Temperature and Heat

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Printed: November 13, 2014

www.ck12.org Chapter 1. Temperature and Heat

CHAPTER 1 Temperature and Heat

Lesson Objectives

• Explain the relationship between temperature and thermal energy.• Define heat and specific heat.

Lesson Vocabulary

• heat• specific heat

Introduction

The hot air and sand in Death Valley have a lot of thermal energy, or the kinetic energy of moving particles. But evencold objects have some thermal energy. That’s because the particles of all matter are in constant random motion.If cold as well as hot objects have moving particles, what, if anything, does temperature have to do with thermalenergy?

Temperature

No doubt you already have a good idea of what temperature is. You might define it as how hot or cold somethingfeels. In physics, temperature is defined as the average kinetic energy of the particles in an object. When particlesmove more quickly, temperature is higher and an object feels warmer. When particles move more slowly, temperatureis lower and an object feels cooler.

Temperature and Thermal Energy

If two objects have the same mass, the object with the higher temperature has greater thermal energy. Temperatureaffects thermal energy, but temperature isn’t the same thing as thermal energy. That’s because an object’s mass alsoaffects its thermal energy. The examples in Figure 1.1 make this clear. In the figure, the particles of cocoa aremoving faster than the particles of bathwater. Therefore, the cocoa has a higher temperature. However, the bathwater has more thermal energy because there is so much more of it. It has many more moving particles. Bill Nyethe Science Guy cleverly discusses these concepts at this URL: http://www.youtube.com/watch?v=f1eAOygDP5s&feature=related (2:06).

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If you’re still not clear about the relationship between temperature and thermal energy, watch the animation "Tem-perature" at this URL: http://www.sciencehelpdesk.com/unit/science2/3 .

FIGURE 1.1The cocoa is scalding hot. The bath wateris comfortably warm. Why does the bathwater have more thermal energy than thecocoa?

Measuring Temperature

Temperature is measured with a thermometer. A thermometer shows how hot or cold something is relative to tworeference temperatures, usually the freezing and boiling points of water. Scientists often use the Celsius scale fortemperature. On this scale, the freezing point of water is 0°C and the boiling point is 100°C. To learn more aboutmeasuring temperature, watch the animation “Measuring Temperature” at this URL: http://www.sciencehelpdesk.com/unit/science2/3 .

Did you ever wonder how a thermometer works? Look at the thermometer in Figure 1.2. Particles of the red liquidhave greater energy when they are warmer, so they move more and spread apart. This causes the liquid to expandand rise higher in the glass tube. Like the liquid in a thermometer, most types of matter expand to some degreewhen they get warmer. Gases usually expand the most when heated, followed by liquids. Solids generally expandthe least. (Water is an exception; it takes up more space as a solid than as a liquid.)

Heat

Something that has a high temperature is said to be hot. Does temperature measure heat? Is heat just another wordfor thermal energy? The answer to both questions is no. Heat is the transfer of thermal energy between objects thathave different temperatures. Thermal energy always moves from an object with a higher temperature to an objectwith a lower temperature. When thermal energy is transferred in this way, the warm object becomes cooler andthe cool object becomes warmer. Sooner or later, both objects will have the same temperature. Only then does thetransfer of thermal energy end. For a visual explanation of these concepts, watch the animation "Temperature vs.Heat" at this URL: http://www.sciencehelpdesk.com/unit/science2/3 .

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www.ck12.org Chapter 1. Temperature and Heat

FIGURE 1.2The red liquid in this thermometer is alcohol. Alcohol expands uniformlyover a wide range of temperatures. This makes it ideal for use inthermometers.

Example of Thermal Energy Transfer

Figure 1.3 illustrates an example of thermal energy transfer. Before the spoon was put into the steaming hot coffee,it was cool to the touch. Once in the coffee, the spoon heated up quickly. The fast-moving particles of the coffeetransferred some of their energy to the slower-moving particles of the spoon. The spoon particles started movingfaster and became warmer, causing the temperature of the spoon to rise. Because the coffee particles lost someof their kinetic energy to the spoon particles, the coffee particles started to move more slowly. This caused thetemperature of the coffee to fall. Before long, the coffee and spoon had the same temperature.

FIGURE 1.3A cool spoon gets warmer when it isplaced in a hot liquid. Can you explainwhy?

Specific Heat

The girls in Figure 1.4 are having fun at the beach. It’s a warm, sunny day, and the sand feels hot under their barehands and feet. The water, in contrast, feels much cooler. Why does the sand get so hot while the water does not?The answer has to do with specific heat.

Specific heat is the amount of energy (in joules) needed to raise the temperature of 1 gram of a substance by1°C. Specific heat is a property that is specific to a given type of matter. Table 1.1 lists the specific heat of four

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FIGURE 1.4Sand on a beach heats up quickly in thesun because sand has a relatively lowspecific heat.

different substances. Metals such as iron have relatively low specific heat. It doesn’t take much energy to raise theirtemperature. That’s why a metal spoon heats up quickly when placed in hot coffee. Sand also has a relatively lowspecific heat, whereas water has a very high specific heat. It takes a lot more energy to increase the temperatureof water than sand. This explains why the sand on a beach gets hot while the water stays cool. Differences in thespecific heat of water and land also affect climate. To learn how, watch the video at this URL: http://www.youtube.com/watch?v=dkBStF2Rnu4 (7:07).

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In Table 1.1, how much greater is the specific heat of water than sand?

TABLE 1.1: Specific Heat of Some Common Substances

Substances Specific Heat (joules)iron 0.45sand 0.67wood 1.76water 4.18

KQED: Bridge Thermometer

The roadway across the Golden Gate Bridge rises and falls as much as 16 feet depending on the temperature. Whenthe sun hits the bridge, the metal expands and the bridge cables stretch. As the fog rolls in, the cables contract andthe bridge goes up. Curators from the Outdoor Exploratorium in San Francisco have set up a scope two miles awayso you can see how the bridge is moving up or down depending on the weather. For more information on how thebridge moves due to temperature, see http://science.kqed.org/quest/video/quest-lab-bridge-thermometer/ .

MEDIAClick image to the left for use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/131612

Lesson Summary

• Temperature is the average kinetic energy of particles of an object. Warmer objects have faster particles andhigher temperatures. If two objects have the same mass, the object with the higher temperature has greaterthermal energy. Temperature is measured with a thermometer.

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• Heat is the transfer of thermal energy between objects that have different temperatures. Thermal energy alwaysmoves from an object with a higher temperature to an object with a lower temperature. Specific heat is theamount of energy (in joules) needed to raise the temperature of 1 gram of a substance by 1°C. Substancesdiffer in their specific heat.

Lesson Review Questions

Recall

1. What is temperature?2. How is temperature measured?3. Define heat.

Apply Concepts

4. Give an example of heat that you didn’t read about in this lesson.5. Glass has a specific heat of 0.84 J/g·°C. Copper has a specific heat of 0.39 J/kg·°C. Which material takes more

energy to warm up?

Think Critically

6. Explain how a cooler object can have more thermal energy than a warmer object.7. Relate heat to temperature.

Points to Consider

In this lesson, you read that heat is the transfer of thermal energy from one object to another.

• How do you think the transfer of thermal energy occurs? For example, how does thermal energy move fromhot sand to bare feet when someone walks on a beach?

• Do you think there might be more than one way that thermal energy can be transferred? For example, howdoes thermal energy move from a bonfire to a nearby person who isn’t touching the flames?

References

1. Cocoa: F_A (Flickr:seelensturm); Boy in bath: Brandon Doan (Flickr:bdogggut34). Cocoa: http://www.flickr.com/photos/seelensturm/8401926552/; Boy in bath: http://www.flickr.com/photos/bdogggut34/3749195206/. CC BY 2.0

2. Jessica Mullen. http://www.flickr.com/photos/jessicamullen/130940810/ . CC BY 2.03. Basheer Tome. http://www.flickr.com/photos/basheertome/6971028847/ . CC BY 2.04. Mike Baird. http://www.flickr.com/photos/mikebaird/7476468820/ . CC BY 2.0

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