Current and resistance

When the switch in this circuit is closed, there is a good flow of electricity through the bulb so it glows brightly.
When a second or third bulb is included in the circuit, they each glow less brightly i.e. the current in the circuit is smaller.
The size of the current in any circuit is affected by the number of components in the circuit and what they are. The components oppose the flow of electricity. They have resistance.
The effect of including a component which has resistance into a circuit can be explained by picturing the current as runners on an athletics track, and the component’s resistance as an obstacle such as a set of step ladders. Without the component, the electricity flows freely around the track. When the component is included in the circuit, the flow of electricity, i.e. the current, is reduced.
Resistors:Components called resistors are included in some circuit to control the electricity that flows. Some resistors have a resistance that can be charged or altered. They are called variable resistors and are extremely useful for altering the size of current in a circuit.
The variable resistor in the circuit in figure 5, is being used as a dimmer switch to control the brightness of the bulb. Variable resistors are also used to alter the loudness of the music from a stereo system, the colour and brightness of the picture on a TV set, and the speeds of electric motors.
We measure the size of a resistor in ohms Ω for example a 10Ω resistor will offer half the resistance to the flow of current of a 20Ω resistor