Domestic Uses of Electricity

• Direct and Alternating Potential Difference

  . A direct current (d.c.) supply:

  - has a direct potential difference, i.e. one that is always positive or always negative - this makes the current direction constant

  - is the type of current that is supplied by cells and batteries.

  . An alternating current (a.c.) supply:

  - has an alternating potential difference, i.e. one that alternates from positive to negative - this makes the current direction alternate

  - is the type of current used in mains electricity.
• What is the voltage of mains electricity in the UK?

  230V
• How many times does mains electricity change direction in the UK per second?

  50 times
• What is the frequency of mains electricity in the UK?

  50Hz
• How many wires are in a three-core cable used for mains electricity?

  Three wires
• What does the live wire in a three-core cable carry?

  230V potential, alternating between positive and negative
• What is the potential of the neutral wire in a three-core cable?

  At or close to the 0V earth potential
• What is the purpose of the earth wire in a three-core cable?

  It stops the exterior of an appliance becoming live
• What happens during operation regarding the live and neutral wires?

  The potential difference causes current to flow through them
• What does the live wire do in the circuit?

  It carries the alternating potential from the supply
• What is the role of the neutral wire in the circuit?

  It completes the circuit
• When will current flow in the earth wire?

  If there is a fault connecting it to a non-zero potential
• How does the earth wire function as a safety feature?

  It prevents the appliance exterior from becoming live
• What are the three wires in a three-core cable and their functions?

  • Live wire (brown): 230V potential, alternating
  • Neutral wire (blue): Close to 0V earth potential
  • Earth wire (green/yellow): Safety wire at 0V potential
• What can an electric shock from mains supply easily be?

  Fatal
• What happens when touching the live wire?

  A large current flows through the body
• Can the live wire be dangerous if the switch is open?

  Yes, it can still be dangerous
• What happens to a television that is switched off but still plugged in?

  The live wire remains an alternating potential
• What is needed for electricity to flow through a live wire?

  A path for the electricity to flow
• How can a damaged cable affect safety?

  It can expose the live wire
• What occurs if someone touches the exposed live wire?

  They create a potential difference and get shocked
• Key Point

  Any connection between live and earth can be dangerous, because it creates a potential difference, causing a current to flow.
• What is power defined as?

  The rate at which energy is transferred
• How is 1W of power defined in terms of energy transfer?

  1J of energy per second
• How can you demonstrate the meaning of power?

  By comparing energy use over different times
• What does it indicate if one kettle boils water faster than another?

  It has a higher power rating
• What is the formula for efficiency in energy transfer?

  • Efficiency = Useful energy out / Total energy in
• What does an efficiency of 0.5 or 50% mean?

  Half the energy is useful, half is wasted
• What does an efficiency of 0.75 or 75% indicate?

  Three-quarters of the energy is useful
• How can you increase the efficiency of an energy transfer?

  By reducing the amount of wasted energy
• Key Point

  Power is the rate at which energy is transferred.