The conservation of energy: nothing is created or destroyed

A torch battery goes "flat"; a bouncing ball stops bouncing. It is tempting to say the energy was used up.

Physics says something stronger and more useful: the energy is still somewhere. It has been transferred to another store or transformed into another form, often warming the surroundings.

Because nothing is lost, you can always balance the books.

Write the energy going in on the left and everything coming out on the right:

$\text{total energy in} = \text{useful energy out} + \text{energy dissipated}$

Dissipated energy has spread out into the surroundings, usually as thermal energy (often with a little sound). It has not vanished — it is simply no longer useful for the job.

Two everyday balances:

• a motor: electrical energy in = kinetic energy of the load + thermal energy in the motor;
• a ball falling: stored energy at the top = kinetic energy at the bottom + thermal energy from the air.