Transcript Physics 2 - Mains Supply

Additional Science
Chapter 14
SAFETY FEATURES USED IN THE
CIRCUITS OF THE MAINS
SUPPLY
Physics 2
Electrical Power
Additional Science
Chapter 14
Remember:
Electrical Power (W) = Current (A) × Voltage (V)
or
P = IV
Current = 0.25A
Power = Current × Voltage
Voltage = 240V
= 0.25 × 240
= 60W
Power = Current × Voltage
Current = 12.5A
= 12.5 × 240
Voltage = 240V
= 3000W = 3kW
Electrical Power
Additional Science
Chapter 14
Remember:
Electrical Power (W) = Current (A) × Voltage (V)
or
P = IV
Current = 0.5A
Current = 2A
Voltage = 240V
Voltage = 240V
Power =?
Answer: 120W
Power =?
Answer: 480W
Current = 0.4A
Current = 3A
Voltage = 240V
Voltage = 240V
Power =?
Power =?
Answer: 720W
Answer: 96W
Electrical Power
Additional Science
Chapter 14
You must remember:
Electrical Power (W) = Current (A) × Voltage (V)
or
P = IV
Appliance
Power(W) Current(A) Voltage(V)
Hair dryer
3
heater
1200
iron
960
personal
stereo
2.4
microwave
800
240
Electrical Power
You must remember:
Electrical Power (W) = Current (A) × Voltage (V)
or
P = IV
Additional Science
Chapter 14
Appliance
Power(W) Current(A) Voltage(V)
Hair dryer
720
3
240
heater
1200
5
240
iron
960
4
240
personal
stereo
2.4
0.01
240
microwave
800
3.33
240
Electrical plug
Remember:
Te live wire carries the electricity
The fuse protects the appliance from damage
The earth wire protects us from danger from the circuit
The plastic and the rubber is the material for the plug and wire cover
Earth
Live
Additional Science
Chapter 14
Fuse
Neutral
Cord grip
Cable
Electrical plug
Additional Science
Chapter 14
Which plug has been correctly
wired?
A
B
C
D
E
F
Electrical Power and the Fuse
Additional Science
Chapter 14
You must remember:
Electrical Power (W) = Current (A) × Voltage (V)
or
P = IV
Current = ?
Current = ?
Voltage = 240V
Voltage = 240V
Power = 720W
Power = 120W
Current = ?
Current = ?
Voltage = 240V
Voltage = 240V
Power = 3000W
Power = 480W
What fuse must be used?
3A
5A
13A
The work of the fuse
Current travels along the live wire, through the
appliance (bulb in this case) and back along the
neutral wire.
Additional Science
Chapter 14
`
The work of the fuse
This shows what happens when there is a fault in the
circuit.
Additional Science
Chapter 14
The current is too
strong for the
fuse. It blows
causing a break
in the circuit.
Part of the live
wire touches the
side of the
appliance.
A lot of current
now flows along
the earth wire.
Circuit Breaker
Miniature circuit beakers (MCB) are used in
modern consumer units. They replace
fuses.
Additional Science
Chapter 14
When the current exceeds the rated
value, the strength of the electromagnet
is enough to separate the contacts. The
circuit is switched off.
Circuit breakers, like fuses, do not
provide protection should you touch
a live component.
Advantages of the fuse:Click here for further
information from the
web
1) It works a lot faster
2) It is possible to reset it.
(Residual Current Devices RCDs)
These devices are designed to protect against electrical shock. This type
of circuit breaker measures the current in the live and neutral wires. If
everything is working correctly, then these two are equal.
If there’s a fault, you get a small electrical shock but not strong enough
to kill you. You should always use a RCD every time you work with
outdoor equipment.
If there’s a difference between the live wire and the neutral, the
electromagnet in the live side will pull the iron bar downwards, pushing the
button up to create a break in the circuit. The effect of this is to prevent
electricity from reaching the appliance. Switch on the drill to see the RCD at
work.
You must reset the switch before using the RCD again.
Additional Science
Chapter 14
Break in the circuit
B
pivot
N
Iron bar
Wire coils
(electromagnet)
Switch on the drill
Current in
Current out
neutral
live
Power supply
DC and AC
1.5V
1.5V
Voltage (V)
DC means “Direct Current” – the
current travels in one direction only
e.g. in a battery
Time (T)
Oscilloscope
1 th/ s
230V
Voltage (V)
Additional Science
Chapter 14
50
Time (T)
Oscilloscope
AC means “Alternating Current” –
the current changes 50 times per
second (frequency = 50Hz). This
is the frequency that comes from
the mains supply.
Why 230V?
The UK and European countries have
used 230V since the start.
It would be very costly to change this to
a lower voltage – appliances use 230V.
Many transformers would be required for
our appliances to work at a lower voltage.
Additional Science
Chapter 14
Thicker and more flexible cables would
be required (lower voltage higher current
to ensure the same power.
Many builders use tools
that use 110V. This
voltage reduces the risk
of fatal electrical shocks.
The USA has chosen a voltage of 120V for its mains
supply. Because of this, some electrical appliance
manufacturers include a switch so that the correct
voltage can be chosen for the appliance – using the
appliance with a voltage of 120V would be damaging
in this country.