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Nothing is lost

Energy and power · level 3, lesson 1 of 6

A question to start

A toy car's battery goes flat after an hour of driving. The car is back where it started, not moving. So where did all the battery's energy go? Did it vanish?

The idea

One of the great discoveries of science, made in the 1840s, is the conservation of energy:

Energy cannot be made or destroyed. It can only change from one form into another, or move from one place to another.

So the energy that goes in always equals the energy that comes out, once you count every form. The toy car's battery energy did not vanish. It became:

  • heat in the motor and wires;
  • heat in the wheels and floor, from friction;
  • sound, the whirring of the motor;
  • a little heat in the air, pushed aside by the car.

All that heat spread out into the room, warming it by an amount far too tiny to notice.

battery: 100toy carheat, motor: 55heat, friction: 40sound: 5

This rule never fails. Scientists use it to check their sums: if energy seems to have gone missing, there is a form they have not counted yet.

On Mars, a habitat's engineers would count energy just as carefully: every watt that comes from the solar panels or the power unit ends up, sooner or later, as heat inside the habitat or leaking out through its walls.

Worked examples

1. A kettle. 100 units of electrical energy go in. About 90 warm the water; about 10 warm the kettle itself and the air around it. \(90 + 10 = 100\). None vanished.

2. A rolling ball that stops. Its kinetic energy became heat (in the ball and the floor) and a little sound.

Try it

  1. State the law of conservation of energy.
  2. A lamp takes in 60 units of energy. 10 come out as light. Where did the other 50 go?
  3. A sledge slides down a hill and stops at the bottom. Where did its height energy go?
  4. Why doesn't a room get noticeably warmer when a toy car's battery runs flat in it?
Answers
  1. Energy cannot be made or destroyed, only changed from one form to another.
  2. Into heat, warming the lamp and the air.
  3. It became moving energy, then heat (and a little sound) through friction with the snow.
  4. The energy is very small and spreads out through the whole room.

Common mistake

"Energy gets used up." In everyday talk we say so, but energy never disappears. "Used up" means it has turned into spread-out heat, which is hard to use again.

Where it leads