You connect four 100-ohm resistors in parallel across a 12-volt battery. How many milliamperes of current are drawn from the battery?

There are two effective ways to solve this parallel circuit problem. The first is to calculate the current for a single branch. Each 100-ohm resistor experiences the full 12 volts of the battery. Using Ohm's Law (I = E / R), the current through one resistor is 12 V / 100 ohms = 0.12 amperes, which is 120 milliamperes. Because there are four identical branches in parallel, you simply sum these individual branch currents together. Multiplying 120 mA by 4 gives you the total current of 480 mA. Alternatively, you could first find the total resistance. For four identical 100-ohm resistors in parallel, total resistance is 100 / 4 = 25 ohms. Using Ohm's Law for the whole circuit, 12 V / 25 ohms = 0.48 amperes, which easily converts to 480 mA.