Transcript Mains electricity - Thomas Tallis Science Department

Mains electricity
Revise the following
•
•
•
•
•
Direct and alternating current
3-pin plugs and electrical cable
Safety – fuses and earth wire
Period and frequency (Higher tier)
Calculating Power
Direct current
Alternating current
What are the parts of a plug?
earth
terminal
live
terminal
fuse
neutral
terminal
cable
cable
grip
Label the plug parts
The names and jobs of each plug part
What’s inside an electrical cable?
copper
insulating
plastic
• An electrical cable contains three wires.
• These wires are made of copper because it is a good conductor of
electricity
• Each wire is made of thin strands of copper to keep the cable flexible
• Each wire is encased in plastic to stop the wires touching and causing a
short circuit
• Plastic is used as it is a good insulator, as well as being tough and
flexible
• The whole cable is encased in another layer of plastic
What does each wire do?
The live wire (brown)carries a
current that alternates between
a negative and positive voltage.
The earth wire
(green and yellow) is a
safety wire that is needed
to earth appliances with
a metal case.
This makes it safer to
touch the appliance if it
develops a fault.
The neutral wire (blue) completes
the circuit. It is kept at a zero
voltage by the electricity company.
Safety using Fuses and Earthing
FUSES
•
•
A fuse contains a thin wire, which
melts if the current is too high.
This breaks the circuit and so
electricity is unable to flow
through the appliance.
•
The appliance stops working and
any danger has been averted.
•
Fuses act as an early warning
system, preventing appliances
from being damaged by surges in
electricity and warning owners of
faults.
EARTHING
• Cables contains an earth wire to
provide an alternative path for
current if appliances develop
faults.
• An earth wire is essential for an
appliance with a metal case.
• If the live wire becomes loose
and touches the metal case, a
very large current flows to earth
and blows the fuse, breaking the
circuit
• When an appliance is working
correctly, no electricity flows
through the earth wire.
Frequency and period
(Higher tier)
• We can use an oscilloscope to work out
the frequency of the ac supply by
measuring the period (time taken for
one complete cycle)
• Frequency and period are connected by
the equation:
Frequency (Hertz) =
1
.
period (seconds)
Using oscilloscope traces to work out period
and frequency
Each division
will have a
potential
difference
(voltage) value
Zero voltage
(0V)
The 0 voltage
represents the
Neutral
terminal
(stays at a
potential close
to zero –
compared to
the Earth)
Time
Work out the frequency of the supply shown below?
Each time division =1 milliseconds
Period = 4 divisions x1 = 4ms
4 ms = 0.004 seconds
Frequency = 1/Period (seconds)
Frequency = 1/0.004
= 250Hz
Calculating Power
• Electric current is the rate of flow of charge.
• When an electrical charge flows through a resistor,
electrical energy is transformed into heat energy.
• The rate at which energy is transformed in a device is
called the power.
Power
= current x potential difference
(Watts/W))
(Amps)
(Voltage/V))
energy
transformed
(Joules/J)
=
potential difference
(Voltage/V)
charge =
current x
(Coulomb/C) (Amps)
x
charge
(Coulombs/C)
time
(secs)