Main->Readings->4th Grade Readings->Matter in Motion- >Part 4: Putting Energy to Work
Vocabulary |
Perhaps 15 billion years ago, the universe began. Since then, there has been about the same amount of energy in the universe. Energy is the ability to do work. When the universe ends, there will be the same amount overall. In the big picture of things, energy cannot be created or destroyed. However, it can be changed into other forms of energy. In this section, you will learn about some different types of energy and how they can be changed to make them more useful.
When something is moving, we say it has kinetic energy, (kin-ET-ick) the energy of a moving object. Two objects of the same mass going the same velocity have the same amount of kinetic energy. If something is going fast, it has more kinetic energy, and if it is going slowly, it has less kinetic energy. Two objects of the same mass going different speeds have a different amount of kinetic energy.
If you drop a ball that has 20 g of mass and a ball which has 10 g of mass, they are going the same rate, but the 20 g ball has more kinetic energy. How is that possible?
It helps if you think about the amount of damage something can do. A bowling ball and a marble, dropped at the same time from the same height, will accelerate at the same rate and will land at the same time. Suppose you dropped them each on a banana. The bowling ball would crush the banana, splat! The marble would not. This is because the bowling ball has more mass and more kinetic energy than the marble.
Kinetic energy is something all moving objects have. A sixteen-wheeler truck going 30 miles an hour has kinetic energy. So does a kid on a bicycle going 30 miles an hour. Which one do you think has more kinetic energy, the truck or the bicycle? Well, which one would do the most damage if it hit?
Suppose you were holding a bowling ball and a marble at arm's length. You already know which one will do more damage before you let go. That is because the bowling ball has more mass. When you hold a bowling ball in the air, it has the potential to do more damage if gravity acts on it, and therefore it has more potential (poe-TEN-shul) energy. Potential energy is the stored energy of an object.There are many kinds of potential energy, including chemical potential energy and the energy of a wound-up spring.
When you let go of the bowling ball and the marble, the potential energy is changed to kinetic energy. When you drop something or knock it off a shelf, you are changing energy.
Here is another example of changing potential energy to kinetic energy. Suppose you are on a swing. You apply force to the swing using your muscles and gravity, and now you are swinging back and forth. At the top of your swing, you come to a stop and start going the other direction. At the bottom of the swing, gravity has accelerated you to your top speed. You are changing kinetic energy to potential energy and potential energy to kinetic energy, over and over. At what point in your swing do you have the most potential energy? At what point do you have the most kinetic energy?
There are many ways to change energy. A wind-up toy is another example. When you wind the toy, you are using kinetic energy to tighten the spring. When the spring is tightened, it has potential energy. When you release the toy, the potential energy is changed back to kinetic energy.
It takes energy to get energy. Electrical energy is another type of energy which we use for many purposes. How do we get it? There are many ways, and they all involve changing energy from one form to another.
One way of making electricity is using the energy of moving water. When water flows downhill, it is being moved by the force of gravity and so it has kinetic energy. If you make a dam and send the flowing water through a wheel called a turbine, the turbine turns and makes a big magnet spin inside some loops of wire. This creates electricity. The kinetic energy of the water has been changed to electrical energy.
Another way of producing electricity is by burning fuel, which may be coal, oil, or natural gas. Usually, the fuel is burned, which heats water, which produces steam, which turns a turbine, which makes electricity. There are problems with this method. Burning fuel makes many kinds of pollution which can cause disease and which may be causing the Earth's climate to get too warm. Also, the fuels must be mined. Some people believe we are using up the Earth's fuels and will have nothing left in the future.
Most of the Earth's energy comes from the sun. All of the fuels we burn come from fossilized plants, which used the sun's energy to grow. Even water energy comes from the sun. In order for rain to fall, it must first be evaporated into the air by the sun.
Where does the sun get its energy, then? The sun is so large that it is pressing in on itself with a huge force of gravity. The force is so big that it pushes the sun's atoms together and they fuse, releasing some of the energy of the atoms. The sun will eventually run out of fuel itself, but it will take 4 or 5 billion years.
Electricity by itself is not useful. It has to be changed again to make it useful. A lamp changes electrical energy into light energy. A stove changes electrical energy into heat energy. A blender changes electrical energy into kinetic energy.
Which of the appliances above does any work? According to a scientist, only the blender does work, because its blades spin. Work, to a physicist, is done when a force moves an object. If you are sitting at your desk doing your homework, you are doing very little work according to a scientist. If you are playing soccer, or running around, on the other hand, you are doing work.
There are many ways of making work easier. For instance, the simple machines (inclined plane, wheel and axle, pulley, and lever) change the size or direction of a force. Also, there are many ways of putting simple machines together in compound machines.
Work is measured by the amount of force used and the distance moved. Therefore, if you move a box of bricks up the street using your toy wagon, you have done the same amount of work as someone driving the box of bricks up the street in a car. However, the car was probably much faster. Many times, making something useful means making it work more quickly. Power means the rate at which work is done. The car has more power than your legs. The more powerful an engine is, the less time it takes to do the same amount of work. Usually, when something is powerful, it is both fast and strong, because it takes greater strength to do quick work.
Whenever energy is changed from one form to another, some of the energy is always turned to heat. This energy often goes into the matter around it and cannot be used again.
Entropy (ENN-trow-pee) is the word we use for the loss of energy to heat. Eventually, because of entropy, scientists believe all of the energy in the universe will be spread out as heat, and will be unusable. Luckily for us, this will be many billions of years from now.
Questions: For your first assignment of the week, answer these questions in complete sentences on a sheet of loose-leaf paper, with a proper header:
Notes: For your second assignment of the week, in your journal on the next clean page, write the vocabulary words from this section and their definitions.
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This page last modified August 15, 2002
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