The document discusses mains electricity, which provides alternating current to homes and workplaces. It describes electrical safety hazards like overloaded sockets, frayed cables, and metals in appliances. Fuses and circuit breakers are explained as protecting circuits from overcurrent. The document also covers double insulation, calculating electrical power and energy, and extended power calculations.

1. presents
a production
Everything Electric
Episode 3
Mainly Mains

2. Mainly Mains
The Mains is the electric power
supply to a house or workplace.
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It is alternating current (AC) which means the electricity
repeatedly changes direction in the circuit (50 times per
second). Batteries provide direct current (DC); always in
the same direction.

3. The hazards of mains electricity
Overloaded socket Fire Shock
Images: Health and Safety signs
Image: Citizencorps.gov
Water near sockets
Images: OCAL @ clker.com
Image: Childsafetyaustralia
Frayed cables Metals in appliances Image: Safework SA

4. Electrical safety
Image: aka @ wikipedia
Double
insulation
Fuse
Image: Jared C Bennett @ wikipedia
Earth
wire
Circuit
breaker
Image: wn.com Image: Aussiedingo @ flickr.com

5. Fuses and circuit breakers
When the current is too high,
they melt or break the circuit.
Image: wn.com
Q: A heater has a maximum
current of 10 A; what is the
appropriate circuit breaker or
fuse for safety?
Choose from 8, 16, 20 or 32 A
HINT: the ‘rating’ must be
greater than the max.
8 A - too low current, but not by too much.
16 A - this one
20 A, 32 A - too high; maybe good for an electric oven.

6. Earth wire
Normally there is no
current in the earth wire.
A fault can cause the
metal case to be live.
The earth conducts the
current away from the case.
The current in the earth
wire is often high enough
to melt the fuse.
Images: OUPChina

7. Double insulation
Image: Jared C Bennett @ wikipedia
Many appliances have an insulating cover (eg plastic);
this is called double insulation since the wires inside
are also insulated. So a fault could not give the user a
shock from the case. No earth wire is necessary.

8. Electrical heating
Components transfer some or all of the electrical energy
to heat energy which then warms the surroundings.
Energy transfer
by radiation
Image: opencage.info
Image: Fireflyco.com
Energy transfer Image: Pokayoke @ wikispaces
by conduction Energy transfer
(and convection by convection
in the water)

9. Calculating electrical power
Power is the rate of transfer of energy to other forms
P
I xV
Power, P is measured in watts (W)
Voltage,V is measured in volts (V)
Current, I is measured in amps (A)
To calculate power in watts, multiply volts times amps.
Look at the label on a computer or kettle etc.

10. Calculating electrical energy
Energy, E is measured in joules (J)
Voltage,V is measured in volts (V)
Current, I is measured in amps (A)
Time, t is measured in seconds (s)
Power, P is measured in watts (W)
Reminder: Power = Energy/ Time
P E E
I xV Pxt
VxIxt

11. Power calculations
A movie projector converts 4.32 MJ showing
a 3 hour movie. What is its power?
A kettle rated at 1.5 kW boils water for 2
minutes. How much energy is converted?
What current passes through a 12 V bulb
which is rated at 3.0 W?
How much energy is converted in 30 min by
a laptop whose supply voltage is 12 V and
which takes a current of 1.0A?
ANSWERS ON NEXT BUT ONE PAGE

12. Extended Power calculations
More than IGCSE demands: just for fun
A van de Graaf generator develops a PD of 200 kV. It
sparks with a current of 2.0 mA in 2.0 ms.
A: How much energy is converted in joules?
B: How much electric charge is discharged in coulombs?
C: The charge on one electron is 1.6 x 10-19 C. How
many electrons are discharged?
D: One electron-volt (eV) is a unit of energy. An electron
accelerated through 200 kV converts 200 keV. The Large
Hadron Collider gives protons energies of 4 TeV (2012).
How many times more energetic is this than the vdGG?
ANSWERS ON NEXT PAGE

13. Extended Power calculations
400 W
180 kJ
0.25 A
21.6 kJ 0.8 J
4.0 µC
2.5 x 10 13
2 x 10 7

14. a production
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