Practical Electricity

Uses of electricity

1. Electrical appliance use heating effect of electrical current

   • made of nichrome

     • high resistivity

     • high melting point

     • does not oxidise easily at high temperature

2. Filament lamp convert electrical energy to light energy and heat

   • made of tungsten

     • high resistivity

     • high melting point

Damaged insulation

• Exposed live wire can cause electric shock when touched

• damaged insulation between live and neutral wires may result in a short circuit leading to a fire

Overheating of cables

Can cause fire when

• electrical cable passes current that exceeds its limits

• a large current flows through the wires due to too many plugs being connected to a socket

• contact between plug and socket is poor

• inappropriate wire used; thinner wire higher resistance, generates more heat. Appliances that require high power need thicker wire

Damp environments

Water from taps and other sources contain charged ions and impurities that make it a good conductor of electricity

Water in contact with exposed electrical wires provides a conducting path for electric current and can lead to electric shock

Circuit breaker

electromagnetic switch that can automatically switch off electrical supply in a circuit when excessive current flow through them

Advantage: can be manually reset without having to replace wires

Circuit breaker trips when:

• circuit overload

• short circuit occurred somewhere in system

Fuse

safety device to prevent excessive current flow

needs to be replace after it blows

Fuse will melt and opens the circuit when current exceeds its rated value

Earth metal casing

if live wire touches metal casing of appliance (fault), large current will flow to the earth through the earth wire and fuse in live wire will blow or the circuit breaker will trip

user avoids electric shock

Double insulation

for 2 pin plug

enclosed in a plastic casing which is not earthed

three pin plug

1. Live wire (brown)

   • 240V

   • Switches, circuit breakers, fuses must be connected to live wire

2. Neutral wire (blue)

   • 0V

   • Completes circuit; provides returning path to supply

3. Earth wire (green and yellow)

   • Connected to metal casing

   • no current

Why safety features connected to live wire

1. When switch turned off of fuse is blown, appliance is disconnected from high voltage of live wire

2. If connected to neutral wire, appliance still connected to high voltage even when safety feature is off.

   If user touches exposed live sections, a large current flows through the user to the earth and he experiences an electric shock