To help visualise electricity flowing through an electric circuit, consider how water flows through pipes in your home. The water is stored in a tank, normally in the loft space and feeds down through pipes to your bathroom, kitchen and other water outlets

Increasing the pressure of the water in  the pipe speeds up the flow of water from the outlets, see fig 1. 

One way  to increase the water flow from the outlet tap is to increase the height where the the water tank feeding into the pipes is stored. 

Study the water flowing from the various taps in your house, for example look at the water flow from the hot tap in the upstairs bathroom of your home then look at the flow of water from a hot tap in the downstairs kitchen or bathroom, you will see a difference in the rate at which the water flows from both taps.  The water flowing from the downstairs tap runs faster than that from the upstairs tap. 

This difference is caused by the pressure between the outlet at the lower level tap being greater than the pressure of the water fed to the upper level tap. This difference in pressure is caused by height difference between the taps and the water storage tank.

The Greater the height between the outlet tap and the water source, the greater the water pressure and the faster the water will flow from the outlet.

Tanks A and B above are identical,

• both feed into pipes of the same diameter,
• the only difference is that Tank A is higher than tank B.
• This causes an increase in the water flow from the pipe due to the increased water pressure (the drawings are not to scale).

Water always takes the easy route

• filling every available area
• flowing fastest through the wider pipes
• flowing slower through narrower pipes.

Fig 2 illustrates a situation where two identical water tanks have drain pipes fitted to their base,

• both tanks are identical in volume
• both tanks are mounted at the same height,
• the only difference is; the outflow pipe from the tank on the left is much wider than the outflow pipe of the tank on the right.

Water will flow from both tanks and run out of the outlet pipes. The outlet pipe from the left hand tank is twice the diameter of the outlet pipe fitted to the tank on the right. This will allow approximately twice as much water to flow from the tank. 

By doubling the internal diameter of the pipe we have halved the resistance offered to the water escaping from the tank. This is true for electric current flowing through a conductive circuit.

Fig 2

The flow of both water and electrons is controlled by the amount of resistance placed into the circuit or network.

• In a water network resistance is mainly determined by the width of the pipe.
• If a narrower pipe is used the flow of water will be reduced, If a wider pipe is used, water flow will increase
• The wider pipe offers less resistance resulting in a greater flow from the tank.

The relationship between current flow pressure and resistance is extremely important and a grasp of this relationship  is fundamental to understanding the actions of an electrical circuit.

• If output resistance remains constant and the input pressure is increased, the current flow will increase.
• If the pressure remains constant but the resistance is increased the current flow will be decreased 
  

Although this example deals with water flow, the same principles apply to electrical current flow where:

• greater electron pressure (Voltage) causes greater electron flow 
• greater resistance causes reduced current flow.

In an electrical circuit, the force of electron pressure is called a power source.

The pressure applied by this power source is measured in volts.

These Voltage sources act like a water pump in a central heating system, where the pump remains in the one place but forces the water to flow through the network of pipes. The voltages source performs the same king of action by remaining in the one spot and forcing electron current to flow through the network of conductors.