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. Since fewer electrons pass through any loop, they will find it easier to travel and the total resistance of the circuit will be lower than any of the individual resistors in the circuit.

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:

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

This is when:

  • current ( \text{I}) is measured in amps (A)
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.

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:

\text V_{s} = \text V_{1} = \text V_{2}

This is when:

  • potential difference ( \text{V}) is measured in volts (V)
Circuit containing a switch, battery and two resistors labelled R1 and R2 in parallel. There are voltmeters in parallel to the resistors and battery.

Resistors connected in parallel

When resistors are connected in parallel, we know that the potential difference across each resistor is the same but the current through each resistor adds up to the current through the supply. By having more resistors that are connected in parallel, the more current flows for the same potential difference. If more current flows for the same potential difference, then the resistance has gone down. Overall then, more resistors connected in parallel means less overall resistance.

Jonny Nelson introduces an animated explanation of circuits

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