In this case, a distinction can be made between the emf of the battery and the actual
voltage it provides once the current has begun. Since I = 2 A, the voltage drop
across the internal resistance is Ir = (2 A)(2 Ω) = 4 V, so the effective voltage
provided by the battery to the rest of the circuit—called the terminal voltage—is
lower than the ideal emf. It is V = ε− Ir = 20 V − 4 V = 16 V.
- A student has three 30 Ω resistors and an ideal 90 V battery. (A
battery is ideal if it has a negligible internal resistance.) Compare
the current drawn from—and the power supplied by—the battery
when the resistors are arranged in parallel versus in series.
Here’s How to Crack It
Resistors in series always provide an equivalent resistance that’s greater than any
of the individual resistances, and resistors in parallel always provide an equivalent
resistance that’s smaller than their individual resistances. So, hooking up the