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# Physics- what determines the brighntess 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?

thanks in advance
2. Increasing the PD will increase the current. Increasing the current will increase the PD.

You can't increase the voltage over a bulb without the current increasing, and vice versa. (except for special, useless exceptions)
3. (Original post by fisherman)
Increasing the PD will increase the current. Increasing the current will increase the PD.

You can't increase the voltage over a bulb without the current increasing, and vice versa. (except for special, useless exceptions)
but doesn't the bulb have a fixed power rating? and as P = IV, if I increases, then V will decrease for P to be constant...
maybe i am wrong about the constant power??

so is it the p.d. or the current which is responsible for brightness?
4. (Original post by Mr Nonsense)
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?

thanks in advance
youre right it is the power. If the resistance of the resistor increases, then current will decrease which leaves you with a constant power. As the power supplied by the battery is constant, the bulb wont change its brightness.
5. how good the idea was.
6. (Original post by Mr Nonsense)
but doesn't the bulb have a fixed power rating?

and as P = IV, if I increases, then V will decrease for P to be constant...
maybe i am wrong about the constant power??
The bulb's resistance is constant (ish, it might change a little with temperature). Your logic is flawed. To show you why, if you drop the current down to zero, your way of thinking means that V = infinity to keep the power down. And if your thinking was correct, the bulb would never go off. Unfortunately, this is not the case.
7. No - it doesnt have a fixed power rating. It has a specified power at a given PD.

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