The variables I will be changing are the distance of the bulb from the L.D.R because I need to read the mA at every 5cms towards the L.D.R from 55cms.
We will have to measure how much daylight there is and what volt the bulb is at and keep it at that to it to be a fair test.
I predict that the nearer the bulb to the L.D.R the higher the amp reading and the further away the L.D.R is, the duller the light to the L.D.R This is because it all depends on how much the light spreads out, when the LDR is near to the light there is a small distance so it misses less light. But when the LDR is further away from the light there is a bigger distance and so it misses more light. See diagram on next page.
Equipment-
1 L.D.R
1 bulb
2 voltmeters
1 ammeter
2 power packs
1 metre ruler
Wires
Method-
Attach all of it together
Put the bulb 55cms away from the L.D.R, we put it 55cms away because that’s the maximum length, otherwise if we go over that then the readings will just be 0 all the time and it would be pointless. So we had to find the reading that was the lowest.
Set the bulb to 10v and keep it on that.
Find out what the ammeter reads without the bulb light (this is the daylight).
Then record what the ammeter reads as you move the bulb 5cms nearer to the L.D.R each time, but take the reading of no bulb light away from them.
Observing
We found out the reading for daylight which is 6.8mA and took this away from the above results to leave us with results without daylight on the next page.
See graph on graph paper on next page.
Then I have to work out the surface area of the bulb at each distance.
The surface area of a L.D.R is 1cm, so I have to work out what 1cm of mA received by the L.D.R is, to do this I have to divide 1cm by the surface area for each distance in the table above.
Analysing
The closer the bulb to the L.D.R, the higher the average distance measured by mA.
At the end, the further away from the L.D.R, the closer together the results are. When light shines on the LDR, the photons making up the light beam supply the energy needed to break the bonds in the cadmium sulphide. Electrons become free and can help conduct the current.
If the light beam is more intense, then more bonds are broken. So the LDR’s resistance decreases when you bring the light source closer. And the resistance increases as you move the light away. This is because it all depends on how much the light spreads out, when the LDR is near to the light there is a small distance so it misses less light. But when the LDR is further away from the light there is a bigger distance and so it misses more light. See diagram on next page.
Evaluating
I think maybe at 45cms we had shadows covering the light a bit so the recordings (mA) varied.
Overall the experiment was fairly accurate. A thing that may vary the recordings to make the points off the best fit line is the angle of the bulb as it could be a few millimetres off. To extend this investigation we could put tissue paper or something in front of the bulb to measure how much light is absorbed.
