Parallel Circuits - Resistive

A simple parallel circuit is made up of a source and resistors connected in parallel across the source by "ideal conductors." This diagram shows two ways of drawing the same circuit.


There is a separate independent path from the source to each resistance and back to the source. This means that the voltage must be the same at each point in the parallel circuits. Therefore,

E8 = E1 = E2 = E3 = E ...

In the parallel system, each of the load elements is connected across the same terminals, therefore each has the same voltage drop:

There are three rules governing simple parallel circuits of resistive elements:

1. Voltage across each resistor is the same as the voltage across the parallel combination.
2. The current flowing through the parallel combination is the sum of the current in the separate branches.
3. Summing resistance of a parallel circuit can be stated as follows: The reciprocal of the total resistance is equal to the sum of the reciprocals of each of the individual resistances.

The total current from the source, however, divides at one terminal and recombines at the second terminal. For the example, the current divides at point A, with part of the current flowing through each resistor. The current then recombines at point B. This can be described using Kirchoff's Current Law which states that the sum of the current leaving a junction must equal the sum of the currents entering a junction. This can be expressed for the example as:

The total resistance in ohms is calculated by multiplying the resistances and dividing by the sum of the resistances.

If the voltage produced by the source is 240 volts, then each load has 240 volts applied to it. Ohms Law allows us to calculate the total current (amps) flowing in the circuit.

Since 8 amps is the total current flow through the circuit, the current through each load is equal to the circuit voltage divided by its resistance.