What is energy?

What is energy?

Energy is an abstract idea that is difficult to understand. There are a few common observations that we commonly associate with energy: a body moving, a hot object, a glowing body, and electricity.

There appears to be something common among these. We call this thing energy. The energy in a moving object is called kinetic energy. The energy in a hot object is called heat. That of a glowing body is light energy. That in electricity is electrical energy.

We may have seen that energy can be converted from one form to another. When two objects rub against each other, kinetic energy is converted to heat energy. If a body get so hot that it glows, heat is converted to light energy. If you switch on the electric fan, electrical energy is converted to kinetic energy.

It took physicists a few hundred years to realise that we can associate a number to each form of energy, and that when energy is converted from one form to another, the total energy remains the same.

For example, when a body of mass m moves with velocity v, what is the kinetic energy? Is it v, or is it mv? Both look possible, because they are larger for larger v, suggesting more energy.

However, the physicists found that it should be mv²/2. For example, body sliding along a table slows down and stops, the friction converts the kinetic energy to heat. Only the formula mv²/2 gives the correct amount of heat produced.

Heat, on the other hand, is given by the formula mcθ, where c is the specific heat capacity and theta the temperature increase. The conservation of energy means that if kinetic energy and heat are the only two forms of energy involved, then any decrease in kinetic energy must all be equal to the heat produced. So mv²/2 = mcθ.

The Physics Notes