Chapter 9. The Earth's Energy Sources

9.1 Introduction

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Author: Jordan Raddick, Johns Hopkins University

Editor: Beth Hufnagel, Anne Arundel Community College

The goals of this module: At the end, you should be able to:

Identify three of the five sources of energy that drive motion and change in the Earth.
Explain how solar energy and the rotation of the Earth work together to create the motions of the Earth's atmosphere.
Explain how solar energy drives ocean currents.

In this module you will explore:

Three of the five energy sources for changes in the Earth: solar energy, Earth's rotation, and tidal forces. (The other two are heat of formation and radioactive decay.)
Ways that this energy works to create motion and change in the Earth's atmosphere and oceans.

Why you are doing it: By understanding the Earth's energy sources and how this energy is transferred, you can better understand the processes that cause motions and changes in the Earth.

9.2 Background

The Earth as seen from the Moon

The Earth is our home planet. Nearly all of the events of human history, all the vast changes that we have experienced, have taken place here on the surface of the Earth. But the Earth itself is constantly changing too. Earth is a complex system that includes land, water, and air, and all of these parts interact with each other to create motion and change. All these motions and changes require energy to continue.

Different energy sources act on different parts of the Earth. In this activity, you'll explore how different energy sources drive different motions.

Let's start with the atmosphere.

9.3 Solar Energy Heats the Atmosphere

In the animation below, on the right, you see a pot of water heating on a stove. Turn on the stove and watch what happens!
You have probably seen something like it yourself. The circular motion of the water in a heating pot is called convection. It turns out that the motion of air in the Earth's atmosphere can also be explained by convection.

On the left, you see the Earth's surface and its atmosphere. "Light" the sun and watch what happens!

Question Sequence

Question 9.1

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Try again. It is rising to the top, and some changes to steam, but this process cools the water off. What happens to cooler, denser water?

Correct. The stove burner heats the water. Near the flames at the bottom of the pot, the heated water is less dense and rises. Once it reaches the top, it spreads out and becomes cooler. The cooler water is more dense, so it sinks. Overall, the water makes a circular path, rising above the flame, spreading out to the sides of the pot, sinking to the bottom, and flowing to the middle.

Incorrect. The stove burner heats the water. Near the flames at the bottom of the pot, the heated water is less dense and rises. Once it reaches the top, it spreads out and becomes cooler. The cooler water is more dense, so it sinks. Overall, the water makes a circular path, rising above the flame, spreading out to the sides of the pot, sinking to the bottom, and flowing to the middle.

Question 9.2

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Try again.Think about this as similar to what happens in the boiling pot, where the equator is the bottom of the pot and the poles are the surface of the water.

Correct. The Sun heats the surface of the Earth. The energy from the Sun is more concentrated near the equator, so the air above the equator heats up the most. The hotter air becomes less dense and rises. It spreads out, moving toward the poles. The air gives off heat to its surroundings, so it becomes cooler. The cooler air is more dense, so it sinks.

Incorrect. The Sun heats the surface of the Earth. The energy from the Sun is more concentrated near the equator, so the air above the equator heats up the most. The hotter air becomes less dense and rises. It spreads out, moving toward the poles. The air gives off heat to its surroundings, so it becomes cooler. The cooler air is more dense, so it sinks.

Summary

But you may have noticed that the animation of the Earth above isn't quite realistic - this Earth doesn't rotate, while our real Earth does! The animation shows what would happen if the Earth didn't rotate, but the Earth's rotation does have some effect on how the atmosphere moves. Continue to find out how.

9.4 Earth's Rotation Affects the Motion of the Atmosphere

The animation below is a more realistic diagram of the Earth, with the Earth's rotation included. Click the Sun to heat up this Earth, and watch what happens.

Atmospheric Motion of the Earth Animation

Question Sequence

Question 9.3

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Try again. This is a correct answer, but not the only one!

Correct. But Earth's rotation causes the air to spread out in the direction of rotation too.

Incorrect. But Earth's rotation causes the air to spread out in the direction of rotation too.

Question 9.4

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Take another looks at the animation and note that Central America is southwest of Spain. Try again!

Correct. The movement of air in the atmosphere results in winds. The animation above and to the right shows air moving along the Earth's surface from the northeast to the southwest - so the winds along the surface blow from northeast to southwest. When the sailors sailed from Spain to its colonies, they traveled southwest, so these winds were at their back. Traveling with the winds at their back, they could move more quickly.

Incorrect. The movement of air in the atmosphere results in winds. The animation above and to the right shows air moving along the Earth's surface from the northeast to the southwest - so the winds along the surface blow from northeast to southwest. When the sailors sailed from Spain to its colonies, they traveled southwest, so these winds were at their back. Traveling with the winds at their back, they could move more quickly.

9.5 Solar Energy Moves the Oceans

The atmosphere is not the only thing in constant motion on Earth - the oceans move too. The energy that drives this motion also comes from the Sun. Study the animation below to see how the oceans move.

Question Sequence

Question 9.5

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Try again. Watch the animation again and notice how the currents flow.

Correct. The warm currents start near the equator.

Incorrect. The warm currents start near the equator.

Question 9.6

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Try again. The Sun is never directly overhead of any latitude above В±23.5 degrees north or south, so there is less solar energy there.

Correct. Solar energy is more concentrated near the equator, and that energy heats up the ocean there more. As you move toward the poles, the sunlight spreads out and is less effective.

Incorrect. Solar energy is more concentrated near the equator, and that energy heats up the ocean there more. As you move toward the poles, the sunlight spreads out and is less effective.

Question 9.7

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Correct.

Incorrect.

Question 9.17

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Correct.

Incorrect.