Chapter 4
Understanding Energy
I am very energetic. Are you too? What is energy? Why do we use this term in our daily lives? Actually, physics involves a different meaning of the word energy. But one can use this word without any fear in his or her life. You eat food, you get energy. Why do you get energy and where do you get it from? It just so happens that plants, roots and trees in our surrounding are capable of gathering sunlight and they convert it into food energy. This fact is known to every person in the world. Plants make food. Then you eat food, you get energy. You get energy to do work.
What is work? A man holds two baskets filled with water for two hours. He is standing under the sun. He is covered all in sticky sweat. Yet he did no work if physics is considered – in real life, he was given a punishment. In physics, work and energy are very different terms from what we know in real life. Here we are going to talk about energy but in this way only, you will get an idea of what work really is.
Imagine a car travelling at 40 mph. You may have heard about kinetic energy which is energy possessed by an object due to its linear motion. What is it and how do we know that this car has kinetic energy? We know this because this car has ability to do work. If this car collided with a bicycle, energy will be transferred. We discussed in the first chapter that when a body collides with another body, momentum is transferred as per the given conditions. Then, the bicycle will fall at some distance after the impact. The car would come to a halt. The energy due to its speed has vanished. This energy has to go somewhere, it cannot vanish like that because nature loves harmony. What happened? Actually, the bicycle gained the energy. The car slowed down, say to 15 mph, but it still has kinetic energy equivalent to 15 mph, the remaining energy had been transferred to the bicycle (the bicycle moved too!) and also to the heat and sound energy during the accident.
This is a fundamental result. Energy cannot be created or destroyed. Energy can only be converted from one form to another. Sun does not create light energy arbitrarily. How does sun make light? Do you think it creates energy? No, never, never, never. Sun, in fact all the bodies in the universe are only capable of converting one form of energy into another form. Before understanding these conversions, we must know something about these forms.
The first form of energy that we talked about was heat energy, in the previous chapter. In this chapter, we began with kinetic energy, i.e. energy due to motion. The next type of energy is potential energy. Potential means ability or capability. To understand potential energy, we must understand about references. Imagine that you are living on a three floored building. This does not include the ground floor. Thus, this building actually has four floors. The ground floor is used as a reference. One can say that ground floor is the lowest level or the zero level. Then, as you go up, the levels increase. With respect to the zero level, the top level is called the third level. So you go like this: 0, 1, 2, 3 and you are at the top. If three objects are on the ground floor, i.e. they are on the zero level, they have zero potential energy. If these objects are on the first level, their potential energy is no longer zero. One of the objects is a leather ball. This ball can fall off from this floor and hurt somebody on the zero level (on the ground floor) but what would be the consequence if this leather ball were to fall from the third level to the zero level? This could lead to more severe injury. The ball has gained further potential energy. One can say that potential energy is energy due to height. But potential energy is relative, one has to first choose the zero level. This zero level serves as the reference. This level has zero potential energy and thus, we call it zero level.
The next type of energy is sound energy. How does sound energy do work? Sound is a wave and a wave is equivalent to energy. A wave carries energy. Sound energy can shatter tall buildings. It requires energy to make sound. How do we make sound? We make sound by speaking. But media are required to convey sound energy. Sound cannot travel without a medium. When a bomb explodes, much of its energy is converted into sound energy. This is one of the many examples of why sound is a form of energy.
Light is also a form of energy. Then there is electrical energy.
You will be amazed to know that mass is another form of energy. If you have read about Einstein’s energy-mass equivalence, mass is a compact form of energy. This leads us to atomic bombs, really devastating for the society. Now that we know about different forms of energy, we can learn about conversions. We will learn mutually. You teach me and I teach you.
The first example that I want to give you is hydroelectric power. Water falls from height (having potential energy) which is used to gain electrical energy. The potential energy of water is used to run turbines and current electricity is produced and stored. In this case, potential energy is converted into electrical energy.
The second example is of an electric bulb. When current runs (electrical energy) it heats the tungsten wire inside the bulb (heat energy) and eventually, the heat is enough to get light (light energy) In this case, there are two conversions. Electrical energy is first converted into heat. Heat is then converted into light energy.
The next example that I want to give you is about a falling body. Imagine that a body is at the edge of a cliff. The cliff is 100 meters high. Thus, this body has initially got some potential energy with respect to the ground (here is our zero level!) now let this body fall. As it falls, gravitational acceleration will increase the speed of the body. Thus some of the potential energy is converted into kinetic energy – energy of motion. In the end, when the body hits the ground, it hit it with some velocity – having kinetic energy but it will have zero potential energy on the ground.
The last and the most bizarre example of conversion I want you to know is about how sun makes light. Sun actually uses its mass to make sunlight. It is converting its mass (energy source) into light (conversion) – eventually after millions of years, there will come a time when sun will use all of its mass and would meet its end. Imagine how massive sun should be to light for another million years!
We have seen multiple examples of how energy is converted from one form to another. These different forms of energies exist between us but we hardly give any importance to them – we hardly appreciate their existence. Now that you know a lot about energy, it is time for revision 2.
Revision 2
1. What is heat and what is temperature? Are the two same? Explain your answer.
2. Try to touch a hot cup of tea with a metal rod. You hold the rod and let one of the ends touch the cup. After some time, what do you feel?
3. Take another hot cup of tea, this time touch this cup with a plastic stick. What do you feel this time? Compare this with what you felt in activity (2)
4. List all the forms of energy that you see in your surroundings.
5. What is conservation of energy?
6. List any two examples of conversion of energy.
7. How can you use sunlight to make electrical energy? What is this process called? Are you destroying any energy?
8. What is energy?
9. If you are on the top floor of a 100 m tall building and your brother (you and your brother have the same mass – just for the sake of simplicity) is on a floor such that his potential energy is exactly half of that of yours. This building is 10 floors high. Where is your brother?
10. How do stars make light?
11. Why should energy be conserved?