Simple Parallel Circuits

Let's start with a parallel circuit consisting of three resistors and a single battery:

The first principle to understand about parallel circuits is that the voltage is equal across all components in the circuit. This is because there are only two sets of electrically common points in a parallel circuit, and voltage measured between sets of common points must always be the same at any given time. Therefore, in the above circuit, the voltage across R1 is equal to the voltage across R2 which is equal to the voltage across R3 which is equal to the voltage across the battery. This equality of voltages can be represented in another table for our starting values:

Just as in the case of series circuits, the same caveat for Ohm's Law applies: values for voltage, current, and resistance must be in the same context in order for the calculations to work correctly. However, in the above example circuit, we can immediately apply Ohm's Law to each resistor to find its current because we know the voltage across each resistor (9 volts) and the resistance of each resistor:

   REVIEW:

•    Components in a parallel circuit share the same voltage: ETotal = E1 = E2 = . . . En

•    Total resistance in a parallel circuit is less than any of the individual resistances: RTotal = 1 / (1/R1 + 1/R2 + . . . 1/Rn)

•    Total current in a parallel circuit is equal to the sum of the individual branch currents: ITotal = I1 + I2 + . . . In.