Transcript Alternating Current - School

http://www.youtube.com/watch?v=MrNdMoiMh-Q
P2 - Physics
Mains Electricity
P2 – Physics - Aims
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to recognise errors in the wiring of a three-pin
plug
to recognise dangerous practice in the use of
mains electricity
to compare potential differences of d.c. supplies
and the peak potential difference of a.c.
supplies from diagrams of oscilloscope traces
(HIGHER ONLY) to determine the period and
hence the frequency of a supply from diagrams
of oscilloscope traces
Mr. P. R. Collins
P2 – Physics – Mains Electricity
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Mains electricity is useful but can be very
dangerous. It is important to know how to use
it safely
Mr. P. R. Collins
P2 – Physics – Currents
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Cells and batteries supply current which
always passes in the same direction. This is
called direct current (d.c.)
An alternating current (a.c.) is one which is
constantly changing direction. Mains electricity
is an a.c. supply. In the UK it has a frequency
of 50 cycles per second (50 hertz)
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UK mains supply is about 230 volts
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Most electrical appliances are connected to the
mains using cable and a three-pin plug
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Mr. P. R. Collins
P2 – Physics - Currents
Voltage
Direct Current – flows in
one direction only
Time
1/50th second
230V
Time
Alternating Current –
changes direction 50
times every second
(frequency = 50 hertz)
Voltage
Mr. P. R. Collins
P2 – Physics – The Three-Pin Plug
Earth Wire
Live Wire
Fuse
Neutral Wire
Insulation
Cable Grip
Mr. P. R. Collins
P2 – Physics – The Three-Pin Plug
The neutral wire of a plug stays at a
potential close to zero with respect
to the earth
The live wire of a plug alternates
between positive and negative potential
with respect to the neutral wire
Mr. P. R. Collins
P2 – Physics - Safety
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If an electrical fault causes too great a current the circuit
should be switched off by a fuse or a circuit breaker
When the current in a fuse wire exceeds the rating of the
fuse it will melt, breaking the circuit
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Appliances with metal cases are usually earthed
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The earth wire and fuse together protect the appliance
and the user
Mr. P. R. Collins
P2 – Physics - Earth wires
Earth wires are always used if an appliance has a _____ case. If
there is a _____ in the appliance, causing the live wire to ______ the
case, the current “_______” down the earth wire and the ______
blows.
Words – fuse, fault, metal, surges, touch
Mr. P. R. Collins
P2 – Physics - Fuses
Fuses are _______ devices. If
there is a fault in an appliance
which causes the ____ and
neutral (or earth) wire to cross
then a ______ current will flow
through the _____ and cause it to
_____. This will break the _______
and protect the appliance and
user from further _____.
Words – large, harm, safety, melt, live, circuit, fuse
Mr. P. R. Collins