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Physics Heat
Class 7 Beginner About 12 min Premium

A solar cooker: catching the Sun's heat

Heat a pot of water in a box solar cooker. Try a black, a white and a shiny pot, add a foil reflector and a glass lid, and find out how to make the water boil with sunlight alone.

Core concept

Dark surfaces absorb more of the Sun's radiation than light or shiny ones; a reflector sends in more sunlight and a glass lid traps the heat

Topic: Heat

The Sun sends us energy: What is needed to make things move, heat up, light up or grow. All key words as light and heat, which travel through space as radiation. When sunlight falls on something, some of it is absorbed and turns into heat, and the rest is reflected. Dark, dull surfaces absorb most of the light, while white and shiny surfaces reflect most of it. A box solar cooker uses these ideas to cook food with free sunshine: a black pot to absorb, a shiny reflector to send more sunlight in, and a glass lid that lets sunlight in but stops the warm air escaping. Solar cookers are used in sunny parts of Nepal to save firewood and gas.

Key words explained

What you will learn

  • Explain that the Sun's heat reaches us by radiation.
  • Compare how black, white and shiny surfaces absorb radiation.
  • Explain how a reflector and a glass lid help a solar cooker.
  • Describe how the water's temperature rises with time.

Before you start

  • Read a thermometer in degrees Celsius.

Materials

  • Water, three small pots (painted black, painted white, and shiny metal)

Equipment

  • A box solar cooker with a foil reflector and a glass lid, a thermometer, a clock

Procedure

  1. Leave the black pot in the cooker for 30 minutes with no reflector or lid, and record.
  2. Do the same with the white pot and the shiny pot, and record both.
  3. Add the foil reflector to the black pot and record.
  4. Add the glass lid too and record.
  5. Record the temperature after 0, 15, 30, 45 and 60 minutes.

How this simulation is simplified

  • The day is sunny and the air is 20 °C; the cooker holds 1 L of water.
  • The black pot absorbs 90% of the sunlight, the white pot 30% and the shiny pot 10%.
  • The foil reflector sends in 2.5 times as much sunlight; the glass lid keeps 60% of the heat that would escape.
  • The water warms towards its highest temperature, quickly at first and then more slowly, and cannot get hotter than 100 °C (boiling).
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