The rules for current, potential difference and resistance in series and parallel circuits, and how they differ between the two arrangements.

What this dot point is asking

CCEA Double Award wants you to know the rules for current, potential difference and resistance in series and parallel circuits, and to explain how the two arrangements differ. These rules let you work out unknown currents and voltages, and explain everyday wiring.

Series circuits

Adding more components in series increases the total resistance, so the current falls.

Parallel circuits

Comparing the two

Working out a parallel current split

In a parallel circuit each branch behaves as if connected straight to the supply, so you can find each branch current with Ohm's law using the full supply voltage, then add the branch currents to get the current drawn from the supply. For example, with a $12\ \text{V}$ supply, a $6\ \Omega$ branch carries $12/6 = 2.0\ \text{A}$ and a $4\ \Omega$ branch carries $12/4 = 3.0\ \text{A}$, so the supply provides $2.0 + 3.0 = 5.0\ \text{A}$. Notice the total current ($5.0\ \text{A}$) is larger than through either branch, which is why the overall resistance is lower than either resistor on its own.

Examples in context

Example 1. Christmas lights

Old-style lights wired in series all go out if one bulb fails, because the circuit is broken; modern sets avoid this with parallel wiring or special bulbs.

Example 2. House wiring

Sockets and lights are wired in parallel so each has the full mains voltage and can be switched on and off independently without affecting the others.

Example 3. A torch with two cells

Two cells in series add their voltages to give a bigger push (a brighter bulb), while connecting two identical bulbs in parallel across one cell lets each bulb get the full cell voltage so both shine at normal brightness, though the cell runs down faster because it supplies more current.

Try this

Q1. What is the same everywhere in a series circuit? [1 mark]

• Cue. The current.

Q2. Two $3\ \Omega$ resistors are in series. Find the total resistance. [1 mark]

• Cue. $R_{\text{total}} = 3 + 3 = 6\ \Omega$.

Q3. Why is house lighting wired in parallel? [2 marks]

• Cue. So each lamp has the full mains voltage and one lamp failing does not switch off the others.