P2 Physics Aims to recognise errors in the wiring of a threepin plug to recognise dangerous practice in the use of mains electricity to compare potential differences of dc supplies and the peak potential difference of ID: 790703
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Slide1
P2 - Physics
Mains Electricity
Slide2P2 – Physics - Aims
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
Slide3P2 – Physics – Mains Electricity
Mains electricity is useful but can be very dangerous. It is important to know how to use it safely
Slide4P2 – Physics – Currents
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)
UK mains supply is about 230 volts
Most electrical appliances are connected to the mains using cable and a three-pin plug
Slide5P2 – Physics - Currents
Voltage
Time
Direct Current
– flows in one direction only
1/50
th
second
230V
Voltage
Time
Alternating Current
– changes direction 50 times every second (frequency = 50 hertz)
Slide6P2 – Physics – The Three-Pin Plug
Earth Wire
Fuse
Live Wire
Neutral Wire
Insulation
Cable Grip
Slide7P2 – 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
Slide8P2 – Physics - Safety
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
Appliances with metal cases are usually earthed
The earth wire and fuse together protect the appliance and the user
Slide9P2 – 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
Slide10P2 – 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