7th Gr. Unit 3 (3-4)

The diagram shows a model of an air mass. The dots represent molecules of the gases that make up air. A student is changing this model to represent what happens to the air mass when it comes into contact with a warm front. Describe how a student could best change the model.

• Add squares to represent particles of warm air.

• Increase the size of half of the dots to show warm air.

• Add arrows to each particle to show increased motion.

• Increase the size of the box so that there is more space between the dots.

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MS-PS1-4

In electric ovens, the heat in the oven comes from the bottom when you bake something. However, cookies baking on the top shelf get done before cookies on the bottom shelf, even though the bottom shelf is closer to the heat source. Why does this happen?

• The heat rises from the cookies on the bottom shelf, so some of the heat from the cookies helps to cook the cookies on the top shelf.

• The oven heats the air at the bottom. The air becomes less dense than the air at the top of the oven and rises. The hotter air stays at the top of the oven and so it transfers more energy to the cookies, and the cookies on the top shelf cook faster.

• The oven walls absorb a lot of the heat and the top shelf is closest to the top wall.

• There is more air above the lower pan so it loses more heat to the air so the pan on the upper shelf appears to heat faster.

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MS-PS1-4

If you hold your finger next to the flame on a candle like the one in the picture, you can feel the heat, but you will not get burned; however, if you hold your finger above the candle, you will get burned.  Why?

• As the heated air rises, cooler air moves in to take its place. This causes convection and more hot air to rise. Some thermal energy is radiated from the flame to the side of the candle, and that is why you feel the heat. It is hotter above the candle because of convection.

• The shape of the candle flame directs the heat out through the top so if you place your hand at the top of the flame you could get burned.

• The sides of the candle are cooler than the flame so it doesn’t get as hot on the sides.

• The candle absorbs the heat from the lower part of the flame so the top part of the flame is hotter.

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MS-PS1-4

In class, you investigated what happens when air is heated and cooled.

Which images of the air molecules in the balloon represent the following observations. When the system is heated, the warmer air in the bottle expands into the balloon as they are less dense, moving faster, and farther apart.

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• 
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• None of these diagrams represent the situation.

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MS-PS3-5

In class, you investigated what happens when air is heated and cooled.

Which images of the air molecules in the balloon represent the following observations. When the system is cooled, the air molecule become more dense, move less, and are closer together.

• 
• 
• 
• None of these diagrams represent the situation.

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MS-PS3-5

How was energy transferred in the convection box?

• Energy was transferred by radiation from the heat source to the air. When the air was heated it became less dense and rose by convection.

• Energy was transferred by conduction from the heat source to the air. Matter and energy were moved by convection because the heated air became less dense and rose.

• When thermal energy is transferred from the heat source to the air, it decreases the kinetic energy of the air molecules so that the air becomes much more dense and will sink.

• The flame of the top part of the candle heats the air through radiation. When the air is warmer the molecules move faster because the thermal energy is converted into kinetic energy.  As the air collides with the walls of the convection box, the air molecules are pushed upward.

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MS-PS3-5

How was energy transferred in the bottle experiment?

• Energy was transferred from the hot water to the bottle to the air inside the bottle by conduction.

• Energy was transferred from the hot water to the bottle by conduction and then to the air in the bottle by convection.

• Energy was transferred from the hot water to the bottle by convection and then to the air in the bottle by conduction.

• The hot water transfers heat to the water bottle which causes the bottle to contract and the balloon deflates.

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MS-PS3-5

What differences in air masses can cause wind? Use the model to help you explain why this happens.

• When two warm air masses come together the more warm mass moves over the less warm and causes wind.

• Where there is a temperature or density difference in two air masses, the warmer air rushes to the colder air to help it increase its temperature. When the warm air moves to the cold air, that is what causes wind.

• The temperature/density differences between cooler and warmer air cause wind. Hotter, less dense air rises. When that happens the cooler, denser air sinks and moves in to take the place of the rising air. The movement is wind.

• Wind happens when the higher air mass sinks toward the ground and pushes the lower air mass away.

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MS-ESS2-6

Below is a model that illustrates why hotter air rises in the atmosphere. Select the best explanation for why hot air rises. 

• Thermal energy from Earth’s surface is transferred to the air above it by conduction. The air molecules move faster and get farther apart. Because they are now farther apart, gravity no longer has a strong enough pull to keep the air molecules close to the surface so they rise up.

• Thermal energy from Earth’s surface is transferred to the air above it by conduction. The air molecules move faster and get farther apart. When a cold air mass is nearby, the cold air is stronger than the warm air and will push the warm air upward.

• Thermal energy from Earth’s surface is transferred to the air above it by conduction. The air molecules move faster and get farther apart. The warmer ground transfers more energy than the cooler ground, so the air mass becomes hotter and less dense than the surrounding air. Since the warmer air mass is less dense than the air around it, the warm air mass rises.

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MS-PS1-4

Jocelyn used a balloon to model the motion of air molecules in the atmosphere. She measured the diameter of the balloon at room temperature. Then she placed the balloon in a freezer to simulate a strong cold front. After one hour, Jocelyn removed the balloon from the freezer and immediately measured the balloon’s diameter. Jocelyn drew these pictures to show the diameter of the balloon at room temperature and the diameter immediately after removing it from the freezer. The dotted line represents the size of the balloon before it was placed in the freezer. Why is a balloon a good model to use for predicting changes in the motion of air molecules? Select all that apply.

• Investigating air molecules in the atmosphere is on such a large scale that it is difficult to directly observe. Using a balloon puts the investigation to a scale that we can directly observe.

• We cannot directly see how an air molecule is moving but we can observe how the diameter of a balloon changes depending on the movement of the particles.

• A balloon adds a color dimension to air molecules in motion so they can be directly observed.

• Measuring the diameter of the balloon allows us to give a number to how a specific amount of air behaves when heat is added or removed.

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MS-PS1-4