Electricity in the home
Many household items run from batteries, but the many larger electrical
appliances like heaters, lights, cookers, refrigerators, computers and
so on require a continuous power supply than batteries cannot provide.
The electrical energy that can be transferred to an appliance each second
is the product of voltage and current. The higher the voltage and current,
the more energy is transferred. Ordinary dry cells provide only about
1.5 volts with a low current, while the household mains supply provides
240 volts with much higher available currents.
The other major difference is that dry cells deliver their energy using
direct current (DC), while the mains in your home is alternating current
(AC).
Direct current
As the name implies the current flows directly around a circuit, with
electron charges traveling from the (flat) negative terminal of a cell
to the (protruding) positive terminal in one direction only. Dry cells,
car batteries, and solar panels are examples of DC electrical sources,
and the telephone exchanges in Australia operate on a 48 volt DC supply.
Alternating
current
Alternating current in your home reverses the current 50 times each
second. It has a frequency of 50 Hz. The unit is named after Heinrich
Hertz (1857-1894).
There are several advantages to using AC current, including:
- Alternating current is much easier to generate in high voltages
and currents.
- AC can readily be transformed to higher or lower voltages. High
voltage transmission of power to your home reduces energy loss in
the cables.
- AC motors are cheaper and easier to maintain than DC motors.
Power in your home
Another electrical unit that you will see in your home is the watt.
This unit was named after James Watt (1736-1819), and is an indication
of how much electrical energy per second is transformed by an appliance
into another form of energy. For example, a standard light globe might
have 60 watts marked on it, meaning it will transform 60 Joules of energy
each second into light and heat. A heater labeled 2500 watts transforms
2500 Joules of energy each second into heat .
Power is related to voltage and current by the formula power = voltage
multiplied by current. For a heater of 2400 watts, with a voltage of
240 volt, the current is 2400/240 = 10 amp. This is the maximum current
that can safely pass through a conventional power point, so the maximum
power that can safely be transformed, without blowing a fuse or jumping
the circuit breaker, is usually 2400 watt.
How much current will a 60 watt globe draw when connected to a 240
volt power supply? To find out you have to rearrange the formula. You
need to know the current, so by simple transformation, the formula becomes:
current = power divided by voltage. In this case it is 60 watt / 240
volt = 0.25 amp.
One reason why piggyback plugs are not a good idea in a power point
is that although any individual item may draw less than 10 amp, the
combination of several items could well exceed the limit and cause a
fire in the wiring. Each home and factory has a safety system in place
to prevent fires from happening, this safety feature is called a fuse
or circuit breaker.
Fuses
The word fuse means to melt and that is exactly what a fuse in
your household supply box does. As charge flows through the fuse wire,
it heats up. If the circuit is overloaded, then the fuse wire gets very
hot, melts, and breaks the circuit, cutting off current to the appliances
connected on that circuit. A common phrase is that the fuse has "blown".
Sometimes a fuse blows because there is a fault in an appliance and
a 'short circuit' has been created producing a sudden surge of current
in the circuit. Circuit breakers work in a similar way to fuses, but
physically jump and break the circuit. The appliance is usually protected,
but you may not be! That's where another safety feature comes into play
- the Earth connection.
Earthing
Most electrical plugs have three pins. Two of them are for the
charge to flow through and the third, a longer one, is connected to
a wire in your house that leads straight to an Earth connection - often
a metal pipe buried in the ground. If a wire comes loose in an appliance,
it could touch a metal part and pass through you to Earth, with possible
fatal results. The Earth connection provides an easier path for the
current to flow, so it takes that path instead.
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