Measurements to guide policy – or at least a model measurement
What the activity is for
In this activity a solar cell is illuminated with light from a 12 V bulb. The distance from the bulb to the cell is increased so that the light intensity decreases, although not in a linear way. Measure the power delivered to an external load to produce a graph relating distance to power delivered.
What to prepare
- a 12 volt power pack
- 8 leads
- 2 crocodile clips
- a fixed resistor below 100 ohm (sample used was 33 ohm)
- 2 SEP energy meters (although one is sufficient if moved from the bulb connection to the solar cell connection during the demonstration)
- 1 12 V incandescent bulb in holder (the bulb glass should be transparent so that the filament is visible)
- 1 solar cell (sample used was 9.5 cm by 6.5 cm)
- some black card, large enough to form a closed box around the cell and bulb (sample used was 15 cm by 40 cm)
- some Blu-tack
- a strip of graph paper 15 cm long with lines marked at each centimetre along the length
What happens during this activity
Set the power pack at 10 volt so that the bulb is near to maximum brightness. When collecting data, the black card surrounding the bulb should exclude light from the room. The solar cell is fixed to stand at right angles to the benchtop. The graph paper strip is fixed to the base of the black card to read the distance between the cell and bulb filament. The filament bulb should be moved along the length of the black card, Blu-tacked into position each time it's moved.
Your own values are likely to differ from those shown and should be used. Show the students the set-up and explain that you will be investigating how well a solar cell can deliver power to a load when illuminated by an external source.
Using, and measuring the output from, solar cells
Teacher: Here we have a solar cell. Has anyone ever seen one in use?
Jenny: Yes. I have one on my calculator.
Julian: And I have seen one connected to the bus stop where I live. It's connected to the sign that says what time the bus is due to arrive.
Teacher: Good. They're designed so that when they're illuminated by light they deliver power to devices such as the ones you have mentioned. During the day the light conditions are changing if we think about the bus sign. What causes this to happen?
Jenny: Loads of things, like how cloudy it is or if a bird is sitting in front of the cell.
Julian: And it is obviously sunnier at midday than in the afternoon.
Teacher: You're right. More generally, the ability of the cell to deliver power to a device depends on the brightness of the lighting on the cell. We'll use the
intensity as a measure of the brightness of the light – that'll be a precise enough definition for our use. We'll find the power delivered to a resistor when the distance from the lamp filament to the cell is increased.
Julian: Why did you say filament? Can't we just use the edge of the bulb?
Teacher: No. That's because the source of the lighting is the filament.
Begin by placing the filament 3.5 cm from the cell. Switch the bulb on and have both SEP meters set to read power. It's useful to write these figures on the board:
At 3.5 cm, power delivered to bulb is 24 watt
At 3.5 cm, power delivered to load is 12.6 milliwatt
Teacher: Any thoughts on these values?
Jenny: The power delivered to the load is half as much as to the bulb.
Linda: Don't be silly! It's much, much less because one number is measured in watts and the other milliwatts which is 1000 times smaller than watts! This means that the power delivered to the load is 2000 times smaller than delivered to the bulb.
Teacher: That's excellent! Now why do you think that's so?
Pulling ideas together
Julian: Well, the bulb is being continuously supplied with energy through the electrical working pathway. It must be that not all of the energy shifted from the original store is being shifted to the light pathway. The bulb is getting hot so the thermal energy store is also filling.
Teacher: That's also an excellent point.
Linda: If we're talking about efficiency, surely the solar cell itself is not 100 percent efficient when switching power to the electrical pathway.
Teacher: You're also right. There's another significant point too. The filament is emitting light in all directions away from it over a large area, but the solar cell is only collecting a very small percentage of the total light being emitted over a small area, this being the area of the solar cell. All of the other light is not collected by the cell and so ends up heating the surroundings along the heating by radiation pathway.
Teacher: Now let's see what happens if we double the distance between the filament and the bulb. Any guesses?
Jenny: I think the power delivered will be half that of before because the light intensity must be less, so about 6 mW.
Put the bulb now a distance of 7.0 cm from the cell and notice that the power delivered to the load is around 1.5 mW.
Jenny: That's not what I expected!
Teacher: This is because you imagined that the brightness of the light reaching the cell would halve if we moved the bulb twice the distance from the solar cell. This might seem sensible but actually it's not the case. The light intensity doesn't fall in a linear way as we increase the distance from the cell to the bulb.
It's sufficient to leave this point as it is and not to explain further why this is the case. A more able group might want to know what the relationship is between distance and light intensity. If so, see the note about taking matters further.
Teacher: Let's see how the cell responds as we increase the distance in a regular way.
Begin with the bulb 3.5 cm away from the cell and record the power delivered to the load as you increase the distance in 1 cm increments. You'll see that the power delivered will fall quite rapidly as the distance increases. You might want to run through the process two or three times, finding average values of power delivered to the load at each distance. Students should be asked to plot a graph of the data with the distance between the filament and the cell on the x axis and power delivered to the load on the y axis, as in the sample data.
Teacher: We can see from the data that the amount of power being delivered to the load decreases in a non-linear way as the light intensity or brightness falls.
When increasing the distance between the filament and the cell by a factor of 2, say from 3.5 cm to 7.0 cm, the light intensity will decrease by a factor of 22, which is 4. When increasing the distance between the filament and the cell by a factor of 3, say, from 3.5 cm to 10.5 cm, the light intensity will decrease by a factor of 32, which is 9, and so on.