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For me, understanding energy and power and the difference between them falls into the category “essential knowledge”.

First a misnomer: “Energy creation”. How often have your read or heard somewhere about “energy creation”? Probably a lot. Well, guess what: we cannot create or destroy energy. Nope, its not possible, no matter what anyone says. In fact, the first law of thermodynamics is also called the law of conservation of energy.

What the first law means is simple: we cannot create or destroy energy. All we can do is transform energy from one state to another. Either the density of energy is transformed or the kind of energy is transformed. Often the two go hand in hand. We can illustrate this by the internal combustion engine: The petrol is ignited and burns in a cilinder, converting chemically stored energy into hot gasses. The hot gasses are allowed to expand by moving a piston. This expansion cools the gasses and transforms part of the energy of the gas molecules into piston movement. Motion energy is called kinetic energy. The piston receives kinetic energy from the expansion of gasses. The kinetic energy of the piston is transformed through mechanical means into motion energy of the car. The kinetic energy of the car is transformed into a slight temperature increase of the air and road. No energy was lost (or destroyed) and no energy was created. In the end all what was done is the conversion of chemically stored energy in the petrol into a warming of the environment. Note: If a change in elevation occurred during the motion an additional energy called potential energy is involved as well.

Energy transformation is called “work”. We are all familiar with the term ‘work”, but in physics the term “work” is used to describe energy transformations. The amount of energy transformed (work) per second is the “power” of the transformation. There are many different units available to express power, the most universal is probably “Joule”. 1 Joule is equivalent to 1 W per second.

Energy and power are thus related by the passing of time. A device with a power of 1000W that is active for 1 second will have consumed 1000J of energy. If that same device is active for 10 seconds, it will have consumed 10 times as much. I.e. 10,000J.

It is said that an energy transformation is never 100% efficient. However that too is not correct. Since energy cannot be created or destroyed, the result of all transformations performed during the work is by necessity exactly the same as what went into it. However the useful amount of work is always less than 100%. But this is of course a subjective look at the energy transformation from the human perspective.

This subjective viewpoint is of course important. We want to do stuff (work), and need energy for that. But technological limitations prevent the use of all the available energy for the purpose at hand. There is always a loss involved. Sometimes large, sometimes small. But the loss is always there.

The loss is usually in the form of heat. But sometimes there is a loss due to chemical processes or radiation as well. Great ingenuity is often necessary to minimize the loss. Sometimes people can get so exited that they think to have invented a perpetuum mobile. A device that -left to its own- will keep on moving forever. In reality though such devices are impossible. Any claim for such a device must be met with the utmost skepticism. Especially when money (investment) is involved. In the past people have invariably lost their bets against the thermodynamic laws.


The main point I would like to make here is this: In any given problem, always consider the energy picture. And keep in mind that both sides must always balance each other out. Energy in = Energy out. Just because we sometimes cannot see all the energy sources and sinks does not mean that they are not there.

TANSTAAFL: There Aint No Such Things As A Free Lunch.

Originally posted at: 2016-11-19
Last modified on: 2016-11-19