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Can any one help, I'm wanting to do the experiment with a small can of water and let it heat up from sun light to work out the power of the Sun. The one that Brian Cox carried out on tv some time ago. I need to know step by step as I want to do this with my Scout group while doing some solar observing.

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I imagine such experiment would be quite inaccurate, but interesting one nonetheless.

I'm speaking of the top of my head here, so hopefully others will fill in the blanks.

First thing is to insulate water from surroundings as best as you can. Maybe attempt experiment in thermos bottle of sorts or similar. You'll need thermometer, magnifying glass and piece of good thermal conductor - like copper strip or something like that. 

Measure weight of the water. You'll need to know this.

Put one side of copper strip inside water canister and put thermometer in water canister as well. Some insulation of rest of the copper strip - one exposed to air and not hit by sun rays will be beneficial as well.

Read of temperature of water at start of time interval (for example 5 minutes?). After that take magnifying glass and secure it so that it is focused on exposed piece of copper strip. You might want to do test prior to experiment to see approximate surface of focused sunlight to allow that much copper strip to be exposed (actually make exposed part of copper strip about 20% larger just in case).

Wait some time (mentioned 5 minutes?) and read off water temperature at the end of this period. If you want to do more precise experiment - have stopwatch and do "continuous" measurements at some time interval - like every 2 minutes for 20 minutes.

After this it is the matter of doing some basic calculations. You will need to measure surface area of your magnifying glass, and heat capacity of the water (https://en.wikipedia.org/wiki/Heat_capacity) which is 4185.5 J/(kg⋅K) (15 °C, 101.325 kPa)

From this you can get energy flux from sun per unit area (it takes ~4200J to heat up 1Kg of water 1C and that much energy is radiated thru surface of magnifying glass).

Alternatives / improvements would be:

- heating water directly without conductor

- taking into account heat capacity of copper strip

- using some sort of filter to pass only interesting band of light (IR part of spectrum)

- putting everything in dark box and just having magnifying lens as "window" to the box. Measuring temperature of the air inside of the box and accounting for that energy as well.

- Doing the same as above but having insulated box to minimize energy exchange with environment.



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Thanks for this, but what I'm looking for is to carry out if possible William Herschel experiment from the 1800's. Simple letting a said volume of water heat up over a given time by sun light alone and then some how do ( very rough calculations) to work out how much KW of energy the Sun kicks out.

What's your maths like?

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Let's assume you have 0.5Kg of water and temperature of it raises by 5C in half an hour.

Heat capacity of water is like mentioned above (from wiki page) 4185.5 J/(kg⋅K).

This means that you need 4185.5 Joules of energy to raise temperature of one kilogram of water 1 degree (K or C - same units).

When we plug in numbers we get 4185.5 (J/kg*K) * 0.5 kg * 5C = 10.46375 KJ of energy needed for this.

Power is energy per unit time. So we have 10.46375 KJ of energy in half an hour, or 1800s. Power will be 10465.75J / 1800s = ~5.8132W

So 5.8W heater will need half an hour to heat half a liter (roughly half a kilo) by 5C. Or alternatively Sun gave out 5.8W of heat. Now saying that Sun gave of that much power does not make sense - not unless you define it via flux, or integrate over whole sphere to get total solar power output. This is why it is important to define surface area for heat collection - easiest by use of lens to converge all rays to small surface. In that case area of lens is collecting area.

Also, if you have slow heating and water is not well insulated from environment - some of the heat will escape to surroundings and your measurement will be off.

Also to get total EM power output of the Sun you would need more calculations and a bit specific gear - like IR pass filter. In that case you need to integrate flux over sphere with radius earth - sun distance, and also know surface temperature of the Sun and apply Plank's law for black body radiation and apply section of IR pass filter. That way you can get total power output of the Sun over all frequencies / wavelengths.


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A little bit  different to what you have in mind but you could use a piece of spray painted (matt black) aluminium foil and monitor its temperature as it is heated by the sun with an infrared thermometer. You could then shade the foil and measure the temperature drop. You could combine the two sets of results to obtain an approximation of the intensity of the sunlight.

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Two experiments we ran at school.

1/ Not precise but gives the idea. Can be fun.
Place a volunteer(?) against a wall in the shade, palm of one hand facing out.
Equip the rest of the group with small mirrors.
Persuade the group to shine the mirrors at the subjects hand. Not eyes, not other people, etc.
As the number of mirrors increases, the subject is going to report heating. Maybe even say ouch!

2/ A more precise measure.

Something along the lines of vlaiv's first suggestion. 
We had a metal can with a small hole in the lid, for the thermometer.
This was placed inside a larger can which had a small (1cm-ish) hole in the side.
The gap beteen the cans was filled with sponge insulation.

Set up a magnifiying lens to focus the sun in the exposed area.

Known weight of water in the smaller can. Take the temperature at the start.

Run the experiement for a few minutes until there is a reasonable temperature rise.

Energy received in joules is temp rise centigrade x  water grammes x 4.2.

Divide this by the number of seconds to give the rate of energy supply in watts.

Now calculate the lens area to give the heat supply watts per square metre.


Experiment 2 is more educational and teaches various skills.
But unless you have enough kit to allow pairs to run the experiment, there is a lot of standing around.

Experiement 1 is more fun and everyone joins in.

From my (limited) experience of arranging scout activities, experiment 1 is going to receive more attention.

Hope this helps,






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