Corel Corporation
Corel Corporation
You’re sitting around a roaring campfire with family and friends. The night is cold and so is your back, but the part of your body facing the fire is toasty warm. You then start thinking about the fire and the heat. What’s behind these phenomena? How does the heat from the fire get to you? Why is your back so cold?
These are all good questions! Questions you will explore in this lesson. You will use a model (or representation) of matter to help you get at the nature of thermal energy and how it differs from temperature.
The particle model of matter explains the make-up of all matter. Many scientific concepts are based on this model. This model is introduced on pages 202 and 203 of the textbook. Read this introduction.
1.Atoms make up the particles of the particle model.
The diagram on the left is a simple model of an atom. An atom is the smallest unit of matter. They are so small that they are invisible. Atoms are the building blocks of the particles in the particle model of matter. The diagram on the right is a model of a water particle. It is comprised of two hydrogen (H) atoms and one oxygen (O) atom.
You know what solids, liquids, and gases are. But what makes them a solid, liquid, or gas? How does energy fit into the picture? The next activity will focus your thoughts on how the particle model explains the three states of matter.
Read the activity on page 202 of the textbook. Study the diagrams very carefully.
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Tara has found a connection between a rubbing motion and heat. Do you think the particle model of matter can explain this?
What did you learn from the diagrams and questions in the last activity? Did you see that if the motion of the particles increased, the temperature increased? If you did, good job! This is, in fact, the fourth point of the particle model and is stated as follows. (The first three points are at the top of page 203 in the textbook.)
The more energy particles gain, the faster they move.
Think about it! This statement makes a lot of sense! You move more when you have a lot of energy, don’t you?
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But what is energy? More specifically, what is heat energy?
Many years ago, scientists thought that heat was a fluid that, although invisible, had mass. They called this substance caloric. (You may have heard people talk of the number of calories in a certain food. Now you know where the name came from.) According to these scientists, the substance—caloric—moved from a hot material into a cold material. It stood to reason that a beaker with 100 mL of 80°C water in it would have more mass than 100 mL of water at 20°C. For hundreds of years, this was assumed to be true until it was tested with the more precise balance scales that became available. Surprise surprise, no difference in the masses was found. (Modern devices would in fact find the 80°C water to be lighter.)
Scientists now believe that heat is a form of energy. Energy has no mass. Remember that every tiny particle has a tiny bit of energy. An object is made of multitudes of tiny particles. Their average energy can be measured with a thermometer. Temperature is an indicator of the average thermal (heat) energy of the particles in a substance. The more energy the particles have, the faster they move. This gives a higher reading on the thermometer.
Turn to page 204 of the textbook. Read “Temperature and the Particle Model” and the information in both “Did You Know?” boxes.
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Ms. Burton: Do you see any attraction between these people?
Austin: I’m not sure what this has to do with science, but the smiles indicate some sort of attraction. I’d say they look like friends.
Ms. Burton: I agree. Did you know, in science, there are also forces of attraction between particles of a substance. This force may be strong or it may be weak.
The fifth statement of the particle model of matter is as follows:
There are attractive forces between the particles. These attractions may be strong or weak.
Do you remember the second “Did You Know?” box on page 204 of the textbook? It says why the strength of these attractive forces is important. The strength of these attractive forces relates to characteristics of materials that you can observe, like state and solubility.
Attractive forces hold substances together. If the attractive force between two substances is strong, one substance will dissolve into the other. Sugar and water particles are strongly attracted to each other, so sugar dissolves into water. (The sugar breaks up into separate particles and diffuses into the spaces between the particles of water.)
If the attractive force within a substance is stronger than the attractive force between two substances, the particles will not dissolve. They will remain clumped. For example, the attractive forces between oil and water are weak, so they do not mix.
These attractive forces are also an important factor in changes of state. Remember, it is attractive forces that hold matter together. As particles gain more and more energy, the particles move faster and faster. They hit each other harder and harder. If a particle gains enough energy, it can break away from the attraction of the other particles. As it frees itself, it changes state (solid to liquid or liquid to gas).
Note: You will deal with changes of state again later in this module.
The opposite is also true. As particles lose energy, they move more slowly and get closer together. The attraction between the particles at some point overcomes a particle’s energy level. It is captured by the group and changes state in the process (gas to liquid or liquid to solid).
Mikaila: What is energy?
Mr. Casey: Energy is defined as the ability to do work.
Mikaila: So, just what does that mean and how does it relate to heat and temperature?
Mr. Casey: Let’s look at specific examples. It takes energy to lift a rock; lifting the rock is work. It takes energy to keep a car going at highway speeds. That’s why the gas gauge goes down when driving from Calgary to Edmonton. The energy used to move the car is stored in the chemicals of the fuel that is burned in its engine. The engine then moves the car. So, in this case, work is done (the car is moving) using the chemical energy released from the fuel.
PhotoDisc Collection/Getty ImagesMikaila: What about a baseball hit to the outfield? Does it really have energy? It does not have any fuel to keep it going.
Mr. Casey: Yes, the baseball has energy simply because it’s moving. Such energy is called kinetic energy.
Mikaila: Does this mean that even particles of matter have kinetic energy, too? If so, they are so small that they shouldn’t have much.
PhotoDisc Collection/Getty ImagesMr. Casey: You have made an interesting connection. You are right that each individual particle would not have much kinetic energy. They’re just too small. Now, add up all the kinetic energy of all the particles in a visible object. You now have a lot of energy. This total kinetic energy of the particles is known as the thermal energy of the substance.
Mikaila: I see. When an object gets warmer, its particles move faster. Hmm . . . that means they all gain energy . . . well . . . kinetic energy. That also means the total kinetic energy of the object increases. In other words, the thermal energy of the substance goes up with temperature.
Mr. Casey: Well done.
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Turn to pages 204 and 205 of the textbook and read “What Is Energy?” It expands on the ideas just discussed.
9. Write definitions for the following terms:Turn to pages 206 and 207 of the textbook. Read “Thermal Energy and Temperature Changes.” It explains how thermal energy and temperature are related.
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Have you ever burned your tongue on hot soup? This activity should help you determine why that happens. Do you remember the following?
Temperature only reflects how fast the particles are moving. Thermal energy reflects how fast the particles are moving as well as the number of particles in the substance or object.
Procedure
Part A
Consider the following diagrams. One is a small cup of chicken broth; the other is a large pot of the same chicken broth. Both are at 85°C.
13.Use the diagrams to answer the following.
- Compare the thermal energy in both diagrams. Explain.
- The particles in both the pot of soup and the cup of soup have the same average kinetic energy. How do you know this?
- In each diagram, energy is being transferred to the air. What does that do to the speed of the air particles? What does it do to the speed of the soup particles?
Part B
Now, consider this situation. Four cups have exactly the same amount of chicken broth. Each cup is at a different temperature.
14.
Use the diagrams to answer the following.
- Compare the average energy of the cups of chicken broth. Explain.
- Which sample contains the most thermal energy? Why?
Part C
Now, consider the following diagrams. A bowl of chicken broth is at 70°C. A teaspoon of the same chicken broth is at 85°C.
15.
Use the diagrams to answer the following.
- Which has the greater average kinetic energy? Why?
- Which of the samples has the greater total kinetic energy? Why?
What Did You Find Out?
Now, it’s time for you to answer the final question: Why does hot soup burn your tongue? To help you with this, answer the following.
16.
- Which of the substances shown—the chicken broth or the tongue—has a higher temperature?
- Which direction does heat energy move?
- Why can you burn your tongue with hot soup?
As a toddler, you learned that most things continue to exist even when you can’t see them. They don’t magically disappear into thin air or come from nowhere. This applies to energy as well. Scientists have stated it clearly in the particle model of matter.
Turn to page 207 of the textbook. Read “What Energy Is . . . and Is Not.” You will learn about the Law of Conservation of Energy. It is the sixth and final statement of the particle model.
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In this lesson you studied the particle model of matter. You used it to tell what being at different temperatures means. The major concept of this lesson is something that you know from daily experience. Heat moves from areas of high temperature to areas of lower temperature. (Thermal energy moves from a substance with a higher average kinetic energy—higher temperature—to one with a lower average kinetic energy—lower temperature.) You were also introduced to three forms of energy: chemical energy, kinetic energy, and thermal energy. The focus of the lesson was thermal energy.
Matter is anything that takes up space and has mass.
As particles gain energy, they move faster and hit each other harder. At some point, they are able to escape from the attractive forces between the particles. They change state (from solid to liquid or from liquid to gas).
The particles of a solid are close together. They are held in place by the attractive forces between particles. Liquid and gas particles have more energy. They have enough energy to remain free and move around on their own.
The more energy the particles have, the faster they move; the harder they hit, the more the spaces between the particles increase.
b. As the motion of the air particles increases, the temperature increases.
b. The increase in the motion of the water particles causes the temperature of the water to increase. It’s just like how the beating of the wings of the bees increases air temperature.
b. Friction between the two pieces of wood caused the wood particles to increase their motion. Therefore, they increased their temperature.
Thermal (heat) energy is the total kinetic energy of all the particles in a substance.
A joule (J) is the standard SI unit of energy.
