# Q & A: does inserting a resistor change the brightness of a bulb

Q:
I was asked for a science project... How does resistance in a wire affect the brightness of a bulb? can anyone please answer this A.S.A.P ! thanks :)
- Angy (age 14)
Belton, MO, U.S
A:

Hi Angy

There are a couple of different interpretations of your question, so I am going to have to guess at what electric circuit you are thinking of.   If your source of electricity has a specified voltage ( as it would if coming from a battery or the wall ) and if you insert an extra resistance in series with your light bulb, then the current is diminished by that insertion.   The brightness of an incandescent light bulb will depend on the temperature of the filament, which in turn will depend on the (square of) the current through the filament  Thus inserting the resistor will diminsh the brightness.

If you insert the resistor in parallel with the bulb, then the current through the filament is unchanged and the brightess is unaffected.

(More discussion on this may be found at https://van.physics.illinois.edu/qa/listing.php?id=16891)

- Richard

(published on 03/08/2016)