(SME 2.3) Electrical Power and Mains Electricity

Electric Power

Rate of energy transfer or amount of energy transferred per second

• depends on:

  • voltage (potential difference)

  • current

Watt = Joule per second [1W=1J/s]

Power Equation

P = IV

• P: power in watts (W)

• I: current in amps (A)

• V: potential difference in volts (V)

Fuse

Safety device designed to cut off electricity flow to an appliance if the current becomes too large (because of fault or surge)

• glass cylinder containing a thin metal wire

Fuse Working:

• current becomes too large

  • wire heats up and melts

  • wire breaks, breaks circuit, stops current

• ensure current doesn’t continue flowing through the circuit

• prevents damage to equipment and fire

Fuse Sizes

• 3A, 5A and 13A

• find current of appliance with I = P/V

• fuse should have current rating of the next size up

  • too low: circuit will break even with an acceptable current

  • too high: circuit will not break in enough time before damage occurs

Work

Done when charge flows through a circuit

Equal to energy transferred

(Electricity) Energy Transferred

• depends upon:

  • current

  • potential difference

when charge flows through a resistor, energy transferred makes the resistor hot

Energy Equation

E = P x t

• E: energy in joules (J)

• P: power in watts (W)

• t: time in seconds (s)

OR

E = I x V x t

• I: current in amps (A)

• V: potential difference in volts (V)

Electrical Safety Symbol

Common electrical safety hazards

• damaged insulation: exposed piece of wire → lethal shock

• overheating of cables: too much current through too small wire → wire overheating and causing fire or melting insulation

• damp conditions: moisture comes into contact → short circuit (fire) or electrocution risk

Domestic appliances safety features

• double insulation

• earthing

• fuses

• circuit breakers

Insulation

• wire conducting part is usually made of copper or some other material

• person comes into contact → electrocution risk

• electrical safety wires are covered with an insulating material like rubber

Double Insulation

• appliances without metal casing to prevent risk of getting electrified

• these are called double insulated:

  • insulation around wires themselves

  • non metallic casing: second layer of insulation

• don’t need earth wire or designed so earth wire can’t touch metal casing

Earthing

• many appliances have metal cases

• potential electrical safety hazard:

  • live wire comes into contact with case → case becomes electrified, electrocutes person touching it

• earth wire reduces this risk

Earth Wire Working

• live wire comes into contact with case

• earth wire provides low resistance path to the earth

• causes surge of current in earth wire and also in live wire

• high current through fuse causes it to melt and break

• electricity supply to appliance gets cut off

• appliance is safe

Circuit Breaker

automatic electromagnet switch that breaks the circuit if current exceeds a certain value

Circuit Breaker vs Fuse

• Advantage:

  • doesn’t melt or break

  • can be reset and used again

  • works much faster

used in mains electricity in homes as the most important electrical safety device

Electric Heating

• temperature of a resistor increases due to collisions of the free electrons within the wire

• energy is dissipated into surroundings by heating

Electric Heating: domestic contexts

• electric heaters

• electric ovens

• electric hob

• toasters

• kettles

Direct Current

d.c : a steady current constantly flowing in the same direction in a circuit from positive to negative

Alternating Current

ac: a current that continuously changes its direction, going back and forth around a circuit

Direct Current Working

• voltage in a dc circuit travels in 1 direction only

  • from positive to negative

• dc power supply has a fixed positive terminal and a fixed negative terminal

• example of d.c. current: electric cells and batteries

Alternating Current Working

• ac power supplies have 2 identical terminals that change from positive to negative and back again

  • ac always travels from positive to negative

  • current changes direction as terminals’ polarity changes

Alternating Current Frequency

number of times the current changes direction back and forth each second

• ex: UK mains frequency is 50 Hz and potential difference ≈230 V