Today is a big day, even if it completely doesn't feel like it. Today, we're going to learn THE MOST IMPORTANT LAW OF ALL PHYSICS.

As far as we can tell, this law is true absolutely everywhere in the universe. It's a big deal.

Here it is, the law of conservation of energy: Energy cannot be created or destroyed. It can only change from one form to another.

This means that for any energy we can think of — the kinetic energy of a car, the sound energy of a speaker, the thermal energy of a fire — this energy has existed for the entire age of the universe, and it will continue existing till the end of time. However, the form these energies take can change over time.

Another way of stating this law is that total energy in = total energy out.

Let's understand this better by looking at some examples...

Speakers produce sound. So do speakers create sound energy?

No, energy cannot be created or destroyed, so this energy must come from somewhere else. Of course, for a speaker to work it needs to be plugged in (or have batteries). So a speaker works by changing electrical energy into sound energy.

For example, if a speaker produces 100J of sound energy, then all 100J of this energy came from electrical energy.

Cars are useful because they move. Things that move have kinetic energy. So do cars create kinetic energy?

No, energy cannot be created or destroyed, so this energy must come from somewhere else. For a car to work it needs fuel (e.g. petrol or diesel). The chemical energy of these fuels is released when the fuel is burnt. So a car works by changing chemical energy into kinetic energy.

For example, if a car has 100,000J of kinetic energy, then all 100,000J of this energy came from chemical energy.

(NB: Much of the chemical energy also converts into sound energy and thermal energy, as well as kinetic energy).

Light bulbs produce light and heat. So do light bulbs create light energy and thermal energy?

No, energy cannot be created or destroyed, so this energy must come from somewhere else. For a light bulb to work it needs to be plugged in (or have batteries). So a light bulb works by changing electrical energy into light energy & thermal energy.

For example, if a light bulb produces 300J of light energy & thermal energy, then all 300J of this energy came from electrical energy.

On the next page, we will discover a neater way to draw energy transfers.