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Heat Transfer – Conduction, Convection, Radiation

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.

Related Posts

Browse Course Material

Course info.

• Prof. Kripa K Varanasi

Departments

• Mechanical Engineering

As Taught In

• Thermodynamics

Learning Resource Types

Introduction to heat transfer, course description.

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Lienhard Research Group

Seeking energy efficient solutions for clean water supplies, recent papers in heat transfer, heat diffusion during thin-film composite membrane formation.

Schematic illustration of heat transfer in during interfacial polymerization for the fabrication of thin-film composite membrane selective layers

Thin-film composite (TFC) membranes, the backbone of modern reverse osmosis and nanofiltration, combine the high separation performance of a thin selective layer with the robust mechanical support. Previous studies have shown that heat released during interfacial polymerization (IP) can have a significant impact on the physical and chemical structure of the selective layer. In this study, we develop a multilayer transient heat conduction model to analyze how the thermal properties of the materials used in TFC fabrication impact interfacial temperature, focusing on support-free (SFIP), conventional (CIP), and interlayer-modulated IP (IMIP). Using a combination of analytic solutions and computational models, we demonstrate that the thermal effusivities of fluid and material layers can have a significant effect on the temporal evolution of interfacial temperature during IP. In CIP, we show that the presence of a polymeric support adjacent to the reaction interface yields a 20% to 60% increase in interfacial temperature rise, lasting for ∼ 0.1 s. Furthermore, we demonstrate that inorganic or metallic interlayers, which have high thermal effusivities, can lead to short-lived order-of-magnitude reductions in interfacial temperature rise. Finally, we provide analytical approximations for transient heat conduction through multilayered systems, enabling rapid evaluation of the thermal impact of novel membrane support and interlayer materials and structures on interfacial temperature during TFC fabrication. Quantifying how the thermal properties of solvents, support layers, and interlayers affect interfacial temperature during IP is critical for the rational design of new TFC membranes.

A. Deshmukh, J.H. Lienhard, and M. Elimelech, “Heat Diffusion During Thin-Film Composite Membrane Formation,” J. Membrane Science , 696: 122493, March 2024. Editor’s Choice Article for March 2024. There are still fun things to do with classical heat conduction!

Heat transfer in flat-plate boundary layers: a correlation for laminar, transitional, and turbulent flow

Proposed correlation, Eq. (9), compared to constant heat flux data of Blair for three levels of free stream turbulence.

J.H. Lienhard V, “Heat transfer in flat-plate boundary layers: a correlation for laminar, transitional, and turbulent flow,” J. Heat Transfer , online 31 March 2020, 142 (6):061805, June 2020. ( doi: Open access ) ( presentation ) ( one-page summary ) (DSpace)

Accurate linearization of non-gray radiation heat exchange

The internal emissivity based on mean temperature is in good agreement with the exact heat exchange for even extremely non-gray surfaces.

J.H. Lienhard V, “Linearization of Non-gray Radiation Exchange: The Internal Fractional Function Reconsidered,” J. Heat Transfer , online 3 Dec. 2018, 141 (5):052701, May 2019. ( OPEN ACCESS ) (preprint) (presentation)

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Heat Transfer Paper 101: The Ultimate Guide for Beginners

In the fast-paced world of custom printing, selecting the right heat transfer paper is the first step toward achieving exceptional results. With various types of paper available, it's essential to understand their features, advantages, and limitations.

I'll be your Transfer Sensei, guiding you through a journey to unlock the secrets of heat transfer paper and help you embrace your inner Transfer Ninja, creating powerful and stunning custom prints that captivate and inspire. Let's begin.

What is heat transfer paper?

Heat transfer paper is a specially-coated print medium for transferring designs onto various materials, from garments to hard goods. When an image is printed onto heat transfer paper, the ink bonds with the paper's coating.

When the printed transfer paper is placed on the desired material and exposed to heat and pressure (using a heat press or iron), it's released from the paper and transferred onto the material, creating a customized product.

Fast facts about heat transfer paper

• Works on cotton, blends, and most synthetic fabrics.
• Common sizes are letter (8.5 x 11 inches) and tabloid (11 x 17 inches).
• Inkjet, laser, and sublimation transfer papers are not cross-compatible.
• Transparent heat transfer paper is for white or light-colored fabrics.
• Opaque white heat transfer paper is for black or dark-colored fabrics.
• Sublimation transfer paper is designed for light color materials.
• Sublimation heat transfers only work on polyester materials.

Heat transfer paper vs. iron-on transfer paper

The two terms are often synonymous, and both types of transfer paper are close relatives designed to transfer images and designs onto various surfaces. However, a subtle distinction lies in their intended audience:

Iron-on transfer paper caters to artists and hobbyists who might not possess the tools of a seasoned Transfer Ninja, such as a heat press. These creative souls use a regular iron or Cricut to apply their designs.

Heat transfer paper is often of higher quality, offering superior results to those who have invested in the power of a heat press. This quality stems from the paper's design, intended for professionals and serious practitioners of the transfer arts who can apply at higher temperatures and pressures.

How to choose the right heat transfer paper

Asking yourself the following questions will help you make an informed decision, ensuring your projects turn out as professional and eye-catching as possible:

What are your goals?

Consider the purpose of your custom printing projects. Are you starting a business or launching a new product line? Or are you simply creating a few personalized gifts? Some paper types are better suited for high-volume, professional applications, while others are good enough for smaller projects.

What kind of printer do you have?

The type of printer you have is the most critical factor in determining the right heat transfer paper. Inkjet, laser, sublimation, and DTF printers require specific types of transfer papers designed to work with their respective technologies. If you're uncertain about your printer, check the model name and manufacturer's specs. If you don't have a printer, read about choosing the right heat transfer printer .

What's your budget?

Different heat transfer papers cater to various budgets. Some are geared toward hobbyists, and others target professional or high-volume users. Inkjet is typically best for smaller budgets. Laser paper is a bit bit pricier, along with sublimation paper. DTF has become affordable and is an increasingly attractive option. Choose a transfer paper that aligns with your financial constraints without sacrificing quality.

What kind of design are you printing?

Your design's complexity, size, and color vibrancy also play a role in selecting the right heat transfer paper. Simple or complex designs, small or large prints, and vibrant or subdued colors require different considerations. Additionally, determine if your design is photographic or comprised of solid colors. Some heat transfer papers excel with raster images, while others are better suited for vector-based designs.

What kind of materials are you printing on?

The type of material is a major factor in your choice of heat transfer paper. Consider your fabric type (cotton vs. polyester), color (dark vs. light), desired quality, and durability. Different heat transfer papers are designed to work best with specific materials and colors, while certain techniques like DTF can work on light or dark and almost any type of material.

By carefully considering these factors, you'll be well-equipped to make the wise choice. Now let’s venture deeper into the world of heat transfer paper to compare the strengths and weaknesses of each type.

Types of heat transfer paper

Heat transfer paper is available in four primary types, each catering to specific printer technologies, applications, and substrates, each with pros and cons:

Inkjet transfer paper

Inkjet heat transfer paper is designed specifically for inkjet printers, but the printers are known for handling various paper types. These papers are available for both light and dark-colored garments. A wide range of papers are available, including opaque and transparent options.

• Easy to use, ideal for beginners or hobbyists
• Affordable and supplies widely available
• Can achieve vibrant colors and detailed images
• Mixed results on some synthetic fabrics
• Limited durability and stretchability
• Time-consuming cutting and "weeding" (opaque only)
• Can leave white "halo" edges on dark garments (opaque only)

Laser transfer paper

Laser heat transfer paper is specially designed for laser printers, offering sharp, colorful, detailed images and excellent durability. Suitable for both light and dark fabrics.

• Excellent durability and washability
• Works for various fabric types
• Faster printing speeds compared to inkjet
• Certain papers can work on dark or light
• More expensive than inkjet
• May require specialized paper for the best results
• Time-consuming cutting and "weeding" (opaque paper)
• Can leave white "halo" edges on dark garments (opaque paper)

Sublimation transfer paper

Sublimation transfer paper is used with sublimation ink in a dedicated sublimation printer. This process works best on polyester or polyester-blend fabrics and is ideal for creating all-over prints or vivid designs.

• High-quality, professional results
• Excellent durability and stretchability
• Never any cutting, weeding, or trimming edges
• Requires specialized equipment and inks
• Limited to synthetic fabrics
• Limited to light-colored substrates

DTF transfer film

Direct-to-Film transfers are a revolutionary method that offers impressive durability, stretchability, and versatility. DTF printing uses a specialized printer to apply ink directly onto a thin film, which is then heat-pressed onto various materials for high-quality, professional results.

• Excellent for complex-shaped designs and details
• Superior durability and stretchability compared to other methods
• Versatile, works on various materials and colors
• Excellent color vibrancy, gradients, and range
• Requires investment in specialized printer and inks
• A steeper learning curve for beginners

Heat transfer paper for light and dark fabrics

Your design, fabric color, and desired finish will guide your choice between transparent paper (for light colored fabrics) and opaque white paper (for dark colored fabrics). Both types are essential tools in your printing arsenal, and understanding their differences is vital to achieving the best results.

Transparent vs. white heat transfer paper

Transparent heat transfer paper is made for light-colored fabrics and has a thin, see-through layer. It's perfect for white fabrics and can work on other light colors, if you’re going for a vintage look. However, on darker colors, the fabric show through, giving it a dark, low-contrast look.

Although the heat transfer paper is see-through, neat trimming is recommended for a clean finish. Precision cutting machines are useful for this task, but careful hand-cutting with scissors or an Exact-o knife can work too.

White, or opaque heat transfer paper is best for darker fabrics due to its opaque layer, which serves as an underbase. This prevents dark fabric colors from showing through, maintaining the design's original colors. Designs on dark transfer paper can feel thicker and will have a noticeable border.

When working with opaque transfer paper, trimming is essential. Any leftover white areas will be visible after heat pressing. Complex designs require meticulous trimming, or “weeding” to avoid this, which can be labor-intensive. We recommend using a cutting machine like a Silhouette Cameo or a Cricut.

With attention to detail, both types of paper can provide great results.

Frequently Asked Questions

Can i use a regular printer for heat transfer paper.

Yes, you can use a regular inkjet printer for heat transfer paper. Just ensure the printer is compatible with the type of heat transfer paper you plan to use.

Do I need special ink to print on heat transfer paper?

You can use standard inkjet printer ink for regular inkjet heat transfer paper. Same with a laser printer. However, for sublimation printing, you need to use specific sublimation inks designed to turn into a gas when heated and transfer onto the desired surface, and for DTF printing, you need a specialized DTF printer.

Can I use laser paper in my inkjet printer?

No. Laser paper is specifically designed for laser printers, while inkjet printers require inkjet paper. Using laser paper in an inkjet printer may lead to poor print quality, smudging, and even printer damage.

How do I determine whether my printer is an inkjet or a laser printer?

You can identify your printer type by checking the printer's model name, usually found on the front or back of the device. Inkjet printers use liquid ink cartridges, while laser printers utilize toner cartridges. Consult your printer's user manual or manufacturer's website for more information on the type of cartridges and printing technology used.

Can I use sublimation paper with my inkjet or laser printer?

No. Sublimation paper is specifically designed for use with sublimation printers and dye-sublimation inks. Using sublimation paper with an inkjet or laser printer will not yield the desired results. The sublimation process requires specific inks and temperatures that inkjet and laser printers cannot provide.

Can I use transfer paper for a high-volume project?

While it is possible to use heat transfer paper for high-volume projects, it's not efficient or cost-effective. Heat transfer paper requires manual cutting and weeding, which can be time-consuming for large projects. Instead, consider utilizing DTF transfers or screen printing for higher-volume projects, as these methods are better suited for large-scale production.

The path to heat transfer mastery

Understanding the various types of heat transfer paper is crucial for achieving outstanding results. Consider the main factors of printer type, garment color, intended application, durability, and color vibrancy to determine the right heat transfer paper for your project. Armed with the knowledge and insights in this guide, you're on your way to inspiring the next generation of Transfer Ninjas.

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Introduction to Thermodynamics

Chapter outline.

Heat transfer is energy in transit, and it can be used to do work. It can also be converted to any other form of energy. A car engine, for example, burns fuel for heat transfer into a gas. Work is done by the gas as it exerts a force through a distance, converting its energy into a variety of other forms—into the car’s kinetic or gravitational potential energy; into electrical energy to run the spark plugs, radio, and lights; and back into stored energy in the car’s battery. But most of the heat transfer produced from burning fuel in the engine does not do work on the gas. Rather, the energy is released into the environment, implying that the engine is quite inefficient.

It is often said that modern gasoline engines cannot be made to be significantly more efficient. We hear the same about heat transfer to electrical energy in large power stations, whether they are coal, oil, natural gas, or nuclear powered. Why is that the case? Is the inefficiency caused by design problems that could be solved with better engineering and superior materials? Is it part of some money-making conspiracy by those who sell energy? Actually, the truth is more interesting, and reveals much about the nature of heat transfer.

Basic physical laws govern how heat transfer for doing work takes place and place insurmountable limits onto its efficiency. This chapter will explore these laws as well as many applications and concepts associated with them. These topics are part of thermodynamics —the study of heat transfer and its relationship to doing work.

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FREE K-12 standards-aligned STEM

curriculum for educators everywhere!

Find more at TeachEngineering.org .

• TeachEngineering
• Heat Transfer: No Magic About It

Lesson Heat Transfer: No Magic About It

Grade Level: 10 (9-11)

Time Required: 30 minutes

Lesson Dependency: None

Subject Areas: Physical Science, Physics

NGSS Performance Expectations:

• Print lesson and its associated curriculum

Activities Associated with this Lesson Units serve as guides to a particular content or subject area. Nested under units are lessons (in purple) and hands-on activities (in blue). Note that not all lessons and activities will exist under a unit, and instead may exist as "standalone" curriculum.

• What Works Best in a Radiator?

TE Newsletter

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

Heat is a concept that is important to understand in various engineering fields. It is particularly relevant for civil, mechanical and chemical engineers because heat transfer plays a key role in material selection, machinery efficiency and reaction kinetics, respectively. In this lesson, students learn how heat transfer applies to engineering and are asked to consider examples of engineering designs that have capitalized on the scientific principles of heat transfer.

After this lesson, students should be able to:

• Define and explain heat, conduction, convection and radiation.
• Explain the relationship between the kinetic and potential energy of atoms in a thermodynamic system.
• Relate the above concepts to common engineering designs and examples from nature.

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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Do you agree with this alignment? Thanks for your feedback!

State Standards

Texas - science.

Students should be familiar with the concept of energy and the law of conservation of energy. They should also have a basic knowledge of high school chemistry.

(In advance, make copies of the Heat Transfer Guided Notes Worksheet , one per student, and have handy a few ice cubes for a quick demo. Hand out the worksheets now, to help students stay engaged and follow the content as it is presented. The guided notes focus on definitions and heat transfer examples.)

Today we are going to talk about a concept that will sound familiar to you: heat. Although I am sure that you have heard the word heat before, today we are going to discuss the science and physics behind what heat really is. Can anyone describe a situation in which you remember feeling the most cold or most hot that you have ever felt? (Listen to a few students. Avoid storytelling and sidetracking conversations.) Have any of you burned yourself while cooking, or maybe experienced frostbite while in the snow? (Listen to a few student examples.)

When we talk about being hot or cold, we are discussing temperature. Temperature is the measure of the average thermal energy in a system. (Write the definition on the classroom board.) If an object, such as a baking pan, has a lot of energy, then it feels hot, but if it has less energy, then it feels less hot. To help explain this, can anyone tell me what basic elements (building blocks) compose a typical baking pan? (Possible answers: Metal, molecules, atoms.) If you recall from your chemistry class, a baking pan is made of billions and billions of atoms that all vibrate and move in place. As we learned in our unit about energy, anything that is moving has kinetic energy, which is why we define temperature as the measure of the energy in a system. So when the atoms in a system move faster, the system has more energy and a high temperature, and when the atoms are moving slower, the system has less energy and a lower temperature. We know that energy cannot be created or destroyed, but is continuously transferred between different forms of potential and kinetic energy. In today's lesson, we will learn about heat and three ways it can be transferred.

In science, we define heat as the transfer of thermal energy from one system to another. (Add this definition to the board.) I need une volunteer. (Call on a student to come up to the front of the classroom. Give the student an ice cube.) How does the ice feel on your hand? (Answer: Cold) How do you think heat is being transferred between your hand and the ice? (Expected answer: The ice is making my hand cold.) Good, thank you. (Have the student sit down.) This demonstrates an important concept about heat: Heat always goes from high energy to low energy. Because (students name)'s hand had more energy than the ice, energy transferred from their hand to the ice. So the ice makes your hand feel cold, but it is because your hand transfers energy into the ice causing the ice to increase in temperature and your hand to decrease in temperature. So really, your hand is making the ice warmer! Engineers use this detail-level understanding of heat transfer to perform many tasks. For example, let's consider a car's engine.

In order for a car to move, its engine combusts gasoline or diesel and converts the chemical potential energy stored in the fuel into kinetic energy. In that reaction, a tremendous amount of heat is produced and transferred to the engine. If the engine gets too hot, parts begin to break. In order to keep the engine at a more tolerable temperature, engineers designed a system we call the radiator. Fluid in the radiator runs through the engine where heat is transferred into the fluid, which then travels back to the radiator where it is cooled before cycling back to the engine. Students can conduct their own experiment with the associated activity What Works Best in a Radiator? by measuring the differences in transfers of heat energy between different liquids.

This radiator example demonstrates one type of heat transfer. For the remaining part our talk today, we will define and discuss all of the ways heat can be transferred. Additionally, we will see examples of heat transfer in both the magical world of Harry Potter and our daily lives.

(Continue on, presenting students with the Lesson Background content information.)

Lesson Background and Concepts for Teachers

Temperature

Temperature is the measure of the average thermal energy in a system or body. We use three common scales to measure temperature: Fahrenheit, Celsius and Kelvin. The Fahrenheit scale identifies the freezing point of water as 32 ⁰F and the boiling point of water as 212 ⁰F, whereas the Celsius scale identifies the freezing point of water as 0 ⁰C and the boiling point of water as 100 ⁰C. The Kelvin scale is an adaptation of Celsius that sets absolute zero as 0 K. At absolute zero, atoms cease to move and no thermal energy exists. For reference, one of the coldest materials is liquid nitrogen, which has a temperature of 77 K, which is -196 ⁰C or -321 ⁰F.

Heat is an important concept for researchers, scientists and engineers. The term "heat" is different from temperature in that it is not the measure of thermal energy, but rather the measure of the transfer of thermal energy. The three different types of heat transfer are conduction, convection and radiation. When thermal energy is being transferred, it always goes from the state of higher energy to lower energy (as demonstrated by the ice in the student's hand). Although it is common to think that cold objects contribute something to us when touched, it is rather that our bodies lose energy, which results in a decrease in our thermal energy and temperature.

Conduction is the transfer of heat due to direct contact of systems. The atoms vibrating in one object have an effect on the atoms of another object when in contact. For an object with higher thermal energy, the atoms are vibrating faster than an object with less thermal energy, and during their contact, energy is transferred from the object of high energy to the one with less energy. Due to the proximity of atoms and molecules, conduction is most effective in the solid and liquid phases and less effective in the gas phase. This concept can be illustrated by attempting to cool down a drink. A drink cools faster if put in a bucket of ice water (≈4 ⁰C) rather in the refrigerator (≈4 ⁰C) because more molecules are in contact with the drink via the liquid than via the refrigerated air.

Within conduction is the concept of conductance. Conduction is not limited to being between two objects; it can also be the transfer of heat within a single object or material. For example, if a metal skillet is being heated on a stove (the stove top is transferring its thermal energy into the metal skillet), the layer of atoms in the skillet at the point of contact with the stove top begins to vibrate faster, which then interacts with the next layer of atoms, and so on throughout the entire skillet. The conductance, which is the ability to transfer thermal energy, of a material is largely based on the molecular structure and bonding of the material. Conductors are materials that transfer heat quickly, while insulators are materials that do not transfer heat effectively. Materials such as metals, glass and ceramics are good conductors; materials such as plastic, wood and Styrofoam are insulators.

Convection is often observed in the movement of bulk fluids. When liquids or gases flow, they exchange thermal energy with other media with which they come into contact. As discussed earlier, the radiator liquid is cooled once it returns to the radiator; this is done by air passing over the hot radiator and heat moving from the radiator to the air. Additionally, convection can occur within a singular body of fluid through the changing of temperature within a bulk mass of fluid. Much like conductance in solid bodies, bulk systems of fluids can have different temperature gradients, but what is different about fluids is that these temperature differences can lead to significant density changes. Because fluids, such as gases and liquids, are free to move, if one portion of the fluid increases in temperature (due to either conduction or radiation), its density decreases and that portion of the fluid rises. Convection can be illustrated by a fireplace that heats the air in a room, causing it to expand and rise, while the cooler air near the ceiling falls towards the fire where it is warmed. Convection is seen in many scenarios by this continuous heating and cooling process, evidenced by this rising and falling movement.

(To pique students' interest, use examples of spells from the Harry Potter book series to illustrate the concept of convection.) In Harry Potter's magical world, one example of convection is a hot air charm used to cause a blast of hot air to come forth from the wand. The hot air is the movement of a bulk fluid that transfers heat to any object that it encounters. In the muggle world, many engineered and natural examples of convection exist. Engineers have designed ovens, electronic cooling systems and heat exchangers, such as car radiators, to benefit from the concept of convection. As discussed above, the fluid in a car's radiator is heated by the engine, after which it is circulated back to the radiator and cooled by the air passing over the radiator's surface. The radiator is an example of a continuous process of the heating and cooling of a liquid, driven not by density change but by a mechanical pump. Elephant's large ears are an example of a natural radiator; the warm blood is circulated by the heart into the ears and cooled by the air that passes over the ear surface. The continuous process of the warm blood being cooled as it circulates through the ears helps keep the animal cool in the intense African heat.

Radiation is the transfer of heat by electromagnetic waves. Electromagnetic waves can be transferred through space without the presence of matter, but thermal energy is not generated until the waves contact matter. As the electromagnetic waves contact matter, they transfer heat by increasing the thermal energy of the matter. It is interesting to note that the heat transferred by radiation is a function of the matter's absorbance of the electrometric waves. White objects reflect much of the light that hits them, thus absorbing very little of the energy and avoiding the transmittance of heat due to radiation. On the other end of the spectrum, black objects adsorb all of the light and have the maximum heat transfer due to radiation. But radiation does not only occur by visible light; it can also be due to the electromagnetic waves in the range of infrared and others.

(To pique students' interest, use examples of spells from Harry Potter to illustrate the concept of radiation.) In the fictional Harry Potter stories, one example of radiation is a spell called lumus , which causes the wand to transmit a beam of light. Although the spell is used more like a flashlight, it still produces electromagnetic waves that transmit some heat. In the muggle world, many engineered and natural examples of radiation exist, too. One simple demonstration is the concentration of sunlight into a fine point through the use of a magnify glass. If kept steady on a piece of paper, the energy from the sun eventually causes the paper to smoke and catch on fire. An engineered example of the use of radiation to transmit heat is the microwave oven, which transmits microwaves into food, causing the atoms to vibrate more rapidly and the temperature to increase. The primary example of radiation in nature is the sun, which is the source of heat and warmth for all life on Earth. One way to see the effect of the sun in transmitting heat is by comparing climates on the equator to climates closer to the poles. The difference in average temperatures is due to the angle at which the electromagnetic waves encounter the Earth's surface.

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Once students have completed the associated activity, call their attention for a review of all of the main concepts:

• Discuss the importance of understanding the specific heat capacity of different materials and how it is important for engineers to consider these properties when designing structures, devices, chemicals and most products.
• Ask students to explain at least one natural and one engineered example of each of the three types of mass transfer of energy.
• Conclude the lesson by making a brief and initial connection between heat and the larger topic of thermodynamics. Heat transfer is often taught right before or during a unit on thermodynamics, so it is important to help students understand how heat fits in to the larger topic of thermodynamics.

conduction: The transfer of heat by atomic movement due to contact from systems of high temperature to systems of lower temperature.

convection: The transfer and movement of heat by bulk flow of fluids.

heat: The transfer of thermal energy across systems or within a single system.

radiation: The transfer of heat by the absorbance and emission of electromagnetic waves.

temperature: A measure of the average thermal energy in a system or body.

Pre-Lesson Assessment

Energy Review: Verbally review with students the concepts of energy and the law of conservation of energy. Ask questions to refresh their knowledge about energy and connect the concepts of energy and heat. For example:

• What does the law of conservation of energy tell us? (Answer: Energy cannot be created or destroyed.)
• What are the two main forms of energy? (Answer: Potential and kinetic energy.)
• Give me some scenarios in which potential and kinetic energy exist? (Answer: Potential energy: A book on the shelf; kinetic energy: a bowling ball rolling across the floor; both: a bird flying, etc.)

Lesson-Embedded Assessment

Guided Note Taking: During the lesson, move around the classroom to observe each student's progress on the Heat Transfer Guided Notes Worksheet . At lesso end, collect the worksheets to assess student engagement and understanding of the covered content.

Lesson Summary Assessment

Vocabulary: Ask students to define and write short definitions of all the vocabulary words.

Research Examples: Ask students to research engineered and natural examples of conduction, convection and radiation. Have them write short paragraphs explaining their example finding for each.

Students learn about the definition of heat as a form of energy and how it exists in everyday life. They learn about the three types of heat transfer—conduction, convection and radiation—as well as the connection between heat and insulation.

With the help of simple, teacher-led demonstration activities, students learn the basic physics of heat transfer by means of conduction, convection and radiation. They also learn about examples of heating and cooling devices, from stove tops to car radiators, that they encounter in their homes, scho...

Students learn about the nature of thermal energy, temperature and how materials store thermal energy. They discuss the difference between conduction, convection and radiation of thermal energy, and complete activities in which they investigate the difference between temperature, thermal energy and ...

Students are introduced to various types of energy with a focus on thermal energy and types of heat transfer as they are challenged to design a better travel thermos that is cost efficient, aesthetically pleasing and meets the design objective of keeping liquids hot.

Jarvis, Laurie, and Deb Simonson. "Heat Transfer: Conduction, Convection, Radiation." WISC-Online. Posted 2004. Fox Valley Technology College. Accessed December 6, 2012. (Useful for simple definitions and illustrations.) http://www.wisc-online.com/Objects/ViewObject.aspx?ID=sce304

Sonntag, Richard. E., Claus Borgnakke and Gordan J. Van Wylen. Fundamentals of Thermodynamics . 7th edition. Hoboken, NJ: John Wiley & Sons, Inc., 2008.

Contributors

Supporting program, acknowledgements.

This digital library content was developed by the University of Houston's College of Engineering under National Science Foundation GK-12 grant number DGE 0840889. However, these contents do not necessarily represent the policies of the NSF and you should not assume endorsement by the federal government.

Last modified: June 14, 2021