Magnetism and Electromagnetism Exam Practice

This lesson brings together all the key concepts from Topic 8: Magnetism and Electromagnetism in the Edexcel GCSE Physics specification (1PH0) and focuses on exam technique, key equations, common question types, and multi-step worked problems. Use this as your final revision before the exam.

---

Key Equations to Learn

Equation	Meaning	Units
F = BIl	Force on a current-carrying conductor in a magnetic field (Higher)	F in N, B in T, I in A, l in m
Vp / Vs = Np / Ns	Transformer equation — relates voltage and turns	V in V, N = number of turns (no unit)
VpIp = VsIs	Power conservation in an ideal transformer (Higher)	V in V, I in A
P = I²R	Power dissipated (used for National Grid energy loss calculations)	P in W, I in A, R in Ω

Exam Tip: Write all the equations you need on a prompt card and practise using them until substituting values and rearranging become second nature. In the exam, always write the equation first, then substitute values, then calculate. Show every step — even if you get the final answer wrong, you can still earn marks for method.

---

Common Exam Question Types

Type 1: Fleming's Left-Hand Rule

What the question asks: Given the direction of current and magnetic field, determine the direction of force (or vice versa).

How to answer:

1. Identify the field direction (N to S of the magnets).
2. Identify the current direction (conventional: positive to negative).
3. Use Fleming's left-hand rule: First finger = Field, seCond finger = Current, thuMb = Motion.
4. State the direction of the force/motion.

Worked Example:

A wire carries a current from left to right between the poles of a magnet (N pole on top, S pole on bottom — field downward).

• First finger: points downward (field direction, N to S).
• Second finger: points to the right (current direction).
• Thumb: points out of the page (towards you).

The force on the wire is out of the page.

graph TD
    A["Fleming’s Left-Hand Rule"] --> B["First finger = FIELD<br/>(N → S direction)"]
    A --> C["seCond finger = CURRENT<br/>(conventional + → −)"]
    A --> D["thuMb = MOTION / FORCE<br/>(direction wire moves)"]
    B --> E["All three at<br/>RIGHT ANGLES (90°)<br/>to each other"]
    C --> E
    D --> E
    E --> F["Use LEFT hand<br/>for the motor effect"]

    style A fill:#2c3e50,color:#fff
    style B fill:#c0392b,color:#fff
    style C fill:#2980b9,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#8e44ad,color:#fff
    style F fill:#e67e22,color:#fff

Exam Tip: Always state which finger represents which quantity. Do not just say "I used Fleming's left-hand rule and the answer is upward." The examiner wants to see: "First finger points in the direction of the field (downward), second finger points in the direction of current (right), therefore the thumb points in the direction of force (out of the page)."

---

Type 2: Transformer Calculations

What the question asks: Calculate the unknown voltage, number of turns, or current in a transformer.

How to answer:

1. Write the transformer equation: Vp / Vs = Np / Ns
2. Substitute the known values.
3. Rearrange and solve.
4. If asked about current (Higher): use VpIp = VsIs.

Worked Example 1:

A transformer has 500 turns on the primary coil and 50 turns on the secondary coil. The input voltage is 230 V. Calculate the output voltage.

Vp / Vs = Np / Ns

230 / Vs = 500 / 50

230 / Vs = 10

Vs = 230 / 10

Vs = 23 V (step-down transformer)

Worked Example 2 (Higher):

A 100% efficient transformer has a primary voltage of 230 V and a secondary voltage of 23 V. The secondary current is 5.0 A. Calculate the primary current.

VpIp = VsIs

230 × Ip = 23 × 5.0

230 × Ip = 115

Ip = 115 / 230

Ip = 0.50 A

---

Type 3: Explaining How Motors and Generators Work

What the question asks: Describe how a DC motor (or AC generator) works. This is typically a 6-mark extended writing question.

How to structure your answer (DC Motor):

1. Current flows through the rectangular coil (via carbon brushes and split-ring commutator).
2. The coil is in a magnetic field (provided by permanent magnets).
3. Each side of the coil carries current in a different direction.
4. By the motor effect / Fleming's left-hand rule, one side experiences a force upward and the other downward.
5. This creates a turning effect (moment) — the coil rotates.
6. The split-ring commutator reverses the current direction every half turn, ensuring the coil continues to rotate in the same direction.

How to structure your answer (AC Generator):

1. A coil rotates in a magnetic field (driven by an external force, e.g. a turbine).
2. As the coil rotates, each side cuts through magnetic field lines.
3. By electromagnetic induction, a voltage is induced across the coil.
4. The coil sides alternately move up and down through the field, so the induced voltage alternates (changes direction).
5. Slip rings maintain a continuous connection to the external circuit.
6. The output is alternating current (AC) — the voltage follows a sinusoidal pattern.

Exam Tip: For 6-mark questions, use a logical sequence. Start with the energy source (current for a motor, mechanical rotation for a generator), explain the physics principle (motor effect or electromagnetic induction), describe the role of each component, and finish with the output. Use key physics terminology throughout.

---

Type 4: Comparing Permanent Magnets and Electromagnets

What the question asks: Explain why an electromagnet is better than a permanent magnet for a given application (or vice versa).

Template answer:

"An electromagnet is better for [application] because it can be switched on and off using an electric current. This means [specific benefit, e.g. the crane can release the metal when the current is turned off]. A permanent magnet cannot be switched off, so [specific drawback, e.g. the metal could never be released]. Additionally, the strength of an electromagnet can be varied by changing the current, allowing [specific benefit]."

---

Type 5: National Grid and Transformers

What the question asks: Explain why transformers are used in the National Grid.

Model answer:

1. Electricity is transmitted over long distances through cables that have resistance.
2. Energy is lost as heat in the cables. The power lost is given by P = I²R.
3. To reduce energy losses, the current must be reduced (since power loss depends on I²).
4. A step-up transformer at the power station increases the voltage to 400,000 V.
5. Since P = IV, for the same power transmitted, a higher voltage means a lower current.
6. Lower current means much less energy wasted as heat (P = I²R).
7. Near homes, a step-down transformer reduces the voltage to 230 V for safe domestic use.

---

Common Mistakes and How to Avoid Them

Mistake 1: Confusing the Motor Effect and Electromagnetic Induction

Motor Effect	Electromagnetic Induction
Current in a field → force → movement	Movement in a field → voltage → current
Uses Fleming's LEFT-hand rule	Uses Lenz's law
Found in: motors, loudspeakers	Found in: generators, microphones, dynamos

How to remember: Motor = Movement from electricity. Generator = Gets electricity from movement.

Mistake 2: Getting Fleming's Left-Hand Rule Wrong

• Always use the LEFT hand (not the right hand).
• Always use conventional current (positive to negative). If electron flow is given, reverse it.
• Keep all three fingers at right angles — seCond finger is current, not the first.
• First = Field, seCond = Current, thuMb = Motion.

Mistake 3: Forgetting That Transformers Only Work with AC

• Transformers require a changing magnetic field.
• DC produces a constant field → no change → no induction in the secondary coil.
• If asked "Why do transformers not work with DC?" — explain that a constant current produces a constant magnetic field, and electromagnetic induction requires a changing field.

Mistake 4: Incorrect Transformer Calculations

Common errors:

• Mixing up primary and secondary values.
• Forgetting to check whether the answer makes sense (step-up should give a higher voltage; step-down should give a lower voltage).
• Not showing working — always write the equation, substitute, rearrange, calculate.

Mistake 5: Confusing Slip Rings and Split-Ring Commutators