What it shows
Dark and dull surfaces absorb more sunlight than light and shiny ones. The temperature rise of equal cans of water under the same sun shows which colour is best for a solar heater and which suits a roof.
The science
Light that lands on a surface is partly reflected and partly absorbed. Absorbed light becomes heat in the can and warms the water. A black surface absorbs most of the light, while white and shiny foil reflect most, so the black can heats fastest.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Four identical empty tin cans or small steel bowls
- Black, white, silver foil and red paper or paint
- Four thermometers
- Water at room temperature
- A measuring cup
- A watch and a notebook
Safety
For this project: Tin can edges are sharp, so an adult must smooth them with tape. Do not look at the foil glare, and keep thermometers away from the edge of the table.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
7 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Cover the outside of each can with one colour: black, white, silver foil or red.
Pour 200 ml of the same water into each can and place a thermometer in it.
Put all four cans side by side in full sun on a wooden board so none shades another.
Read the starting temperatures of all four cans and write them down.
Read every five minutes for thirty minutes without moving the cans.
Draw a line graph with one line for each colour.
Repeat on another sunny day to check that the order of colours is the same.
What you should see
Temperature curves fan out, with black rising the most and silver or white the least. Repeating the test on a second day should give the same order.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why use equal amounts of water?
A fair test changes only the colour.
2Why is a car in a white colour cooler?
White reflects more sunlight so less heat is absorbed.
3Which colour should a solar water heater pipe be?
Black, because it absorbs the most.
4Why did we keep the cans apart?
So their shadows and reflections do not disturb each other.
Take it further
Test matt black against glossy black paint and decide which finish a heater plate should use.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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