Electrical circuits

When one or more electrical components are joined together to a cell it is called a circuit. Electricity will not flow if there are any breaks in this circuit.

In a metal wire there are millions upon millions of free electrons, tiny negative charges, buzzing around randomly in all directions. If we connect a cell battery between the two ends of the wire these electrons will drift towards the positive terminal. This flow of free electrons is what we call an electric current.

An electron is such a tiny amount of charge that we use a bigger unit when studying actual electric currents. The amount of electrical charge is measured in units called coulombs.

The electric current in a circuit is measured in Amps using an instrument called an ammeter.

Appliance	Current flowing through it
Light bulb ('average brightness' 100 W)	0.5 A
Electric torch	1 A
Washing machine	1 A
Television	1.5 A
Electric kettle	4 A
Mains immersion heater	13 A

Series and parallel circuits

If you have a battery and two bulbs there are two ways of connecting them to make a circuit:

Series circuit

(a) in SERIES - the bulbs are connected one after the other.
In this circuit the current is the SAME AT ALL POINTS IN THE CIRCUIT (see Figure 1)

Parallel circuit

(b) in PARALLEL - the bulbs are connected side by side.
The current SPLITS AT THE JUNCTIONS (S), some going one way and some the other.
If the bulbs are exactly the same then exactly the same current will flow through each bulb (see Figure 2)


If the batteries and bulbs in both circuits are the same then:
(a) the bulbs in the parallel circuit will be brighter than those in the series circuit
(b) the battery in the parallel circuit will run down quicker than the one in the series circuit