Question

say battery = 10v
but bulb only requires 5v (when bulb says it needs 5v does it mean it needs 5v for it to operate? so any smaller it wont operate?)
what if we supply a battery 10v. then will weird things happen?

Answer 1

If you have a bulb rated for 5 V and you connect it to 10 V battery, it will probably burn out, because the power heating the bulb is much more greater, so the filament will melt and the lamp passes out.

5 V bulb means it will work fine with that voltage (it will give bright light but doesn't burn out). If you connect a 5 V bulb to a 3 V battery, for instance, the bulb will emit light but not very bright light.

Answer 2

Note: In some high-school (not university books, I hope :-) physics books there are exercises where you have to calculate things like:

"You have a 5-volt 1-watt bulb. What power it will take if you connect it to a 10 volt battery." Then the "correct" answer in the book is that because
\[P=V*I=\frac{V^2}{R}\]
and you double the voltage, then according to the formula the power will be four times the original power. This is *not* true, because the resistance of the wolfram filament in the bulb is highly dependent on the temperature of the bulb. Therefore, if you raise the voltage, the filament will get hotter and its resistance will increase and the power will decrease (of course the total power increases but not in power of two).

Answer 3

hmmm i'm quite lost on the answer you gave about calculating power..
why do you use those equations? why are there so many equations to choose from for power?