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Wednesday, March 10, 2010

What is a complete circuit?

As you know in order to have electricity you will need a complete. So in order to have a complete circuit there are several features that must be present.

The power supply 

A Power supply may comes in different forms such as:

1. The power pack that you have in your school’s physics lab. An example if in fig 1 below.

power pack

                  Fig 1

2. A dry cell such as the one in your portable radio. An example is shown in fig 2 below.

dry cell

                  Fig 2

3. A battery such as the one that you have in your car.

The purpose of the power supply is to give energy to the electrons such that they can move around the whole circuit and come back to the power supply.

The symbol of the power supply is shown in fig 3.


                                     Fig 3

As you know all power supplies have a positive terminal usually red in colour while the negative terminal is black in colour. Hence in the symbol the long vertical bar is the positive terminal while the short vertical bar is the negative terminal. 

The switch

A circuit must also contain a switch. The purpose of the switch is to allow to to stop the flow of current in the circuit by opening it or to allow the flow of current by closing it. An example of a switch is shown in fig 4.


            fig 4

The symbol of the switch is as follows:

A closed switch –current flowing


An open switch- no current flowing


The load

A load is a circuit component that will use the energy supplied by the power supply and will thus convert the electrical energy. Fig 5 below show a bulb that convert electrical energy to light energy.


                        Fig 5

Each load has a symbol. The symbol for the bulb is as shown below:


The complete circuit

Now let us combine all this together. In order to have a complete circuit we need the power supply the switch and the bulb to be connected together with wires as shown in fig 6.



Fig 6

Let us try to understand what happens when the switch is closed and a current is flowing on the circuit.


Fig 7

As you know an electric current is due to a flow of electrons. When the switch is closed electrons can leave the negative terminal of the power supply as shown in fig 7 and move in a clockwise direction. The electrons will move through the switch, through the bulb and finally will reach the positive terminal of the power supply. The movement of the electrons are represented on fig 7 using arrows.The electron will lose energy to the  bulb and this energy will be lost in the form of light. Should the switch be open then the electron will not be able to move from the negative terminal to the positive terminal.

If the circuit is represented using symbols then the circuit will be as shown below in fig 8.


Fig 8

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