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Energy chains

Energy and power · level 3, lesson 2 of 6

A question to start

The rover Spirit worked on Mars for six years on solar power. Can you follow its energy from the Sun to its turning wheels, step by step?

The idea

An energy chain follows energy from where it starts to where it ends up, one change at a time. Each arrow is an energy change; each box is a form or a store of energy.

For a solar-powered rover:

lightSunelectricalpanelchemicalbatteryelectricalmotorkineticwheels heat leaks away at every step

And where did the Sun's energy come from? Deep inside the Sun, hydrogen is squeezed into helium, which releases nuclear energy. So the chain really starts in the Sun's core.

Most energy chains on Earth also start with the Sun:

  • Food: Sun → plants (chemical) → you (chemical, then kinetic and heat).
  • Wind power: Sun warms the air → wind (kinetic) → turbine → electrical.
  • Coal and oil: Sun → ancient plants and sea life → buried for millions of years → fuel (chemical).

Worked examples

1. Riding a bike. Chemical (food) → kinetic (legs) → kinetic (bike), plus heat in your body and the tyres.

2. A torch at night. Chemical (battery) → electrical → light, plus heat.

3. A Mars greenhouse. Electrical (city power) → light (lamps) → chemical (plants) → chemical (your salad).

Try it

  1. Draw the energy chain for a wind-up toy.
  2. Where does the energy in a battery-powered car come from, if the battery was charged by solar panels?
  3. Where does the Sun get its energy?
  4. Why does almost every chain end in heat?
Answers
  1. Chemical (your muscles) → elastic (wound spring) → kinetic (the toy moving) → heat.
  2. From sunlight, through the panels and the battery.
  3. From nuclear reactions in its core.
  4. At every step some energy turns into heat, and moving things end up stopped by friction, which makes heat.

Common mistake

Leaving out the heat. A chain that goes neatly from light to movement with nothing lost is not right: every step wastes some energy as heat.

Where it leads