Cracking the SAT Physics Subject Test

source must do positive work on the charge, forcing it to move from the negative
terminal toward the positive terminal. The charge is now ready to make another
journey around the circuit.

Energy and Power

When a carrier of positive charge q drops by an amount V in potential, it loses
potential energy in the amount qV. If this happens in time t, then the rate at which
this energy is transformed is equal to (qV)/t = (q/t)V. But q/t is equal to the current,
I, so the rate at which electrical energy is transferred is given by the equation

P = IV

This equation works for the power delivered by a battery to the circuit as well as
for resistors. The power dissipated in a resistor, as electrical potential energy is
turned into heat, is given by P = IV, but because of the relationship V = IR, we can
express this in two other ways.

P = IV = I(IR) = I^2 R

or

Resistors become hot when current passes through them; the thermal energy
generated is called joule heat.

Heat Dissipation

Power in a circuit is very

closely associated with

heat given off. If we were

to touch a light bulb we

would notice it becomes

hot. The greater the heat

given off, the brighter the

light bulb is.