CHAPTER 14 SECTION Heat and Temperature 2 Energy Transfer KEY IDEAS As you read this section, keep these questions in mind: What are three kinds of energy transfer? What are conductors and insulators? What makes something a good conductor of heat? How Can Energy Be Transferred? In the morning, you might turn on the shower and wait for the water to warm up. Outside your home, the dew on the grass evaporates when sunlight hits it. In your freezer, the water in ice trays becomes solid after the water cools to 0 ºC. None of these things could happen without energy transfer. There are three different ways that energy can be transferred between objects: conduction, convection, and radiation. The figure below shows an example of a situation in which all three kinds of energy transfer are occurring. Heat moves through the metal wire in the girl s hand by conduction. Convection in the air carries heat away from the fire. The light from the flames moves by radiation. READING TOOLBOX Compare As you read this section, make a table comparing convection, conduction, and radiation. In the table, define each term and give examples of it. 1. Identify What are the three ways that energy can be transferred? Energy moves through the wire by conduction. Heated air carries energy away from the fire during convection. The fire gives off visible light through radiation. 2. Identify How is visible light transferred from the fire? Interactive Reader 307 Heat and Temperature
3. Describe How is energy transferred during thermal conduction? 4. Apply Concepts Could conduction happen between two objects that were not touching? Explain your answer. ENERGY TRANSFER BY CONDUCTION One way that energy transfers between objects is by conduction. Thermal conduction is the transfer of energy through the collisions of particles. Heat can move by conduction between two objects that are touching one another. Conduction also happens within the particles of a substance. Imagine that you put a marshmallow on one end of a metal coat hanger. Then, you hold the marshmallow over a campfire flame. The wire absorbs heat from the fire and warms up. Then, the end of the wire that you are holding gets warmer. Energy moves from the wire to your hand as heat. This energy transfer through the wire and from the wire to your hand are examples of thermal conduction. Conduction happens when particles collide with one another. For example, the wire nearest the flame is warmer than the wire farther away. Therefore, the particles in the wire near the flame are moving quickly. They collide with other particles further from the flame. Energy moves from the fast-moving particles to the slower-moving particles. The slower particles begin to move faster. In this way, heat moves from one end of the wire to the other. Before the wire is near a flame, it is the same temperature everywhere. Therefore, all the particles in the wire have the same average kinetic energy. 5. Infer In the bottom image, which side of the wire is hottest? Heat flows from the fire into the end of the wire. The particles in the wire near the fire move more quickly. They collide with other particles in the wire. This transfers energy from the warmer parts of the wire to the cooler parts. Interactive Reader 308 Heat and Temperature
ENERGY TRANSFER BY CONVECTION Above a campfire, hot air expands and moves upward, carrying energy with it. This movement of energy through the movement of matter is called convection. Convection is possible only in fluids. Most fluids are liquids or gases. During convection, the density of a portion of a fluid changes. In most cases, a fluid s density changes because its temperature changes. The denser fluid sinks below the less dense fluid. Convecting fluids often form convection currents. A convection current forms when density differences cause a fluid to flow along a circular path. Convection currents can form in any fluid that is not the same density everywhere. An example of how convection currents form in air is shown in the figure below. 6. Define What is convection? Cool air Cool air Convection currents Warm air Convection currents 7. Explain Why does the air directly above the flame rise? ENERGY TRANSFER BY RADIATION If you stand next to a campfire, you may feel its warmth. The fire gives off energy in the form of electromagnetic waves. These include infrared radiation, visible light, and ultraviolet rays. Radiation is energy that is transferred as electromagnetic waves. Radiation is different from conduction and convection because it can transfer energy through a vacuum such as outer space. Almost all of the energy that we get from the sun is transferred by radiation. 8. Compare How is radiation different from convection or conduction? Interactive Reader 309 Heat and Temperature
9. Describe Why does metal get hot quickly when it is placed near a heat source? What Are Conductors and Insulators? If you toast a marshmallow using a metal wire, the wire gets hot quickly. If you used a wooden stick instead, it would not heat up as fast. This is because metal is a good conductor of heat, but wood is not. A conductor is a type of material through which heat can move easily. Most cooking pans are made of metal because most metals are good conductors. Heat does not move through the wooden stick very easily because wood is an insulator. An insulator is a material that transfers energy poorly. Insulation is often used in attics and walls of houses to keep heat from leaving the house. Insulation can also be part of clothing. 10. Identify What is the main way that energy moves through a substance? CHARACTERISTICS OF CONDUCTORS Energy moves through a substance mainly by conduction. For energy to move through conduction, the particles in the substance must collide with each other. The closer the particles are to each other, the more likely they are to collide. Therefore, materials with particles that are close together are generally better conductors than materials with particles that are far apart. Remember that density is a measure of how closely packed the particles in a substance are. The particles in a dense material are more closely packed than those in a lower-density material, as shown below. Metals tend to be good conductors because most metals have high densities. Most gases are poor conductors. This is because the particles in a gas are much farther apart than the particles in a solid or liquid. The particles in a gas do not collide with each other very often. Dense material: particles close together Less-dense material: particles far apart 11. Infer Which of the two materials in the figure is most likely a gas? Explain your answer. Interactive Reader 310 Heat and Temperature
What Is Specific Heat? If you place a metal spoon into a cup of hot tea, the spoon will become warm. The tea will become cooler, because the spoon absorbs heat from the tea. How much will the temperatures of the tea and the spoon change? That depends on three things: the mass of the tea and of the spoon the temperature of the tea and of the spoon the specific heat of the tea and of the spoon 12. Apply Concepts If the spoon was the same temperature as the tea, would the spoon heat up when it was placed in the tea? Explain your answer. Specific heat (c) describes how much heat a substance must absorb in order for its temperature to change by a certain amount. Specifically, it is the amount of heat required to increase the temperature of 1 kg of the substance by 1 K. The higher a substance s specific heat, the more energy it must absorb before its temperature will change. The specific heat of a substance does not depend on how much of the substance is present. For example, 100 kg of water has the same specific heat as 1 kg of water: 4,186 J/kg K. The tables below give the specific heats of different substances at 25 C. Substance Specific heat at 25 C (J/kg K) Substance Liquid water 4,186 Solid copper 385 Specific heat at 25 C (J/kg K) Liquid ethanol Gaseous ammonia Gaseous water vapor Solid aluminum Solid carbon (graphite) 2,440 Solid iron 449 2,060 Solid silver 234 1,870 Liquid mercury 140 897 Solid gold 129 709 Solid lead 129 13. Identify Which substance would require the most heat to produce a temperature change: liquid water or gaseous water vapor? Different substances have different specific heats. The particles in some substances need to absorb only a small amount of energy in order to move faster. The particles in other substances must absorb much more energy before they can move that fast. A substance with a high specific heat must absorb more heat than a substance with a lower specific heat before its particles begin to move faster. Interactive Reader 311 Heat and Temperature
Math Skills 14. Describe Relationships How is mass related to the amount of energy needed to raise a substance s temperature? USING SPECIFIC HEAT It takes 4,186 J of energy to raise the temperature of 1 kg of water by 1 K. If you had 2 kg of water, it would take twice as much energy to raise the temperature by 1 K. It would also take twice as much energy to raise the temperature of 1 kg of water by 2 K. The equation below shows how to use specific heat to relate mass, temperature change, and energy: energy = specific heat mass temperature change energy = cm T In this equation, the delta symbol ( ) represents change in. To calculate the change in temperature of a substance, use the following equation: T = final temperature initial temperature T = T f T i Now, let s look at an example of how to use specific heat to solve problems involving heat and temperature. How much energy is required to increase the temperature of 200 kg of water from 25 ºC to 37 ºC? Step 1: List the known and unknown values. Known: mass of water, m = 200 kg initial temperature of water, T i = 25 ºC final temperature of water, T f = 37 ºC specific heat of water, c = 4,186 J/kg K Unknown: energy Math Skills 15. Calculate How much energy is required to raise the temperature of 755 g of solid iron from 283 K to 403 K? Show your work. (Hint: Use the information in the table on the previous page.) Step 2: Write the equations. Step 3: Insert the known values and solve for the unknown values. Remember to convert all measurements to proper units. T K = T C + 273.16 energy = cm T T = T f T i T i = 25 ºC + 273.16 T i = 298.16 K T f = 37 ºC + 273.16 T f = 310.16 K T = 12 K energy = (4,186 J/kg K) (200 kg) (12 K) energy = 10,000,000 J = 1.0 10 4 kj So, it takes about 1.0 10 4 kj of energy to increase the water s temperature. Interactive Reader 312 Heat and Temperature
What Can Happen When a Substance Absorbs Heat? Imagine placing a pot of cold water on a hot stove burner. The water absorbs heat from the burner. If you put a thermometer in the water, you would see the temperature of the water increase. When the water s temperature reached 100 ºC, the water would start to boil. Many people think that the temperature of the water continues to rise as it boils. However, this is not the case. If you watched the thermometer in the boiling water, you would see that the water s temperature would not rise. To understand why this is, you need to know what can happen to a substance when it absorbs heat. Heat moving into a substance can cause the particles in the substance to move faster. If the particles move faster, the substance s temperature increases. This happens if the substance is far from its melting or boiling point. If the substance is at its melting or boiling point, however, the absorbed energy causes the substance to change in state. When a substance is near its boiling point, it still absorbs energy. However, none of the energy is used to increase the kinetic energy of its particles. The energy is used to change the substance from a liquid to a gas. The same is true for a substance near its melting point. The energy it absorbs makes it change from a solid to a liquid. The graph below shows an example of how temperature changes when energy is added to a substance. Notice that absorbing energy can cause the substance s temperature to increase, or it can cause the substance to change state. However, absorbing heat cannot cause both a temperature change and a change in state. 16. Apply Concepts What type of energy transfer causes heat to move from the side of a metal pan into the water inside the pan? What kind of energy transfer occurs when the heated water rises toward the top of the pan? Boiling point GAS Temperature Melting point SOLID LIQUID 17. Interpret What is happening during the times that the temperature of the substance doesn t change? Added Energy Interactive Reader 313 Heat and Temperature
Section 2 Review Section Vocabulary convection the movement of matter due to differences in density that are caused by temperature variations; can result in the transfer of energy as heat convection current any movement of matter that results from differences in density; may be vertical, circular, or cyclical radiation the energy that is transferred as electromagnetic waves, such as visible light and infrared waves specific heat the quantity of heat required to raise a unit mass of homogeneous material 1 K or 1 ºC in a specified way given constant pressure and volume thermal conduction the transfer of energy as heat through a material 1. Explain Why are most cooking pots and pans made from metal? 2. Identify Fill in the blanks in the table below. Example The moon s surface has a higher temperature on the side facing the sun. When cold water is poured into a glass, the glass becomes colder. Warm ocean water carries heat from the equator toward the poles. The pavement in a parking lot becomes hot on a sunny day. Type of energy transfer radiation 3. Draw Conclusions Convection occurs within the rock in Earth s mantle. What can you conclude about the rock in the mantle based on this information? (Hint: In what kind of matter can convection occur?) 4. Infer Why can energy move through outer space by radiation, but not by convection or conduction? 5. Calculate A container holds 2.0 kg of liquid water. The water absorbs 477 kj of energy. If the water s initial temperature was 298 K, what is its final temperature? Show your work. (Hint: Rearrange the energy equation to solve for DT.) Interactive Reader 314 Heat and Temperature