Magnetism and Electromagnetism - Marking Scheme

This is the marking scheme for quiz: P7 - Magnetism and Electromagnetism Quiz
Topic guide: P7 - Magnetism and Electromagnetism

Multiple Choice uestions (3 × 3 marks = 9 marks)

  1. Forces and poles repel

    • Correct answer: A (Both magnets have the same pole facing each other)
    • Reason: Like poles (N-N or S-S) repel. Opposite poles (N-S) attract.

  2. Transformer turns ratio

    • Correct answer: A (Ns > Np)
    • Reason: Step-up transformer: V_s > V_p when N_s > N_p.

  3. Electromagnet with switch

    • Correct answer: A (Magnetic field disappears immediately)
    • Reason: Electromagnets require continuous current. Removing current removes field instantly.

  4. Step-up Transformer Voltage

    • Formula: V_s = V_p × (N_s/N_p)
    • Substitution: V_s = 20 × (2500/500)
    • Calculation: V_s = 20 × 5 = 100 V
    • Marks: 2 for correct formula, 2 for correct answer with units
    • Acceptable: 100 V

  5. Step-down Transformer Voltage

    • Formula: V_s = V_p × (N_s/N_p)
    • Substitution: V_s = 230 × (250/500) = 230 × 0.5
    • Calculation: V_s = 115 V
    • Marks: 2 for correct formula, 2 for correct answer with units
    • Acceptable: 115 V

  6. Fleming’s Left-Hand Rule

    • Mark 3: State each finger assignment correctly: Thumb = force, 1st finger = field, 2nd finger = current.
    • Mark 2: State that reversing any two reverses the third.
    • Acceptable: Clear description of all three finger roles.

  7. Transformer with d.c.

    • Mark 2: Transformers need changing magnetic field for induction.
    • Mark 2: d.c. produces constant field → no change in flux → no e.m.f. induced.
    • Mark 1: Mention that induction requires changing flux.
    • Acceptable: Constant field → no induction → no voltage.

  8. Solenoid Strength

    • Mark 3: List 3 methods: (1) increase current, (2) increase turns, (3) add iron core.
    • Mark 2: Show understanding of field concentration and amplification.
    • Acceptable: Any 3 valid methods from guide.

  9. High Voltage Transmission

    • Mark 2: High voltage reduces current for same power (P = VI).
    • Mark 2: Calculate loss reduction: Original I²R, New (I/10)²R = I²R/100 → 100× reduction.
    • Mark 1: Mention P = VI relationship.
    • Acceptable: Show 10² = 100× reduction.

  10. Slip Rings vs Commutator

    • Mark 2: Slip rings maintain contact to same side → a.c. output.
    • Mark 2: Comutator reverses current → d.c. continuous rotation.
    • Acceptable: Clear contrast between a.c. and d.c. needs.