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NSW Department of Education and Communities

Curriculum support for NSW Public Schools

Household Circuits


circuit breaker in cupboard

A cupboard where the power comes into a school.
Each circuit has its own circuit breaker.

The electricity that comes into the house enables you to plug any suitable electric device into the wall socket (power point), turn the switch on, and the device starts working. The circuits that are wired into a house by an electrician, all start with the wires coming in from the power lines outside. The wire from the street first runs to the FUSE BOX. Inside the fuse box are the fuses, (or circuit-breakers) which are designed to prevent excess electric current passing into the house. If the current running into the house is TOO large then the heat produced in wires can become excessive and a fire could start. A fuse or circuit breaker is designed to only allow a certain maximum current to flow into a household circuit. If the current in the fuse wire becomes too large, the heat released causes the wire to melt and the circuit is broken preventing a possible house fire.
The circuits in the house are generally wired in PARALLEL, which allows you to operate each light or power point independently of the others. This also means the current running through any one section of the parallel circuit stays small enough to prevent problems. It is important to remember that when you use a powerboard or double adaptor each device plugged in creates a new parallel circuit, but the total current for all of the devices must flow in the one house circuit. This is why it can be very dangerous to piggyback double adaptors or to plug too many devices into a single powerpoint.

The following diagram shows the way most circuit breakers work.  The current from the power supply comes in from the right. The current flows in a coil of wire which has a soft iron core inside it. When current flows through the coil an electromagnet is formed. The strength of the electromagnet is directly related to how large the current flowing in the coil is.  If the current grows too large, the electromagnet becomes strong enough to attract the side of the contacts closest to it, and as this occurs, the contacts are broken and the current stops flowing. A person must then reset the circuit breaker before current can again flow.

circuit breaker 

EXTENSION: The energy produced as heat in a wire depends on the size of:

      * the wires resistance, R, to flow of charge, measured in ohms.
      * the size of the electric current, I, flowing through the wire, measured in ampere.
      * the time the current flows, t, measured in seconds.

This can be expressed by a mathematical equation,

i.e. Energy lost as heat = Resistance x Current squared x the time the current flows,
i.e.   EH = R I2 t 
Note: In this equation the current is squared, this means if you double the current in a circuit,
the energy released goes up by 22, i.e. there is FOUR times more energy released.

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