Relationship between voltage and distance between light bulb and solar cell

[Nash4Lyf]

School experiment:

Shine lamp at a solar cell, and measure the voltage using a multimeter.
Independent variable is the distance between the light bulb and solar cell, dependent variable is the voltage.
What is the relationship between the two? And how exactly do I prove it?

What I have is: since Power decreases as Distance increases, and the bulb is spherical, then Power is proportional to 1/[4 pi (r squared)], r being the distance between the solar cell and light bulb. Also, since Power is IV, and we assume current is constant in this case (?), then V is proportional to 1/[4 pi (r squared)].

Also I have to make adjustments to my graph to make it a straight line; for this, do I have to adjust the one I measure (ie voltage) or do I have a choice of which to adjust (distance or voltage)

Thank you to whoever responds :)

 

[tiny-tim]

Welcome to PF!

Hi Nash4Lyf! Welcome to PF!

(have a pi: π and try using the X² icon just above the Reply box )

What I have is: since Power decreases as Distance increases, and the bulb is spherical, then Power is proportional to 1/[4 pi (r squared)], r being the distance between the solar cell and light bulb.

That looks right.

Also, since Power is IV, and we assume current is constant in this case (?), then V is proportional to 1/[4 pi (r squared)].

You're confusing the two Vs … the V inside the bulb does have P = IV = V²/R, but that V is fixed … the V in the meter is the one you're measuring, and that depends on the intensity of the light, not on current.

Also I have to make adjustments to my graph to make it a straight line; for this, do I have to adjust the one I measure (ie voltage) or do I have a choice of which to adjust (distance or voltage)

Either … choose whichever is simplest (I'd go for the one where the slope is the constant you're actually trying to find).

 

[Nash4Lyf]

You're confusing the two Vs … the V inside the bulb does have P = IV = V²/R, but that V is fixed … the V in the meter is the one you're measuring, and that depends on the intensity of the light, not on current.

Thank you for replying. I understand my mistake with confusing the two V's now.
Since you said voltage depends on the intensity of the light, I thought that V is proportional to the intensity. The intensity of the light is P/4πr², and if the intensity is proportional to V measured on the meter and the power of the lamp stays constant (100W), then that proves the relationship of V is proportional to 1/4πr². Is what I said correct?

 

[tiny-tim]

Hi Nash4Lyf!

(just got up :zzz: …)

Since you said voltage depends on the intensity of the light, I thought that V is proportional to the intensity. The intensity of the light is P/4πr², and if the intensity is proportional to V measured on the meter and the power of the lamp stays constant (100W), then that proves the relationship of V is proportional to 1/4πr². Is what I said correct?

Yes!

The meter measures the amount of light hitting the solar cell …

and that amount is proportional to 1/r². ​

 

[Nash4Lyf]

Hi Nash4Lyf!

(just got up :zzz: …)


Yes!

The meter measures the amount of light hitting the solar cell …

and that amount is proportional to 1/r². ​

Thank you very much! I understand it now! :D