practical electricity

uses of electricity

appliances that have heating element eg nichrome

• high resistivity

• high melting point

• does not oxidise easily at high temp appliances that have light element eg tungsten

• high resistivity

• high melting point

power

rate of energy consumption over time unit: Watt P=I/V P=I^2R (useful in series) P=V^2/R (useful in parallel)

electrical consumption

cost of electricity unit= kWh

hazard of electricity (damaged insulation)

• prolonged use/rats

• exposed live wire

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

hazard of electricity (overheating of cables)

causes fire when

• electrical cable passes current that exceeds its limits

• large current flows due to too many plugs in a socket (overloading)

• contact between plug and socket is poor

• inappropriate wires are used

hazard of electricity (damp environment)

water contains charged ions that can make it a good conductor of electricity

• water offers a conducting path for electricity

• dry skin has high resistivity, wet skin reduces it drastically

safety features

circuit breakers fuses switches earth metal casing double insulation three pin plug

circuit breaker

switch that automatically interrupts or shuts off an electric circuit at the first indication of overload can manually reset without having to replace anything c.b trips when

1. circuit is overloaded

2. short circuit

ELCB (earth leakage circuit breaker)

• found as the main switch

• breaks circuits easily and quickly

fuses

prevent excessive current flow needs to be replaced fuse will melt and opens when current exceeds its rated value typical fuse rating: 1A, 2A, 3A, 5A, 10A, 13A

switch

complete or break an electrical circuit

earth metal casing

metal housing that encloses the electrical part of an appliance if live wire touches metal casing, the current will flow through the earth wire and blow the fuse or trip the c.b avoid user being shocked

double insulation

uses for 2 pins only have live and neutral wire 2 layers of insulation to enclose all exposed metal work and "live parts"

three pin plug

a live pin (brown)

• switches, c.b, and fuses are connected to this wire a neutral pin (blue)

• no voltage

• completes the circuit by returning to the supply earth pin (green-yellow)

• low resistance

• connected to metal part of appliances

• no current