Download Electrical power and energy Electrical power P = t = VI = IzR = R The

Electricity in the Home
(A) Electrical Power and Energy
(1) Electrical Power
Electrical power is the rate of electrical energy consumer per unit time by an electrical
appliance. It is given by the equation
P=
E
 VI ,
t
where V and I are the voltage across it and the current though it respectively.
For an electrical appliance with resistance R, it is also given by
P = I2R=
V2
R
The SI unit of electrical power is watt (W). 1 W = 1 J s-1
The total power can be calculated by using the equivalent resistance or by adding the
individuals.
The rated value of an electrical appliance gives us its power rating, the electrical energy
given to it per unit time when operated at the rated voltage.
(2) Electrical Energy
Electrical energy supplied to, or consumed by or dissipated by it in a time interval t is given
by
E = Pt = VIt
For an electrical appliance of resistance R, the electrical energy is also given by
E = Pt = I 2 R t=
V2
t
R
The SI unit of electrical energy is joule (J).
Electric company measures the electrical energy supplied to consumers in kilowatt-hour
(kW h). 1 kilowatt-hour of energy is the amount of energy consumed by an electrical
appliance of electrical power of 1 kW in one hour. Also, 1 kWh = 3600000 J.
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(B) Mains electricity and household wiring
(1) Direct current and alternating current
Alternating current (a.c.) is current that reverses its direction at a certain frequency.
Direct current (d.c.) is current that never reverses its direction. However, the value of a d.c
may vary with time.
Hong Kong uses 220 V a.c. at 50 Hz. A 220-V appliance cannot function normally when
connected to a 110-V supply. Since P is proportional to V2, the output power is much
smaller than the rated value.
(2) Plug and socket
The live wire goes +V and -V alternately, i.e. the voltage between the live wire and the
earth changes from +V to –V alternatively.
The neutral wire has zero voltage (earthed), i.e. the voltage between the neutral wire and
the earth is zero.
The earth connects the metal case to the Earth and prevents the body of appliance from
becoming `live' if a fault develops.
The earth pin is long
(i) to ensure the appliance is earthed before connected to the live wire;
(ii) to open `shutters' on the socket holes that prevent accidental insertion of metal objects.
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The switch is fitted in live wire. Otherwise, the appliance is still `live' when the switch is off.
The fuse is fitted in live wire and melts when the current is larger than the fuse value.
If the live wire touches the metal case, a short circuit will be set up between the live wire
and the earth. The fuse will blow.
If the neutral wire touches the metal case, the circuit can be still completed, and the
appliance will continue to work.
Double insulation: The live part of an appliance is completely covered by an insulating
plastic case.
Safety rules should be observed when using mains-operated appliances. Electric hazards
include electric shock, burns and fire.
(3) Household wiring
In a consumer unit, each circuit is fitted with a circuit breaker which acts like a fuse.
Electrical energy is supplied to the home by a cable containing a live and a neutral wire.
These wires are connected to the consumer unit via a kilowatt-hour meter. They then
branch out into parallel circuits.
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Mains sockets are connected to a ring main which loops around the house as shown. In
the ring main all the sockets are connected in parallel, so that a fault in any socket will not
affect the others.
In a ring main, thinner and cheaper cables can be used.
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