2- electricity

• mains electricity is what type of supply?

  ac supply
• domestic electricity supply in UK values

  230 V and frequency of 50 Hz
• in direct current supplies

  current is always flowing in the same direction
• what things supply dc current?

  cells and batteries
• in ac supplies

  current is constantly cycling between positive and negative directions (alternating current is supplied by application of alternating voltages)
• 50 Hz means

  50 cycles between positive and negative current per second
• power in terms of p.d and current

  P = IV
• power in terms of current and resistance

  P = (I) squared x r
• amount of energy an appliance transfers depends on

  how long it is switched on for, and power of appliance
• national grid has to

  transmit huge amounts of power
• high current means

  lots of heat, because of resistance of wire

  this means lots of energy could be lost to surrounding when national grid transmits lots of power
• voltage in national grid has to be

  high (as current as to be low to not lose heat energy), increased before sending it across the country
• material of each cable in three-core cables


  copper wire and coloured plastic coating
• live wire function + colour

  brown - provides alternating p.d from power supply (230V)
• neutral wire function + colour

  blue - completes the circuit back to power supply. p.d is 0V
• earth wire function + colour

  green and yellow. safety wire to stop casing of appliance from carrying a current(becoming live). Only carries a current itself when there is a fault in wiring
• p.d between live and earth wire

  230V
• if live wire is touched

  a large potential difference is produced between your body and the wire.

  As a result, current will flow through you

  This will give an electric shock, which can injure and kill
• even when appliance is switched off (open switch in mains circuit), there is still potential difference in the live wire so you can still get an electric shock
• dangers of providing any connection between the live wire and earth

  If the live wire and Earth wire touch, this will create a low resistance path through which the current can flow to the Earth. A very large current may flow through this path and this can cause fires.
• appliances transfer

  electrical energy to useful energy
• a fan transfers electrical energy to kinetic energy, an oven converts electrical energy to thermal energy to heat food, electrical energy is converted to heat energy in a heater, this transfers to heat energy in the air.
• step down transformers

  used to decrease potential difference, and increase current, for domestic use (approx 230 V)
• national grid

  a system of cables and transformers linking power stations to consumers
• how is electrical power transferred to consumers

  from power stations, using the National Grid
• step up transformers

  used to increase the potential difference from the power station to the transmission cables (increases voltage to approx 400,000 volts, decrease current, then transmitted around country between huge pylons)
• • The National Grid system is an efficient way to transfer energy due to

  the use of step up and step down transformers
• • The current generated by power stations is greater than homes require so it must be transmitted through a network of wires that travel across the country
  • When electricity is transmitted over large distances, the resistance in the wires causes heating, which results in wasted energy transfers
  • By increasing the potential difference at which the current is transmitted, the same amount of power can be transferred using a much smaller current (due to P = IV)
  • This results in less heating in the wire and hence less wasted energy
• Explain how the step-up transformer increases the efficiency of the National Grid.

  -The lower the current, the less heating due to resistance there will be in the wires 

  • From the diagram, the step-up transformer increases the voltage and decreases the current in the cables
  • By decreasing the current, the energy dissipated to the thermal store of the surroundings through the power cables is reduced
  • Therefore, there is more energy being transferred to homes which increases the efficiency of the National Grid
• potential difference across primary coil x current in primary coil = potential difference across secondary coil x current in secondary coil
• Fossil fuel and nuclear power stations work by generating thermal energy. This is used to turn water into steam which can then rise and turn turbines.

  The kinetic energy of the moving turbines can be converted to electrical energy, which is finally sent out across the national grid.
• 1. How electrical power gets to homes
  2. Electric power is generated at a power station.
  3. Step  transformers increase the voltage and decrease the current.
  4. The electric power is transmitted via the national grid at low  which minimises power loss. 
  5. Step  transformers decrease the voltage and increase the current near our homes.
  6. Electric power is wired into our homes at a voltage that is relatively safe to use.
• Work is done when charge flows in a circuit
• energy transferred in terms of power and time

  power x time
• energy transferred in terms of charge flow and p.d

  charge flow x p.d
• Energy Transfer in Battery-Powered Domestic Appliances

  Batteries store chemical energy. When an appliance is turned on, the battery transfers that stored chemical energy

  e.g a torch: chemical energy from batteries is transferred into electrical energy for useful light energy and wasted thermal energy
• Describe the relationship between the power rating and the changes in stored energy when a device is used

  A device with a higher power rating will transfer stored energy to other types of energy at a faster rate.
• current

  flow of electrons around a copper wire