \(25^oC\) to \(30^oC\), how much heat is necessary to heat the rock from \(45^oC\) to \(50^oC\)?

To a good approximation, the heat transfer depends only on the temperature difference. Since the temperature differences are the same in both cases, the same 25 kJ is necessary in the second case. (As we will see in the next section, the answer would have been different if the object had been made of some substance that changes phase anywhere between \(30^oC\) and \(50^oC\).)

Temperature-Dependent Heat Capacity

At low temperatures, the specific heats of solids are typically proportional to \(T^3\). The first understanding of this behavior was due to the Dutch physicist Peter Debye , who in 1912, treated atomic oscillations with the quantum theory that Max Planck had recently used for radiation. For instance, a good approximation for the specific heat of salt, NaCl, is \(c = 3.33 \times 10^4 \frac{J}{kg \cdot k}\left(\frac{T}{321 \, K}\right)^3\). The constant 321 K is called the Debye temperature of NaCl, \(\Theta_D\) and the formula works well when \(T < 0.04 \Theta_D\). Using this formula, how much heat is required to raise the temperature of 24.0 g of NaCl from 5 K to 15 K?

Because the heat capacity depends on the temperature, we need to use the equation \[c = \dfrac{1}{m} \dfrac{dQ}{dT}.\]

We solve this equation for Q by integrating both sides: \(Q = m \int_{T_1}^{T_2} cdT\).

Then we substitute the given values in and evaluate the integral:

\[Q = (0.024 \, kg) \int_{T1}^{T2} 333 \times 10^4 \dfrac{J}{kg \cdot K}\left(\dfrac{T}{321 \, K}\right)^3 dT = \left( 6.04 \times 10^{-4} \dfrac{J}{K^4}\right) T^4 |_{5 \, K}^{15 \, K} = 30.2 \, J.\]

Significance If we had used the equation \(Q = mc\Delta T\) and the room-temperature specific heat of salt, \(880 \, J/kg \cdot K\), we would have gotten a very different value.

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Heat Transfer: Infrared Radiation Ppt & Practical

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transfer of thermal heat energy

Transfer of Thermal (heat) Energy

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Transfer of Thermal (heat) Energy. Make a guess for each Conduction Convection Radiation. Conduction. Conduction is the transfer of energy through matter from particle to particle. Conduction involves making contact from hot item to cool item.

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Transfer of Thermal (heat) Energy Make a guess for each • Conduction • Convection • Radiation

Conduction • Conduction is the transfer of energy through matter from particle to particle. • Conduction involves making contact from hot item to cool item. • Metals “feeling cold” are just conducting heat away from our hands • What does conduction look like in the kitchen? • Boiling water on the stove: the water is making contact with the hot stove

Convection • Convection is the transfer of heat by the actual movement of the warmed matter • A heater blowing air is moving the warm matter • What does this look like in the kitchen? • Inside the oven, the warm air is moving around. That’s why you keep the door closed. • Would a cookie “bake” if you put it on the stove? • No, the bottom would just burn.

Radiation • Electromagnetic waves that directly transport ENERGY through space. • There is no contact or movement. • The source of energy can be far away • What does this look like in the kitchen? • Take a pot off the stove and hold your hand over the element. Can you feel the waves of heat warming your hand?!

Energy • Defined as the ability to do work. • Energy is measured in joules (J). • Work done on an object gives energy to that object.

7 Forms of Energy

Electrical Energy • Electrons • energy from electricity • Example: your TV

Sound Energy • Vibrating molecules picked up by our ears.

Chemical Energy • energy released from chemical reactions • Example: batteries, digesting food

Heat Energy • How “fast” molecules are vibrating or moving

Light Energy • Energy transmitted in electromagnetic waves that you can often see • Examples include radio, TV waves and colors represents different amounts of electromagnetic energy.

Atomic Energy • Energy from splitting or combining atomic particles • i.e. nuclear power, sun, radioactive elements

Mechanical Energy • Matter that is in motion OR • Energy from movement

Einstein Seldom Cleans His Large Atomic Machine.

Energy Conversions • All forms of energy can be converted to other forms.

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  • HEAT CONDUCTION AND HEAT TRANSFER IN TECHNOLOGICAL PROCESSES
  • Published: 22 February 2024

Cite this article

  • D. S. Semenov 1 ,
  • A. V. Nenarokomov 1 ,
  • S. A. Budnik 1 &
  • D. M. Titov 1  

The authors have solved the problem of simultaneous determination of a set of coefficients of a mathematical heat-transfer model for semitransparent materials: the thermal conductivity of a material and the coefficient of its heat exchange with the external medium, the heat flux from the heater, that has been absorbed by the material, and the coefficient of linear absorption of the material.

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D. S. Semenov, A. V. Nenarokomov, S. A. Budnik & D. M. Titov

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Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 97, No. 1, pp. 3–12, January–February, 2024.

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Semenov, D.S., Nenarokomov, A.V., Budnik, S.A. et al. Investigation of Radiative-Conductive Heat Transfer by Noncontact Measurements. 3. Identification of Mathematical Models of Heat Transfer for Semitransparent Materials. J Eng Phys Thermophy (2024). https://doi.org/10.1007/s10891-024-02861-x

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