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    Transformer Power Calculator (VpIp = VsIs)

    GCSE PhysicsAQAEdexcelOCRHigher tier, GCSE Physics only

    Find the current or potential difference on either coil of a 100% efficient transformer using Vp × Ip = Vs × Is.

    Quick answer

    For a 100% efficient transformer, power in = power out: VpIp = VsIs. For example, 230 V × 0.5 A = 115 W in, so at 11.5 V the output current is 115 ÷ 11.5 = 10 A.

    By Engr. Mehmood Kakar · Updated

    Vp × Ip = Vs × Is
    Find:

    Simulation

    Same power in, same power out

    For a 100% efficient transformer, power in = power out. Step the voltage up and the current goes down by the same factor.

    230 V
    10 A
    1150 V

    This is why the National Grid uses step-up transformers: a high voltage means a low current in the cables, so less energy is wasted heating them.

    Put these numbers in the calculator above

    How to use VpIp = VsIs

    If a transformer is 100% efficient, the electrical power going in equals the power coming out. Since P = VI:

    Vp × Ip = Vs × Is

    • V is potential difference in volts (V)
    • I is current in amperes (A)
    • p means the primary (input) coil; s means the secondary (output) coil

    A transformer takes 0.5 A from 230 V mains (115 W) and gives an output of 11.5 V. The output current is 115 ÷ 11.5 = 10 A. Stepping the p.d. down steps the current up by the same factor.

    This is why the National Grid steps up the p.d. for transmission: a higher p.d. means a smaller current for the same power, so much less energy is wasted heating the cables.

    Common mistakes that cost marks

    1. Pairing the wrong values. Multiply primary p.d. by primary current, and secondary by secondary.
    2. Not converting kV or mA. 25 kV is 25,000 V; 400 mA is 0.4 A.
    3. Expecting current to rise with p.d. When the p.d. is stepped up, the current is stepped down.

    Practice questions

    Try each one on paper first, then reveal the worked answer.

    Q1A transformer draws 0.50 A from a 230 V supply. Its output p.d. is 11.5 V. Assuming it is 100% efficient, calculate the output current.[3 marks]
    1. Is = Vp × Ip ÷ Vs
    2. Is = 230 × 0.50 ÷ 11.5 = 115 ÷ 11.5
    3. Is = 10 A
    Q2A power station transformer takes 400 A at 25 kV and steps it up to 400 kV. Calculate the output current.[3 marks]
    1. Convert: 25 kV = 25,000 V; 400 kV = 400,000 V
    2. Is = 25,000 × 400 ÷ 400,000
    3. Is = 25 A
    Q3A phone charger gives 2.0 A at 12 V from a 230 V supply. Assuming 100% efficiency, calculate the current drawn from the mains.[3 marks]
    1. Ip = Vs × Is ÷ Vp
    2. Ip = 12 × 2.0 ÷ 230 = 24 ÷ 230
    3. Ip = 0.104 A

    Endless practice

    A new question every time, marked instantly. Work it out on paper, then type your answer.

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    Frequently asked questions

    What does VpIp = VsIs mean?

    For a 100% efficient transformer, the power in the primary coil (Vp × Ip) equals the power in the secondary coil (Vs × Is).

    Why does the current go down when the p.d. goes up?

    Power stays the same, and power = p.d. × current. If the p.d. goes up by a factor, the current must go down by the same factor.

    Are real transformers 100% efficient?

    No, but they are very efficient, so GCSE questions usually assume 100% efficiency.