The Physics Question Bank

Circuit questions come up every year, and nearly all of them depend on knowing how current, potential difference and resistance behave in series and parallel circuits.

Series circuits

In a series circuit, all the components are in one loop.

• Current is the same everywhere in the circuit.
• Potential difference from the supply is shared between the components.
• Total resistance is the sum of the individual resistances: R_total = R₁ + R₂

Parallel circuits

In a parallel circuit, components are on separate branches.

• Potential difference across each branch is the same as the supply.
• Current splits between the branches. The total current is the sum of the branch currents.
• Total resistance is less than the resistance of the smallest individual resistor.

Worked example: series

A 4 Ω resistor and a 6 Ω resistor are connected in series to a 12 V supply. Find the current and the potential difference across each resistor.

1. Total resistance: 4 + 6 = 10 Ω
2. Current: I = V ÷ R = 12 ÷ 10 = 1.2 A
3. p.d. across 4 Ω: V = I × R = 1.2 × 4 = 4.8 V
4. p.d. across 6 Ω: V = 1.2 × 6 = 7.2 V

Check: 4.8 + 7.2 = 12 V. ✔

Worked example: parallel

The same two resistors are now connected in parallel to the 12 V supply. Find the total current.

1. Each branch has 12 V across it.
2. Current through 4 Ω: 12 ÷ 4 = 3 A
3. Current through 6 Ω: 12 ÷ 6 = 2 A
4. Total current: 3 + 2 = 5 A

Notice that the total current is much bigger than in the series circuit. That’s because adding resistors in parallel gives the current more paths to flow through, so the total resistance goes down.

Quick memory trick

Series: current same, p.d. shared.
Parallel: p.d. same, current shared.