Resistors in Series & Parallel Circuits (WJEC GCSE Science (Double Award): Physics): Revision Note

Resistors in Series & Parallel Circuits

• Adding resistors in series increases the total resistance of the circuit

  • This is because in a series circuit, there is only one loop and therefore one path that the electrons can take

  • All the electrons have to pass through all the resistors, reducing the current

  • Therefore total resistance is increased

• Adding resistors in parallel decreases the total resistance of the circuit 

  • This is because in a parallel circuit, there are multiple loops and therefore multiple paths that the electrons can take

  • These extra pathways increase the number of electrons flowing around the circuit, increasing the current

  • Therefore, total resistance is decreased

• These rules are also true for any components (because all components have resistance)

How Adding Resistors in Series & Parallel affects Total Resistance

Adding resistors in series increases the total resistance of a circuit. Adding resistors in parallel decreases the total resistance of a circuit

Calculating Resistance in Series Circuits

• When two or more resistors are connected in series, the total resistance is equal to the sum of their individual resistances

• For two or more resistors of resistance the total resistance can be calculated using:

$R=R_1+R_2...$

• Where:

  • R is the total resistance, in ohms (Ω)

  • R₁ is the resistance of the first resistor, in ohms (Ω)

  • R₂ is the resistance of the second resistor, in ohms (Ω)

• Increasing the number of resistors in series increases the overall resistance and decreases the current

  • This is because the electrons now have more resistors to pass through

Resistance and Voltage of Resistors in Series

The total resistance of resistors in series is the sum of the individual resistors. The total voltage across the resistors is the sum of the voltages across each individual resistor

Calculating Resistance in Parallel Circuits

Higher Tier Only

• When two or more resistors are connected in parallel, the combined resistance decreases

• For two or more resistors connected in parallel, the total resistance can be found by using:

$\frac{1}{R}=\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_3}...$

• Where R is the total resistance measured in ohms (Ω)

Resistors Positioned in Parallel

When resistors are connected in parallel, the total resistance of the circuit is decreased

• The advantages of resistors positioned in parallel are:

  • The components can be individually controlled, using their own switches

  • If one component stops working the others will continue to function