Energy and Mains

• What factors affect the energy transferred when charge flows through a component?

  Amount of charge, the potential difference across the component
• Energy =

  Voltage x current x time
• Define potential difference in terms of charge

  The work done per unit charge
• energy transferred =

  Charge (C) x voltage
• When an electrical current flows through a resistor why does it heat up?

  There are collisions between the electrons and the ions in the resistor’s lattice. This causes a transfer of kinetic energy into thermal energy, which is released into the surroundings.
• How do low resistance wires reduce unwanted energy transfers?

  A smaller resistance will mean there are fewer collisions, therefore less energy will be wasted through heating
• Advantages of the heating effect

  It is useful for appliances such as toasters or electrical fires, where heat is the desired product.
• Disadvantages of the heating effect.

  The loss of energy as heat energy can make an appliance inefficient, if an appliance overheats it can catch fire or overheat, which could ruin the device or injure the user.
• Power is…

  Energy transferred per second
• Define power

  The rate of energy transfer, or the rate at which work is done
• Power =

  current x voltage
• Power =

  current squared x resistance
• Power =

  Energy / time
• Power =

  Voltage squared x resistance
• What is meant by the frequency of a supply?

  The rate at which an AC current changes.
• UK mains supply

  AC current, 50 Hz frequency, 230 V
• What is the power rating of an appliance?

  It shows how much energy a device converts per second.
• What colour is the live wire?

  Brown
• What colour is the neutral wire?

  Blue
• What colour is the earth wire?

  Green and yellow striped
• When does the Earth wire carry current?

  Under circumstances, no current flows through the Earth wire. However, when a fault occurs - e.g. a surge or the casing becoming live - current flows through.
• Potential difference between the live an earth wires

  230 Volts
• What is the neutral wire for?

  To complete the circuit by connecting the appliance back to the mains supply.
• For metal appliances, where is the earth wire connected to?

  To the metal casing, so that if a live wire touches the casing, the current will flow through the Earth wire, preventing electrocution
• Why is it important to connect a fuse wire or switch to the live wire?

  When the switch is turned off, or when the fuse breaks, it will break the circuit and stop current running through the wire. This provides a method to stop current flowing into the appliance.
• Why is it dangerous to have a connection between the earth wire and the live wire?

  It can result in a current surge, causing electrical shocks