# Why do I study heat transfer

### Introduction/Motivation

(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 his/her 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 differneces 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

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

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.

(To pique students' interest, use examples of spells from the Harry Potter stories to illustrate the concept of conductance.) From the Harry Potter world, one spell called flagrante causes an object to burn anyone or anything that touches it. Normally an object would require high thermal energy to transfer that energy through conduction, but in this case it is assumed that magic transfers the energy. In the muggle world (our world), many engineered and natural examples of conduction exist. For example, conduction is illustrated in the example of a car radiator (see Figure 1). One function of the radiator system is to circulate cool fluid through a hot engine block, which transfers its heat by conduction to the radiator fluid, therefore reducing the amount of heat in the engine and reducing the risk of overheating. Another example technology that applies the understanding of conduction is the use of insulating materials, which resist the flow of heat and reduce conduction. People and animals take advantage of insulating materials; we build houses and buildings with insulation in order to regulate the temperature, and animals such as walruses have fatty tissue called blubber that insulate them from the cold environment.

Convection

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.