Electrical Power & Fuses (Edexcel IGCSE Physics)

Revision Note

Katie M

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Katie M

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Electrical power

  • 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:

P space equals space I V

  • Where:

    • = power, measured in Watts (W)

    • = current, measured in amperes (A)

    • = 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

PIV triangle (2), IGCSE & GCSE Physics revision notes

Power, current, voltage formula triangle

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

Worked Example

Calculate the potential difference through a 48 W electric motor with a current of 4 A.

 

Answer:

Step 1: List the known quantities

  • Power, P = 48 W

  • Current, I = 4 A

Step 2: Write down the relevant equation

P space equals space I V

Step 3: Rearrange for potential difference, V

V space equals P over I space

Step 4: Substitute the values

V space equals space 48 over 4

V space equals space 12 space V

Examiner Tips and Tricks

Remember: Power is just energy per second. Think of it this way will help you to remember the relationship between power and energy. You can remember the unit by the phrase: “Watt is the unit of power?”

Selecting fuses

  • 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

Fuse, IGCSE & GCSE Physics revision notes

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:

I space equals space P over V

  • 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

Worked Example

If an appliance uses a current of 3.1 A, what would be a suitable rating for a fuse?

 

Answer:

Step 1: Consider a 3 A fuse

  • A 3 A fuse would be too small

    • The fuse would blow as soon as the appliance was switched on

Step 2: Consider a 5 A fuse

  • A 5 A fuse would be an appropriate choice

    • It is the next size up from the current required

Step 3: Consider a 13 A fuse

  • A 13 A fuse would be too large

    • It would allow an extra 10 amperes to pass through the appliance before it finally blew

Examiner Tips and Tricks

Remember there are two steps involved in selecting a correctly sized fuse for an appliance:

1. Calculating the current required using the electrical power equation

2. Selecting the next size up fuse

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.