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 be able to flow round the circuit through the other loop. If one bulb goes out, the other will stay on.

Current in parallel

Since there are different loops, the current will split as it leaves the cell and pass through one or other of the loops. 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)
Circuit containing a switch, ammeter and cell, all connected in series, and two lamps connected in parallel.

Potential difference in parallel

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)
Circuit containing a switch, 6V battery and two 100 ohm resistors in parallel. Label 1 points to a voltmeter connected across the battery, marked Vs. Labels 2 and 3 point to voltmeters connected across each resistor, marked V1 and V2 respectively.

Resistance in parallel

If resistors are connected in parallel so that the current will flow through either one or the other, but not both, then the overall resistance is reduced as less current is flowing through each.

In parallel circuits:
  • the total current supplied is split between the components on different loops
  • potential difference is the same across each loop
  • the total resistance of the circuit is reduced as the current can follow multiple paths