Electrical Power & Fuses (Edexcel IGCSE Physics)

• Power is defined as

The rate of energy transfer or the amount of energy transferred per second

• The electrical power of a device depends on:
  • The voltage (potential difference) of the device
  • The current of the device

• The power of an electrical component (or appliance) is given by the equation:

• Where:
  • P = power, measured in Watts (W)
  • I = current, measured in amperes (A)
  • V = potential difference, measured in volts (V)

• The unit of power is the Watt (W), which is the same as a joule per second (J/s)

A formula triangle can help rearrange the electrical power equation

Power, current, voltage formula triangle

• For more information on how to use a formula triangle refer to the revision note on Speed

• A fuse is a safety device designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)

Fuse circuit symbol

The circuit symbol for a fuse - take care not to confuse this with a resistor

• Fuses usually consist of a glass cylinder containing a thin metal wire
• If the current in the wire becomes too large:
  • The wire heats up and melts
  • This causes the wire to break, breaking the circuit and stopping the current

• This makes sure that more current doesn't keep flowing through the circuit and causing more damage to the equipment, or, causing a fire

Fuse sizes

• Fuses come in a variety of sizes, typically 3 A, 5 A and 13 A
  • In order to select the right fuse for the job, the current through an appliance needs to be known

• If the electrical power of the appliance is known (along with mains voltage), the current can be calculated using the equation:

• Where:
  • I = current in amperes (A)
  • P = power in watts (W)
  • V = voltage in volts (V)

• The fuse should always have a current rating that is slightly higher than the current needed by the appliance
  • Because of this, the rule of thumb is to always choose the next size up

• If the fuse current rating is too low, it will break the circuit even when an acceptable current is flowing through
• If the fuse current rating is too high, it will not break the circuit in enough time before damage occurs