P5 Electricity in the home

Direct current (d.c) - current flows in one direction only. Supplied by cells or batteries.

Alternating current (a.c) - current repeatedly reverses its direction. Used in mains electricity. 

Mains circuit has live wire - alternately positive and negative every cycle. 

Mains circuit has neutral wire - at zero volts

The National Grid supplies electricity - it is a nationwide network of cables and transformers.

Power station ---> Step-up transformer ---> National Grid ---> Step-down transformer ---> Consumer (e.g. homes)

Step-up transformer increases voltage (very high amounts) because then less current is required to transfer same amount of power. Power loss due to resistance heating in cables is reduced.

Step-down transformer brings voltage down to 230V before supplying to homes/offices etc.

UK's mains electricity supply has three wires - live, neutral and earth wire and each wire has its own coloured insulation. 

Live wire - connected directly to the generators of the electricity supply company.

Neutral wire - returns electricity to the generator after it has been used by the appliance and 0V. Completes circuit

Earth wire - carries no electricity. Safety precaution. Connected to the longest pin in plug and is used to earth the metal case of an appliance. 

Three wires have plastic outer coating (insulation) and held securely by flex grip. 

Wire made from copper (good conductor)

Pins made from brass (good conductor, doesn't oxidise) holds copper wire.

Live wire connected to the live pin of plug by fuse. If too much current passes through the wire in the fuse, it melts and cuts wire off.

Mains Electricity - Safety

• Insulation - putting a material that doesn't conduct electricity around a live conductor. Insulator covering wire is made of plastic (good insulator, flexible and cheap)

• Double Insulation - some appliances have double insulation so only need a live and neutral wire (two-core cable).  Double insulated appliance = inside is contained in plastic. Means that even if the appliance is faulty, no live conductor can touch the outer casing because of the insulating plastic. Used in TVs, computers, printers and hairdryers.

• Earthing - Appliance with metal casing is connected to earth through earth wire. Stops metal case becoming live if live wire breaks and touches case. Live case = electrocution - earthing stops this

• Fuse - made from a thin piece of wire that is designed to melt (and break the circuit) when a current passes through it that is higher than the fuse's rating. Fuse protects rest of circuit from damage. 

Power = energy supplied to a device per second.

• Energy (J) = power (W) x Time (secs) 
• Power (W) = current (A) x voltage (V) 
• Resistance (R) = Voltage (V) / Current (A) 

• Fuses need to be just above the current amount (Amps) - sold in 3A, 5A or 13A. 
• Correct rating for fuse = electrical power (W) / voltage (V) 

• Power = current² (A) x resistance (R) 
• Power = voltage² (V) / resistance (R)

Charge (C) = current (A) x time (secs) 

Resistor in circuit 

Battery does work to push electrons (current) through resistor. 

Electrons collide with vibrating metal ions of resistor, so kinetic energy is transferred to the ions. 

Kinetic energy of ions makes them vibrate more and make resistor hotter. 

When charge flows through a resistor, energy is transferred to the resistor, so the resistor becomes hotter. 

Energy transfer to a component = voltage (V) x charge (C)

• Energy (J) = power (W) x time (secs)

• Power = current (A) x potential difference (V)

• Efficiency = useful energy / total energy supplied x 100%

• Output power = efficiency x input power.