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# stupid question i know... quick bit of help please: brightness of a bulb watch

1. what determines the brightness of a bulb?

i know it is a stupid question and i really ought to know the answer....

i think it is power and P = IV, but which of the current or the p.d. affects the brightness.

This is in the context of a potential divider circuit. So if the bulb is in parallel with one of the resistors, and the resistance of that resistor increases, the pd across the bulb gets bigger, so the current gets smaller. Does the bulb get brighter or dimmer?

2. anyone.....?
3. Because the bulb and the resistor are in parallel, if the resistance of the resistor increases the combined resistance of the resistor and bulb will increase:

(For resistors in parallel)

So if the resistance increases the current will drop, V will stay the same (your input voltage) and the power decreases.

The bulb will dim.

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Updated: November 14, 2008
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