1.) Power dissipated by a resistor is given by P = ?VR2/R. However, your textboo

Question

1.) Power dissipated by a resistor is given by P = ?VR2/R. However, your textbook also says that the power dissipated by a resistor is given by P = I2R. Is the power really proportional to R or is it really proportional to 1/R? Can both be correct? 2.) Calculate the total charge that passes through a typical household light bulb (60 W) in one second (when it is on, of course). Assume that the household voltage is 120 V. 3.) When you start your car, your battery may provide several thousand joules of energy to the starter motor (lets say 3000 J). Estimate the total number of electrons that must move from high to low potential in your car battery when this occurs.

Explanation / Answer

(2) P = I2R --> R = P/I2
V = I R = I(P/I2) = P/I ---> I = P/V
I = 60/120 = 0.5 A = 0.5 C/s
(3) Most car batteries have a voltage of V = 12 V.
P = E/t and P = I V and I = Q/t
From these equations, you should be able to derive the relationship
Q = E/V = 3000 J/120 V) = 25 C
Just divide this by the charge of an electron, 1.602 x 10-19, to get the number of electrons.

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