Series circuits

In series circuits, electrical components are connected one after another in a single loop.

Circuit rules

An electron will pass through every component on its way round the circuit. If one of the bulbs is broken then current will not be able to flow round the circuit. If one bulb goes out, they all go out.

Current in series

Circuit with a cell, two lamps and labels 1, 2 and 3 pointing at various points of the circuit.

A series circuit is one loop - all electrons in that loop form one current. An ammeter will measure the same current wherever it is placed in the circuit:

I_1 = I_2 = I_3

This is when:

  • current (I) is measured in amps (A)

Potential difference in series

The current will transfer energy from the power supply to the components in the circuit. Since energy has to be conserved, all of the source energy is shared between the components. Since potential difference is used to measure changes in energy, the potential difference supplied is equal to the total of the potential differences across all other components:

Circuit containing a switch, 6V battery and two 100 ohm resistors in series. 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.

V_s = V_1 + V_2

This is when:

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

Resistance in series

If two resistors are connected in series, the current must flow through both of them meaning the total resistance is both values added together:

R_{total} = R_1 + R_2

This is when:

  • resistance (R) is measured in ohms (Ω)
curriculum-key-fact
In series circuits:
  • current is the same through each component
  • the total potential difference of the power supply is shared between the components
  • the total resistance of the circuit is the sum of individual resistors