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.