electromagnetism - How do I calculate the power consumed by a lightbulb?

I'm studying a lightbulb and its variable resistance, given by the expression:

$R(T) = Ro[1 + α(T-T_0)]$, where $R_0$ is the resistance of the lamp at $T_0$.

In this case, $R$ is not given by Ohm's law ($V=Ri$). So, which expression can I use to calculate the power consumed by a lightbulb?

• $P = R\cdot i^2$


• $P = \frac{V^2}{R}$


• $P = V\cdot i$ (I don't think I can use this one)

I have measured the current and the e.p.d. at the bulb using instruments.

Answer

All those expressions are equivalent and can be used, if you simultaneously measure, the voltage, current (and know the resistance from the temperature).

Why do you think ohm's law doesn't apply? Resistance is only dependent on temperature in your case (not current or voltage).

In general I would use P = IV, if Ohm's law actually doesn't apply.