Transformers (Physics only)

    AQA
    GCSE

    Transformers utilize electromagnetic induction to modify the potential difference of an alternating current supply without electrical contact between circuits. They consist of primary and secondary coils wound around a soft iron core, which facilitates the transmission of a changing magnetic field to induce a potential difference in the secondary coil. The relationship between potential difference and the number of turns is governed by the transformer equation, while the principle of conservation of energy dictates that for an ideal transformer, input power equals output power. This mechanism is fundamental to the National Grid, allowing high-voltage transmission to minimize thermal energy dissipation in cables.

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    Objectives
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    Exam Tips
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    Pitfalls
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    Key Terms
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    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that an alternating current in the primary coil produces a changing magnetic field
    • Award 1 mark for specifying that the iron core is easily magnetised and transmits the magnetic field to the secondary coil
    • Credit responses that explicitly state a potential difference is induced across the ends of the secondary coil due to the changing magnetic field
    • For calculation questions, award 1 mark for correct substitution into Vp/Vs = Np/Ns prior to any rearrangement
    • Award 1 mark for linking high voltage transmission to reduced current and consequently reduced thermal energy loss (I²R) in cables

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that an alternating current in the primary coil produces a changing magnetic field
    • Award 1 mark for specifying that the iron core is easily magnetised and transmits the magnetic field to the secondary coil
    • Credit responses that explicitly state a potential difference is induced across the ends of the secondary coil due to the changing magnetic field
    • For calculation questions, award 1 mark for correct substitution into Vp/Vs = Np/Ns prior to any rearrangement
    • Award 1 mark for linking high voltage transmission to reduced current and consequently reduced thermal energy loss (I²R) in cables

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When explaining the mechanism, the phrase 'changing magnetic field' is non-negotiable; omitting 'changing' often caps marks at zero for the explanation
    • 💡Memorise that power loss is proportional to the square of the current; explicitly write P = I²R when explaining why high voltage is used for transmission
    • 💡Check your calculated values against the transformer type: if it is a step-up transformer, Vs must be larger than Vp
    • 💡For Higher Tier, be prepared to combine the power equation (P=VI) with the transformer efficiency equation in multi-step calculations

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Stating that the iron core conducts electricity from the primary to the secondary coil rather than conducting magnetic flux
    • Failing to specify that the magnetic field must be 'changing' or 'alternating'; static magnetic fields do not induce potential difference
    • Confusing the roles of step-up and step-down transformers within the National Grid context
    • In power calculations, neglecting to assume 100% efficiency before equating input power (VpIp) and output power (VsIs)

    Key Terminology

    Essential terms to know

    Likely Command Words

    How questions on this topic are typically asked

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