Parallel circuits

In parallel circuits, electrical components are connected alongside one another, forming extra loops.

Circuit rules

An electron will not pass through every component on its way round the circuit. If one of the bulbs is broken then current will still pass round the circuit through the other loop. If one bulb goes out, the other will stay on.

Current in parallel

Circuit containing a switch, five ammeters and cell, all connected in series, and two lamps connected in parallel. Four of the ammeters are labelled as I1, I2, I3 and I4.

Since there are different loops, the current will split as it leaves the cell and pass through one or other of the loops. If one loop has less resistance than the other, there will be more current in the loop with the lower resistance. An ammeter placed in different parts of the circuit will show how the current splits:

I_{1} = I_{2} + I_{4} = I_{3}

This is when:

current (I) is measured in amps (A)

If the resistance of any component in a parallel circuit changes, this will change the value of the current in that loop and the total current supplied by the battery. It does not affect the current in the other loop.

Potential difference in parallel

Circuit containing a switch, battery and two resistors labelled R1 and R2 in parallel. There are voltmeters in parallel to the resistors and battery.

Since energy has to be conserved, the energy transferred around the circuit by the electrons is the same whichever path the electrons follow. Since potential difference is used to measure changes in energy, the potential difference supplied is equal to the potential differences across each of the parallel components:

V_{s} = V_{1} = V_{2}

This is when:

potential difference (V) is measured in volts (V)

If the resistance of any component in a parallel circuit changes, this will have no effect on the potential difference across any of the components.