What it shows
Sunlight carries energy. By measuring how fast the sunshine warms a known mass of water, you can estimate the power per square metre that reaches the ground, which is a guide to solar energy.
The science
Water of mass m needs c = 4.18 J for each gram for each degree of warming, so the heat gained is Q = m c times the rise in temperature. If the face of area A is square on to the Sun and the water gains heat for time t, the power received per area is about Q / (A t). Heat is lost to the air, so the reading is a lower estimate of the true value. Bright sunshine at noon, with the Sun high and the sky clear, delivers around 800 to 1,000 W per square metre on a surface facing it, which is why solar cells and heaters work well in India.
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.
- A small aluminium can or steel katori painted matt black on the outside
- Exactly 100 g of water (100 ml)
- A thermometer reading to 0.5 degree
- A stopwatch
- A ruler to measure the face area of the can
- A card to aim at the Sun, with a stand
- A foam or cloth wrap for the sides to reduce heat loss
Safety
For this project: Do not look at the Sun. Wear a hat and take care with the hot can. Do not use hot water above 60 degrees C.
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
8 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.
Paint the can black on its outside and the side facing the Sun. Wrap the sides in foam or cloth. Let it dry.
Measure the area of the face that will point at the Sun, for example a cylinder of 7 cm diameter shows about 7 cm times its height as the shadow area. Use A = diameter x height in square metres.
Add 100 g of water at room temperature and a thermometer, held in place by a card with a hole.
Aim the face of the can at the Sun so that its shadow is the smallest possible, using the card as a guide. Record the starting temperature.
After exactly 5 minutes, stir gently and read the temperature. Repeat for 10 minutes.
Calculate Q = 100 x 4.18 x temperature rise in joules, and then power per area = Q / (A x time in seconds).
Repeat at 9 am, 11 am, 1 pm and 3 pm and plot irradiance against time of day.
Compare your number with the value of about 1,000 W per square metre and write the main sources of heat loss.
What you should see
A graph with the estimate highest near noon and lower in the morning and afternoon, with values of a few hundred watts per square metre because of heat loss, and a note on how to improve it.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the specific heat capacity of water?
About 4.18 joules per gram per degree.
2Why is the measured value lower than the true value?
The can loses heat to the air and reflects some light.
3Why is the can painted black?
Black surfaces absorb most of the light that falls on them.
4Why is the Sun’s power greater at noon?
The rays are more direct and pass through less air.
Take it further
Compare black, white and aluminium foil cans and find how much of the light each absorbs.
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
Science projects
Investigatory Projects for Class 11 and 12
Topics, method and report structure for physics, chemistry and biology