What is energy? Where does it come from?
Energy isn't a thing that you can hold in your hand. There is no pure form that you can smell, taste, feel, hear or see. Energy is an abstract idea that helps us to understand how the universe works.
The simplest form of energy is mechanical energy.
This is the energy that moving objects have. If an object (such as
a wind-up toy) is standing still, it doesn't have any apparent energy.
But if the toy is moving very fast, we say that it has a lot of energy.
The faster the object moves, the more energy it has. The bigger (more
massive) the toy is, the more energy it has. Energy is a way of describing
how much of something, and how fast that something, is moving.
Conservation of Energy
But where does this energy come from? One minute the toy is still, but if you release a switch, the toy starts moving around. Does this mean that energy was suddenly created? Well, no. One of the most important laws of nature is that you can't get something for nothing. We call this idea the "Law of Conservation of Energy". This law states that energy is never created or destroyed, but is simply transformed (changed) from one form of energy into another (or transferred from one object to another).
The energy of the moving toy had to have been some other form of energy. Where did that energy come from? Answer: the sun. You are probably asking yourself what the sun has to do with it. After all, the wind-up toys that I've seen do not have solar panels. But the energy for almost all the motion on the surface of our Earth originally came from the sun. Let's see if we can trace the path of this energy back to the sun.
If we let the toy run for awhile, eventually it will run down and stop moving. Then we might reset the switch, and wind up the toy. If we again release the switch, the toy will start moving again. So, where does the energy come from? It comes from whomever wound up the toy! But something mysterious also happens. Suppose we leave the wound-up toy on a shelf for a couple of hours, without releasing it. Where's the energy? It's stored inside the toy, in the spring. By winding the toy we squeeze a spring inside the toy, and that spring is just waiting to be released so it can expand back to its normal size. This energy, stored in the spring, is called potential energy. It is called potential energy because it has the ability or potential to become the energy of motion.
Now that we have a name for stored energy, potential energy, we probably need a more specific name for the energy of motion. When something is actually moving we say that it has kinetic energy. Both the potential energy stored in the spring and the kinetic energy of the moving toy are types of mechanical energy. Both are types of energy that mechanical things might have.
So the kinetic energy of our hands changes into the potential energy of the spring when we wind up the toy. And when we release the switch, the potential energy of the spring changes into the kinetic energy of the toy as it moves around. Great! So where does the kinetic energy of our hands come from?
Inside the cells in our bodies, sugar from the food that we eat and oxygen from our breath are changed into carbon dioxide and water. This process causes the release of energy stored as chemical energy. Chemical energy is not a form of mechanical energy. But it can be changed into mechanical energy. Chemical energy is used to cause our muscles to move, enabling us to wind up the toy.
If we try to combine sugar and oxygen to get water and carbon dioxide in a laboratory, we must burn it. This releases a huge amount of heat. Yes, heat is another form of energy. We change chemical energy into heat, and then heat into kinetic energy to make lawn mowers, cars, trains, boats, airplanes and rockets move. When sugar and oxygen combine in our bodies, heat is produced as well. That is why our bodies stay warm, why we are said to be "warm-blooded".
Where does the chemical energy come from? Well, it's stored as chemical energy in the sugar. The sugar and oxygen are made by plants during photosynthesis. They are made from water and carbon dioxide. This process requires energy, and the chemical energy stored inside the sugar has to come from somewhere. It comes from light. Light is another form of energy, and it comes from the sun.
So where does the energy of the moving toy come from? It comes from the sun. Where does the energy that causes trains, boats, planes, and even us to move come from? It comes from the sun. Almost all of our energy comes from the sun. Nature and our machines just change energy back and forth from one type of energy into another.
There are other forms of energy that we have not mentioned. The energy from the sun starts out as nuclear energy. Energy is supplied to our homes as electrical energy. X-rays, light and radio waves are all types of radiant energy. Perhaps you can name other types of energy.
Work
Now that we know something about several different
types of energy, how do we measure and compare them? Since energy
can be changed from one form into another, we measure all forms of energy
by how much mechanical energy we can produce. Any change in the amount
of mechanical energy that we measure is called "work". The amount
of work is the amount of energy that has been converted into mechanical
energy.
Suppose that we burn rocket fuel in a rocket engine. The chemical energy will produce heat energy. Hot gases expand. The gases produced by the burning rocket fuel try to expand, but to expand they must push the rocket forward. This push is a force. Suppose this force is constant as it pushes the rocket forward for 10 meters. Then the rocket engine is shut off. The rocket will have sped up, and will now have more kinetic energy than it had before. If we measure the force, and multiply it by the distance, we obtain the amount of work done on the rocket.
WORK = FORCE x DISTANCE = Change in mechanical energy = DE
W = DE = F x d
Using this formula, we can figure out how much chemical energy is stored in the rocket fuel by measuring the amount of kinetic energy that is given to the rocket. The metric unit of work or energy is the joule. One joule is equal to one newton-meter.
Example: Suppose the rocket's engine produces a force of 20 newtons for a distance of 10 meters. Then the resulting energy of the rocket can be found from the formula:
Force x
Distance = Work
(20 newtons) x (10 meters) = 200 newton-meters = 200 joules.
(20 N)(10 m) = 200 J
Summary
We have learned in this unit that energy comes in
many forms, but it is never created or destroyed. It simply changes
from one form into another. Energy can exist as mechanical energy.
This mechanical energy is called kinetic energy if the object is moving.
it is called potential energy if the energy is somehow stored. One
way to measure energy is by measuring a change in energy or work.
Work is defined to be the product of force times distance.
REVIEW
1. List five different forms of energy.
2. What is the difference between kinetic and potential
energy?
3. State the law of conservation of energy.
4. Find the work done when a crane lifts a steel
beam with a force of 2000 N a distance of 7 m.
5. Apply: Do you think that a car can be designed
that is so efficient that it will go 50 miles at highway
speeds on a single drop of gasoline? Why or why not?
copyright David Carpenter 1990
Addendum: 2001
The "article" above was written as an introduction to energy for a
physical science course. It is included here because the Law of Conservation
of Energy is, in my opinion, the most important and fundamental idea in
physics. A partial listing of applications to topics covered
in Physics 1 would include:
