The motor effect

    AQA
    GCSE

    The motor effect describes the force experienced by a current-carrying conductor when placed perpendicular to an external magnetic field. This phenomenon arises from the interaction between the magnetic field generated by the current and the permanent magnetic field, resulting in a resultant force. The direction of this force is determined using Fleming's Left-Hand Rule, relating the orthogonal vectors of motion, field, and current. The magnitude of the force is calculated using the equation F = BIl, where B is magnetic flux density, I is current, and l is the length of the conductor within the field.

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    Objectives
    3
    Exam Tips
    3
    Pitfalls
    5
    Key Terms
    5
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for stating that the force is caused by the interaction of the permanent magnetic field and the magnetic field around the current-carrying wire
    • Credit responses that correctly identify the direction of force using Fleming's Left-Hand Rule: First finger = Field (N to S), Second finger = Current (+ to -)
    • For calculation questions, award 1 mark for correct substitution into F = B I L and 1 mark for the correct numerical answer
    • Award 1 mark for stating that the force is zero if the current is parallel to the magnetic field lines
    • Credit references to the split-ring commutator reversing the current every half-turn to maintain rotation in the same direction

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for stating that the force is caused by the interaction of the permanent magnetic field and the magnetic field around the current-carrying wire
    • Credit responses that correctly identify the direction of force using Fleming's Left-Hand Rule: First finger = Field (N to S), Second finger = Current (+ to -)
    • For calculation questions, award 1 mark for correct substitution into F = B I L and 1 mark for the correct numerical answer
    • Award 1 mark for stating that the force is zero if the current is parallel to the magnetic field lines
    • Credit references to the split-ring commutator reversing the current every half-turn to maintain rotation in the same direction

    Examiner Tips

    Expert advice for maximising your marks

    • 💡Annotate diagrams with 'N', 'S', '+' and '-' before applying Fleming's Left-Hand Rule to avoid orientation errors
    • 💡Memorise that F = B I L only applies when the wire is at 90 degrees to the magnetic field; if parallel, the force is zero
    • 💡When explaining the DC motor, explicitly link the split-ring commutator to the reversal of current every half-turn to ensure continuous rotation

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Confusing the direction of current (conventional current flows positive to negative) with electron flow, leading to incorrect force direction
    • Failing to convert the length of the wire from centimetres or millimetres into metres before calculating force
    • Using the Right-Hand Rule (generator effect) instead of the Left-Hand Rule (motor effect) to determine motion

    Key Terminology

    Essential terms to know

    Likely Command Words

    How questions on this topic are typically asked

    State
    Calculate
    Explain
    Determine
    Suggest

    Practical Links

    Related required practicals

    • •{"code":"Investigation of Motor Effect","title":"Measuring the force on a current-carrying wire in a magnetic field","relevance":"Demonstrates relationship between current magnitude and force strength"}

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