Energy: forms and changes
The forms of energy, stored and working energy, how energy changes from one form to another, and why some energy is always 'wasted' as heat. With a bouncing-ball experiment.
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Scientists say that energy is the ability to do work. It comes in many forms, for example heat, light, motion, sound, electrical, chemical and gravitational energy.[1][2]
Energy is grouped into potential (stored) energy and kinetic (working) energy. Chemical energy in food, batteries and fuels, a stretched spring and an object high up are all stored energy; light, heat, sound, electricity and motion are working energy.[1][2]
Energy changes from one form to another. A cyclist rolling downhill turns gravitational energy into motion energy. In a hydroelectric dam, falling water turns a turbine that produces electricity.[2]
Energy is never made out of nothing and never destroyed: in any process the total amount stays the same. It only changes form or moves from one place to another.[4]
But energy often changes into forms we did not want, usually heat. An old-style (incandescent) light bulb gives off 90% of its energy as heat instead of light. LED bulbs use at least 75% less energy for the same job.[6]
Friction also turns movement into heat, so no machine passes on all the energy it gets as useful energy. This is why a bouncing ball never bounces back as high as it was dropped.[3][4]
Try it: How high does a ball bounce back?
You need
- A bouncy ball, a tennis ball or a ping-pong ball
- A metre stick or measuring tape fixed to a wall
- A hard floor
- A partner to watch the bounce (a phone video helps)
Safety
Drop the ball, never throw it. Clear the floor around you. Don't stand on chairs; 100 cm is high enough.
What to do
- Hold the ball next to the metre stick with its bottom at 25 cm. Let it drop: do not throw it.
- Your partner watches where the bottom of the ball reaches at the top of its first bounce. Write it down.
- Repeat from 50 cm, 75 cm and 100 cm.
- Do each height three times and work out the average bounce height.
- Fair test: use the same ball, the same floor and always just let go.
- Does the ball ever bounce back to the height you dropped it from? Where did the rest of the energy go?
What happens, and why?
Up high, the ball has gravitational (stored) energy. As it falls this turns into motion energy. In the bounce some energy changes into heat and sound, so less is left to lift the ball again and it never reaches its starting height. The total energy is still the same: it has only been spread out.[2][3][4]
Play: Where does the energy start?
Each card shows an energy change. Which form of energy does it start as?
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Quiz
Other ages: Ages 6–7 · Ages 15–16