Mains Electricity
Mains electricity is an alternating current (A.C.)
power supply
(ac)
Direct Current (d.c.)
one direction only
Alternating Current (a.c.)
constantly changing direction
load
load
(circuit components)
(circuit components)
Mains electricity has a frequency of 50Hz; 50 complete waves per second:
Here the current is flowing one way around the circuit
...and here it is flowing the other way around the circuit
Power
station
Live
Neutral
step
up
transformer
The National Grid
step
down
transformer
earth connection
Live
Neutral
The live wire at your house is kept at 230V and the neutral wire is earthed (connected to the ground) to force it to be 0V
step
down
transformer
earth connection
Power
station
Live
Neutral
step
up
transformer
step
down
transformer
earth connection
Live
Neutral
The live wire at your house is kept at 230V and the neutral wire is earthed (connected to the ground) to force it to be 0V
step
down
transformer
earth connection
Mains electricity in the home
fuse box
live wire
neutral wire
ring mains
There is no circuit until you connect an electrical device between the live and neutral rings
Connecting to the ring mains
M
motor
!!
z
zz
live wire Bzz
neutral wire
Each device is connected to the ring mains in parallel. This means that:
• they can be switched on/off individually
• if one fails it doesn't stop the rest from working
• you can connect as many devices as you have wall sockets
Don't worry about the earth wire yet
earth wire
sTriped
live wire (230V)
bRown
The live & neutral must be correctly connected to make a complete circuit and operate the device
neutral wire (0V)
bLue
The cable grip must grip the main cable not the individual wires
The earth wire is only needed to protect the user against electrocution ­ the device will work even if it is not connected
The earth wire should be excessively long so that it pulls out of the plug last
The Earth Wire
This wire also goes around the room with the ring mains and is connected to the ground by a large copper pipe
l
bo
m
sy
Dangerous currents can be carried down
into the ground through the grounding rod
ac
tu
al
ground rod installation
Normal operation
Complete circuit
via power supply
metal
case
live wire
heating element
insulator
neutral wire
Live wire touches the element case which touches the metal case
Faulty
live wire
neutral wire
Live wire touches
the case
Current flows from the 230V person down to the 0V ground
(current flows from high voltage to low)
The current flows through the earth wire instead of the person because the person has a much higher resistance
My kettle
doesn't work!
The metal case is live
(connected to the live wire/at 230V)...
live wire
h
e
Hig istanc
res
neutral wire
...but the current flows down into the ground through the earth wire
negligible resistance
The Fuse
This is a thin piece of wire with a low melting point.
Electric currents produce heat. If the current gets too big, the fuse melts which breaks the circuit.
Live
metal case
Load
(eg a heating element)
Neutral
insulation
The load has a high resistance which opposes the current and brings it down to a safe value
fuse
Live
metal case
Load
(eg a heating element)
Neutral
This circuit now goes:
Live
insulation
The fuse is put in the live wire because the live wire is the dangerous one.
When the fuse "blows" (melts) the live wire is disconnected
a.c. metal case a.c. neutral
The metal case has a much lower resistance than the load, so the current shoots up. This melts the fuse wire and breaks the circuit.
Choosing a fuse
The fuse must be safe at the the operating current of the device it protects, but must "blow" at a lower current than the device it protects.
Eg, a 13A fuse melts at 13A.
It can protect a computer that runs at 10A but blows at 15A.
A 23A fuse would be useless as the computer would "blow" to protect the fuse!
Circuit Breakers
Power spring pushing supply plunger to the right
iron bolt
spr
spring pushing iron bolt upwards
ing
Reset
button
plunger
Load
These are electromagnetic switches that open when the current is too high.
As the current increases the electromagnet increases in strength and pulls the iron bolt down. This allows the plunger to spring to the right which opens the switch.
The switch is reset by pushing the reset button.
Electrical Power
Power is the rate of doing work.
Work is changing one form of energy into another, so
Power = amount of energy transformed
time taken
because it is a rate
Also:
Electrical power (in watts, W) = current x potential difference
(in A)
(in V)
Electrical power (in watts, W) = current x potential difference
(in A)
(in V)
A bulb operates at 230V and draws 0.44A of current.
What is the bulbs power rating?
Power = current x potential difference
= 0.44A x 230V
= 101.2W
P
Vx I
Electrical power (in watts, W) = current x potential difference
(in A)
(in V)
What current would flow through a normally operating 230V, 500W heater?
Power = current x potential difference
current = power
p.d.
= 500W = 2.17A
230V
What value fuse should you choose for this heater?
3A
5A
13A
Calculating Electrical Charge
Potential difference (voltage) causes charge to flow.
This flow of charge is the current.
Charge is measured in units called coulombs (C)
The symbol for charge is Q
(just live with it!)
Actually, come to think of it, the examiners always use words, not symbols
One coulomb is the amount of charge that flows when 1A flows for 1 second.
ie, 1 coulomb = 1A x 1second
Charge = current x time
so charge = current x time
Charge, Q = current, I x time, t
(in coulombs, C) (in A) (in s)
A current of 4A flows for 10s.
How much charge flows?
Q
Ixt
Charge = current x time = 4A x 10s = 40C
A current of 10A flows for one hour.
How much charge flows?
One hour = 60 minutes = 60 x 60s = 3600s
Charge = current x time = 10A x 3600s = 360,000C
Charge, potential difference and energy
Heat is produced in a circuit component when charge flows through it. This is because the flowing electrons hit the atoms of the component and give them kinetic energy/heat.
The formula
Energy = power x time, and power = current x potential difference
So, Electrical energy = current x potential difference x time
(in J)
(in A)
(in V)
(in s)
...or E = VIt if you prefer (I do, the examiners don't)
Electrical energy = current x potential difference x time
(in J)
(in A)
(in V) (in s)
How much energy is transferred when a current of 3A flows through a potential difference of 5V for 20s?
Energy = current x p.d. x time
= 3A x 5V x 20s
= 300J