4PH1

Electrical Power & Mains Electricity

Electricity · 2 question types

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 10% of your exam marks.

stable
Medium
Stable10%

P = IV and energy calculations, plus mains electricity safety, appear in most series.

Energy from power and time

E = P × t

  • where:
    • E = energy transferred (J)
    • P = power of the device (W)
    • t = time the device runs for (s)
  • Substituting P = I × V gives a second form that uses circuit quantities directly:

E = I × V × t

  • The two equations are equivalent, so pick whichever set of quantities the question hands you

Energy and work

  • The work done by the charge flowing through a component is exactly the energy transferred to that component
  • In a complete circuit, the total energy delivered by the battery equals the total energy transferred to every component on the loop, which is conservation of energy
Worked example

Calculating energy transferred (E = P × t)

A lamp has a power of 60 W and is left on for 5 minutes. Calculate the energy transferred.

Solution:

  • Convert time to seconds: 5 min = 300 s
  • Write the equation: E = P × t
  • Substitute: E = 60 × 300
  • E = 18 000 J