What is energy?
Well, we could go to the New Lexicon Webster's Dictionary to get the definition:
en*er*gy n. the unifying concept of all physical science that associates with any system a capacity for work either as a result of the motion of mass in the system (kinetic energy), the configuration of masses or charges in the system (potential energy), or the presence of photons in the system (radiant energy). Energy is a scalar quantity with dimensions of mass x length2 / time.
What?!?!? What does that mean?
Well, let's break down what the definition is telling us.
Basically, the definition starts off telling us that most sciences have a concept called energy. This idea of energy comes from the notion that something can do work. Work is the ability to cause a transformation in something else. For instance, you do work when you press down on your gas pedal in your car. Your muscles need to contract to get your foot to move downward.
Then we find out that there are three different kinds of energy:
 | Kinetic energy is present when things are moving, like spinning a top or moving your car. As the top slows down it loses more and more kinetic energy until it finally stops spinning and falls over. Where did that kinetic energy go? |
Potential energy is a little trickier to explain. Think of standing on the end of a teeter totter. Now think of a big rock at the top of a cliff way above the other end of the teeter totter. That rock isn't moving. It's just sitting there waiting for somone to push it off the cliff. This rock has potential energy. As soon as it is pushed off the clifftop, it starts to fall and gives up its potential energy. When the rock lands on the other end of the teeter totter, it will have no more potential energy left to give up. What happened to the potential energy?
Well, those two concepts weren't really that bad. Now we need to talk about the last idea of radiant energy. Let's think about the sun for a minute. There are nuclear reactions happening on the sun that release huge amounts of energy, and this energy needs to go somewhere. Well, it doesn't turn into kinetic energy by having the sun all of a sudden change direction or speed up or slow down. And it doesn't go into lifting the sun onto the top of a cliff. So it needs to dissipate this energy by radiation.
We've seen now how the different types of energy can change from one to another. If you'd like to see how this leads to the First Law of Thermodynamics, you can continue.
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