Switch on a flashlight and it glows. Where does the light come from? From energy stored in the battery.
Key idea
Energy is never used up. It changes form or moves from one thing to another.
| Thing | Energy goes in as | Comes out as |
|---|---|---|
| Flashlight | stored (battery) → electrical | light and heat |
| Toaster | electrical | heat and light |
| Your body | stored (food) | motion and heat |
| Fan | electrical | motion and a little sound |
| Wind turbine | motion (wind) | electrical |
| Solar panel | light | electrical |
Why it works
When a ball rolls to a stop, its motion energy did not vanish. It turned into a little heat in the floor and the ball (from friction) and a little sound.
Worked example: Follow the energy
A child eats breakfast, then rides a bike to school. Trace the energy.
- The food holds stored energy.
- The body changes it into motion of the legs and some heat.
- The pedals pass motion to the wheels.
Answer: Stored (food) → motion (and heat).
Common mistake
Saying the battery's energy "disappears". It changed into light, heat, sound or motion.
Try it at home
- Rub your hands together fast for ten seconds. What do you feel?
- Your motion energy turned into heat by friction.
Going further. In Pendulums, watch energy swap between height and motion. Later: Conservation of Energy in Physical Science.