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Where the waste goes

Energy and power · level 3, lesson 3 of 6

A question to start

Touch a phone charger after it has been working for an hour: it is warm. An old light bulb gets too hot to touch. Was that heat any use?

The idea

Every machine changes energy from one form into another. The energy that does the job we want is useful energy. The rest is wasted energy. It is not destroyed; it just goes somewhere we did not want it.

Machine Useful energy Wasted energy
Light bulb Light Heat
Electric motor Movement Heat, sound
Car engine Movement Heat, sound
Phone charger Electrical, into the phone Heat
Television Light, sound Heat

Wasted energy nearly always ends up as heat that spreads into the surroundings. Spread-out heat is very hard to collect and use again.

old bulb: mostly heatLED: much more lightlightheat

Machines that waste less are better for everyone: they cost less to run and need smaller batteries and panels. An LED lamp gives the same light as an old bulb using only a fraction of the energy, because it wastes far less as heat.

On Mars, "waste" heat is not always wasted. A habitat needs heating anyway, so the heat from computers, lamps and machines inside helps keep it warm. Engineers plan for this.

Worked examples

1. An old bulb. Of 100 units of energy in, only about 5 come out as light. The other 95 heat the room.

2. An electric motor. A good one turns about 90 of every 100 units into movement. The other 10 heat it up.

3. A rover in winter. Curiosity's power source gives off far more heat than electricity. Pipes carry some of that heat around the rover to keep its parts warm at night.

Try it

  1. What is wasted energy?
  2. Where does most wasted energy end up?
  3. Why is an LED better than an old bulb?
  4. Why is waste heat sometimes useful in a Mars habitat?
Answers
  1. Energy that does not do the job we want: it goes somewhere we didn't want it.
  2. As heat, spread out into the surroundings.
  3. It wastes much less energy as heat, so it needs far less energy for the same light.
  4. The habitat needs heating anyway, so the heat helps keep it warm.

Common mistake

"Wasted energy is destroyed energy." It still exists, as heat warming the surroundings. It is "wasted" only because it did not do the job we wanted.

Where it leads