Physics: Chapter 17 - Practical Electricity

17.1 Uses of Electricity
Electricity is used for:

1. Heating (Kettle, iron, hotplate)
2. Lighting (Filament lamps, fluorescent lamps)
   
3. Electric Motors (Hair dryer, food mixer)

Filament Lamps
Advantages: Gives a cosy and relaxed atmosphere.
Disadvantages: Only a small % are converted to light energy while the rest are converted to heat energy.

Fluorescent lamps
Advantages: It is energy efficient.
Disadvantages: Costly. Mercury vapour in the lamp is toxic.

17.2 Measuring Electrical Energy
Electric Power P
P= VI
P= I²R
P= V²/R
SI Unit: watt (W)

Electric energy E
E= V²/R (t)
SI Unit: joule (J)

17.3 Dangers of Electricity
Faulty appliances or circuits can cause shock or fire.

1. Damaged insulation (shock)
   
2. Overheating of cables (fire)
   
3. Damp Conditions (shock)

1) Damaged insulation
Insulator can crack and break which exposed the conducting wires inside.
Exposed live wires can cause electric shock.

2) Overheating of cables
The higher resistance of thinner wires will produce more thermal heat that will damage the insulation and may cause a fire.

3) Damp conditions
Water provide a conducting path for a large current to flow. If can therefore shock a person to death.

17.4 Safe Use of Electricity at Home

1. Circuit breakers
2. Fuse
3. Correct placement of switch in the circuit
4. The three-pin plug
5. Earth wire
6. Double insulation of certain appliances

1) Circuit breakers are safety devices that can switch off the electrical supply in a circuit when there is an overflow of current.
a) Miniature circuit breaker MCB
b) Earth leakage circuit breaker ELCB

a) MCB prevents excessive current flow through in the circuit by tripping or breaking it.
It can be reset by switching it on again.

b) ELCB monitors the amount of current flowing from the live wire. Current in neutral wire should be the same as live wire. ELCB detects these small current leakages from the live wire to the earth wire.

2) Fuse
A fuse is a safety device included in an electrical circuit to prevent excessive current flow.
A fuse consists of a short thin piece of wire which becomes hot and melts when the current flowing through it is greater than its rated value.

Fuses should have a current rating just slightly higher than the current an electrical appliance will use under normal conditions.

3) Switches
Switches must be fitted onto the live wire so that switching off disconnects the high voltage from an appliance.

4) Plugs and sockets
Live wire : Brown
Earth wire: Green & Yellow
Neutral wire : Blue

5) Earthing
The earth wire is a low-resistance wire.
It is usually connected to the metal casing of the appliance.

6) Double insulation
Appliance that use a two-pin plug requires double insulation.