Edexcel IGCSE Physics 复习笔记 2.3.1 Electrical Power & Fuses

 

• Power is defined as

 

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

 

• The 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:

 

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

 

 

 

• This equation can be rearranged with the help of a formula triangle:

Power, current, voltage formula triangle

 

Worked Example

 

Step 1: List the known quantities

• • Power, P = 48 W
  • Current, I = 4 A

   

 

Step 2: Write down the relevant equation

 

P = IV

 

Step 3: Rearrange for potential difference, V

Step 4: Substitute in the values

 

 

 

Exam Tip

 

• 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)

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

• Fuses usually consist of a glass cylinder which contains 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

 

 

 

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

 

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

   

• The fuse should always have a current rating that is higher than the current needed by the appliance, without being too high
  • Because of this, the rule of thumb is to always choose the next size up

   

• If the fuse current rating is 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 be breaking the circuit in enough time before damage occurs

 

Worked Example