Unit 16: Heat Transfer
How do objects cool down or warm up? What causes windchill? How does solar energy reach Earth if it passes through empty space? What causes the greenhouse effect? After completing this unit, you should be able to explain the answers to these questions.
Learning Outcomes
By the end of this unit, you should be able to
- define and explain the three forms of heat transfer: conduction, convection, and radiation.
- explain emission, absorption, and reflection of radiant energy.
- explain the nighttime cooling effect caused by radiation.
- state Newton’s law of cooling.
- explain the greenhouse effect and its environmental impact.
- discuss the concept of solar power as an alternative source of energy.
eText Material
Reading Assignment
Read the following sections in Chapter 16 of the eText:
- 16.1: Conduction
- 16.2: Convection
- 16.3: Radiation
- 16.4: Newton’s Law of Cooling
- 16.5: The Greenhouse Effect
- 16.6: Climate Change
- 16.7: Solar Power
- 16.8: Controlling Heat Transfer
Supplementary learning resources are available on the Mastering Physics learning platform.
Additional Reading
Conduction
Three different mechanisms are involved in the transfer of heat: conduction, convection, and radiation. When one end of a metal rod is placed in a hot medium, the other end of the rod also warms up after some time. The mechanism by which heat is transferred from the hot end to the cooler end is called conduction. How quickly the process takes place depends on the ability of the material to conduct heat—a property known as thermal conductivity.
The process of conduction can be visualized using the kinetic model of thermal energy. Molecules at the hot end of the rod have a higher kinetic energy and therefore move faster than molecules at the cooler end. As the molecules with the higher kinetic energy collide with the slower-moving molecules, energy is transferred from the faster to the slower molecules along the rod.
Convection
As the atoms or molecules in a solid transfer energy by colliding with their neighbors in the conduction process, they remain in their equilibrium positions. In fluids (liquids and gases), however, the atoms or molecules can move from point to point. The transfer of heat that accompanies the movement of the fluid material is called convection.
Consider the process of heating a room with a radiator. Air in contact with the radiator heats up and expands. Because of the expansion, the hot air becomes less dense and lighter than the colder air around it. The hot air therefore rises, cold air takes its place at the radiator, and the process continues. This transfer of heat by the displacement of a hot air mass from one location to another is an example of free convection. In forced convection, a fan or pump creates the fluid currents that carry heat with them from one point to another.
Radiation
Radiation is a process of heat transfer that requires no material medium. Heat, often called radiant energy, is carried from a hot body, be it the Sun or an electric light bulb, by electromagnetic waves that propagate through free space and carry energy. All objects can emit, absorb, and reflect radiant energy.
Newton’s Law of Cooling
When an object is placed in an environment of a different temperature, the object gradually warms up or cools down until its temperature becomes equal to that of its surroundings. The rate at which the temperature changes with time is proportional to the temperature difference between the object and the environment.
Assume you move a small pot filled with boiling water at 100°C from the stove and put it inside a refrigerator whose internal temperature is set at 0°C. Initially, the pot will lose heat at a relatively high rate. As the temperature of the pot drops, so does the rate at which it cools down. For example, when the temperature of the pot reaches 80°C, the rate at which heat flows out of the pot drops by 20%, compared with the initial rate. At 50°C, the rate at which the temperature decreases is 50% the initial rate.
Assume that it takes 10 minutes for the temperature to decrease from 100°C to 50°C. Since the cooling rate is now half its initial value, the temperature will drop to 25°C in the next 10 minutes and to 12.5°C after another 10 minutes, as shown in Figure 16.1.
Figure 16.1: Temperature of hot water in a pot as it cools down to the freezing point.
As another example, consider a cup of water at 4°C placed in a room where the temperature is 20°C. Note that the initial temperature difference in this case is 16°C. As in the previous example, when the temperature difference is reduced to 8°C, the rate at which the cup of water warms up is half the value of the initial rate. Therefore, the time it takes for the temperature to increase from 4°C to 12°C is equal to the time required for it to rise from 12°C to 16°C (see Figure 16.2).
Figure 16.2: Temperature of cold water in a cup as it warms up to room temperature.
Questions
The following questions are selected from the end of Chapter 16 of the eText. It is important to your learning that you try to answer each question independently before you read through the answer and explanation given.
For questions that ask you to explain, defend, or discuss your answer, the response revealed by the Answer button would earn you only partial marks on a quiz or exam in this course. Use the Answer to help you formulate a complete answer before you select the Explanation button to check your work.
Chapter 16
Question 45
If 70°F air feels warm and comfortable to us, why does swimming in 70°F water feel cool?
Answer
Water has a higher conductivity than air.
Explanation
Water is a much better conductor of heat than air. Therefore, when you swim in a pool of water at 70°F (or 21°C), your warmer body (98.6°F or 37°C) loses heat faster than when you walk along in 70°F weather.
Chapter 16
Question 47
If you hold one end of a piece of metal against a piece of ice, the end in your hand soon becomes cold. Does cold flow from the ice to your hand? Explain.
Answer
No. Cold is another word for less hot, and it is heat that transfers through a substance.
Explanation
When you hold one end of a piece of metal (a good conductor) against a piece of ice, heat flows from your warm hand to the ice. As a result, your hand becomes less hot and feels cold.
Chapter 16
Question 51
Why are mittens warmer than gloves on a cold day?
Answer
Because mittens have a smaller surface area.
Explanation
A mitten has less surface area exposed to the surrounding cold air than a glove has. As a result, your hand loses heat (through conduction and radiation) more slowly through a mitten than through a glove made of the same material.
Chapter 16
Question 54
Can heat spontaneously flow from an object with less internal energy to one with greater internal energy? Defend your answer.
Answer
Yes.
Explanation
Heat flows from objects with higher temperatures to objects with lower temperatures regardless of their internal energies. For example, if you threw a burning log into a lake, heat would transfer from the hot log to the cold lake water. The internal energy of the lake is, of course, much greater than that of the log.
Chapter 16
Question 66
Ceiling fans can make you feel cooler in a warm room. Do ceiling fans reduce room temperature?
Answer
No.
Explanation
Actually, the opposite can happen! A ceiling fan does not reduce room temperature. The electric energy used to operate the fan dissipates as heat and may actually cause the temperature inside a small, closed room to increase slightly.
For you, however, the situation is different. The rotating blades of the fan produce air currents in the air around your body. As a result, heat transfer from your exposed skin to the surrounding air increases, which makes you feel cooler. This is referred to as the windchill effect, which is not relevant for inanimate objects.
Chapter 16
Question 69
Why does a good emitter of heat radiation appear black at room temperature?
Answer
Because it is also good absorber.
Explanation
Good emitters of heat radiation are also good absorbers. Most of the radiant energy (including visible light) that falls on them is absorbed, with only a small portion reflected toward the observer. The absorbed energy can then radiate as thermal (mainly invisible infrared) radiation.
Chapter 16
Question 75
When there is morning frost on the grass in an open park, why is frost unlikely to be found on the ground directly beneath park benches?
Answer
Park benches radiate and reflect part of the radiant energy toward the ground underneath.
Explanation
The park ground (including benches) radiates heat energy as it cools during the night. However, the area under a bench loses heat more slowly. The bench seat radiates heat in all directions, including downward toward the grass underneath. Also, part of the heat radiated by the ground beneath the bench gets reflected back by the bench seat. As a result, the ground below the bench remains warmer than other open spaces in the park.
Chapter 16
Question 77
Why is whitewash sometimes applied to the glass of florists’ greenhouses in the summer?
Answer
To reduce radiant energy entering the greenhouse.
Explanation
When necessary, florists might apply whitewash to the outside of their greenhouses to reflect a greater fraction of the incoming radiation (or sunlight) before it passes through the glass. Too much incoming radiation can cause overheating inside the greenhouse in summer.
Chapter 16
Question 93
Is it important to convert temperatures to the Kelvin scale when we use Newton’s law of cooling? Discuss why or why not.
Answer
No.
Explanation
A change of 1 degree on the Kelvin scale is equal in size to a change of 1 degree on the Celsius scale. For example, we say 1 cal of heat energy can increase the temperature of 1 g of water by 1 K or (equivalently) 1°C. Note that when a glass of ice water warms up to a room temperature of 20°C (or 293 K), the change of temperature can be described as follows: \begin{align} \Delta T &= 20^\circ\text{C} - 0^\circ\text{C} = 20^\circ\text{C} \nonumber\\[6pt] &= 293\,\text{K} - 273\,\text{K} = 20\,\text{K} \nonumber\\[6pt] \end{align}
Newton’s law of cooling involves differences of temperature where the rate of heat transfer between two objects is proportional to the temperature difference between them. Therefore, we can use either scale.
Chapter 16
Question 94
Suppose at a restaurant you are served coffee before you are ready to drink it. In order that it be hottest when you are ready for it, should you add cream to the coffee right away or wait until you are ready to drink it? Why?
Answer
You should add cream right away.
Explanation
Adding cold cream to a cup of hot coffee reduces the temperature difference between the drink and the surrounding environment. As a result, the warm coffee loses heat at a slower rate. In addition, brown-colored coffee (with cream) radiates less heat per unit time than black coffee.
Chapter 16
Question 96
If you wish to save fuel and you’re going to leave your cool house for a half hour or so on a very hot day, should you turn your air-conditioning thermostat up a bit, turn it off altogether, or let it remain at your desired room temperature? Explain.
Answer
You should turn off the air conditioner altogether.
Explanation
As the house warms up, the temperature difference with the outside environment becomes smaller, which reduces the rate at which heat leaks into the house. So when you turn on the air conditioner after you return home, less energy will be required to cool down the house than would have been used to keep it at the desired temperature during your absence.
Exercises
Spend some time completing the following exercises to test your understanding of the main concepts in Chapter 16 and increase your efficiency in answering exam questions.
End-of-Chapter Practice Questions
Answer questions 5, 13, 19, 29, 35, 37, 41, 55, 59, 65, 79, 83, 87, 95, and 99 in Chapter 16 of the eText. If you require assistance, please contact your tutor. The answers are provided at the end of the eText.