Magnetism

Learning goals

What students should be able to explain, calculate and investigate after completing Magnetism.

• Draw magnetic field patterns around a bar magnet, a straight current-carrying wire, and a solenoid.
• Use the right-hand grip rule to find the direction of a magnetic field due to a current.
• Explain the motor effect and use F = BIℓ to calculate the force on a current-carrying conductor.
• Use Fleming's left-hand rule to find the direction of the force in the motor effect.
• Describe the operation of a simple DC motor, including the role of the split-ring commutator.

Key definitions

Precise terms, symbols and SI units used in Magnetism.

Magnetic Fields Around Magnets, Wires and Solenoids

A bar magnet's field lines emerge from its north pole, curve around, and enter its south pole (outside the magnet).

A straight current-carrying wire produces a magnetic field of concentric circles around the wire; the right-hand grip rule (thumb pointing in the direction of conventional current, fingers curling in the direction of the field) gives the field's direction. A solenoid (coil) produces a field very similar in shape to a bar magnet, with clearly defined north and south ends, and adding a soft iron core inside the coil greatly strengthens the field, since the core itself becomes magnetised and adds to the solenoid's own field.

Secondary-source investigation: choose one permanent-magnet and one electromagnet application. Explain why that type is suitable, identify the energy changes or control involved, and evaluate one benefit and one limitation or safety consideration.

The Motor Effect

When a current-carrying conductor is placed in an external magnetic field, it experiences a force — the motor effect.

The magnetic field around the current-carrying conductor combines with the external magnetic field. On one side the fields reinforce, while on the other they oppose. This unequal field pattern produces a force perpendicular to both the current and the external field. In a motor coil, the forces on opposite sides form a couple and therefore produce a torque (turning effect); the immediate effect is not simply “more speed”. Fleming’s left-hand rule gives the force direction.

The Simple DC Motor

A simple DC motor uses forces on opposite sides of a current-carrying coil in a magnetic field to produce a turning effect.

Opposite sides of the coil carry current in opposite directions, so the magnetic forces produce a turning effect on the coil. After each half-turn, the split-ring commutator reverses the current in the coil; this keeps the torque in the same rotational direction. Brushes maintain sliding electrical contact with the commutator.

Diagrams and formulae

Key diagrams and equations for reviewing Magnetism.

• Field around a bar magnet: lines emerge from north, curve around, and enter south (outside the magnet).
• Field around a straight current-carrying wire: concentric circles, direction given by the right-hand grip rule.
• Field around a solenoid: similar in shape to a bar magnet, strengthened further by a soft iron core.
• The motor effect (force perpendicular to both field and current) and a simple DC motor with a split-ring commutator.

• Key relationships
• Motor effect: F = BIℓ (l perpendicular to B) Force on a moving charge (recap from the Electron and Photoelectric Effect topic): F = qvB (v perpendicular to B)

Worked Example

For a straight current-carrying conductor in a magnetic field, use F = BIℓ sin θ and determine the force direction separately.

Question: A 0.30 m wire carries 4.0 A perpendicular to a 0.50 T magnetic field. Find the magnetic force on the wire.

• Known: B = 0.50 T, I = 4.0 A, ℓ = 0.30 m and θ = 90°.
• Equation: F = BIℓ sin θ. Since the wire is perpendicular to the field, sin 90° = 1.
• Substitution: F = (0.50)(4.0)(0.30).
• Answer: F = 0.60 N.
• Direction: use Fleming’s left-hand rule; the force is perpendicular to both the current and the magnetic field.

Practical / Investigation

Experimental planning, measurement and evaluation for Magnetism.

Experiment: Mapping magnetic field patterns

• Apparatus: bar magnet (or a current-carrying wire/solenoid), plotting compass (or iron filings), paper.
• Method: place the magnet (or current-carrying conductor) under a sheet of paper; use a plotting compass at several points to trace the field line direction, or sprinkle iron filings to reveal the overall field pattern.
• Compare the resulting pattern for a bar magnet, a straight wire, and a solenoid (with and without a soft iron core), noting the difference in shape and relative strength.
• Sources of error: the Earth's own magnetic field can slightly distort compass readings if not accounted for; iron filings clump unevenly, giving an approximate rather than perfectly smooth pattern.
• Improvement: orient the apparatus to minimise interference from the Earth's field, or use a Hall probe for a more quantitative field strength measurement rather than just a qualitative pattern.

• Place a light strip of conducting foil at right angles to a magnetic field and pass a safe low-voltage current through it. The foil moves because a force acts on a current-carrying conductor in a magnetic field.
• Reverse the current while keeping the magnetic field fixed: the motion reverses. Reverse the field instead: the motion also reverses. This tests the direction rule rather than merely showing movement.
• Keep the current within the foil’s rating and switch off between observations to avoid heating.

• Use a plotting compass to mark the direction at each point, move it step-by-step, and add arrowheads from N to S outside a bar magnet.
• Iron filings show pattern and relative crowding but not direction. Keep magnetic materials and other magnets away from the setup.
• This is a mapping demonstration, not an independent/dependent-variable investigation unless field strength is measured with a Hall probe.

Quick-Check Questions

Short retrieval questions covering the essential ideas in Magnetism.

1. Describe the shape of the magnetic field around a straight current-carrying wire.

2. What effect does adding a soft iron core have on a solenoid's magnetic field?

3. State the formula for the force on a current-carrying conductor in a magnetic field.

4. Which rule gives the direction of the force in the motor effect?

5. What is the function of the split-ring commutator in a DC motor?

Common mistakes and exam tips

Common misconceptions and exam-technique checks for Magnetism.

• Avoid: Using F = BIℓ without checking the angle. Correct idea: The simple form applies when the conductor is perpendicular to the field; the specification relationship F = BIℓ applies when the conductor is perpendicular to the field.
• Avoid: Using Fleming’s left-hand rule to find the field around a wire. Correct idea: Use the right-hand grip rule for the field around a current; use Fleming’s left-hand rule for motor-force direction.
• Avoid: Omitting the split-ring commutator from the explanation of continuous motor rotation. Correct idea: The commutator reverses current every half-turn so the turning effect continues in the same rotational sense.

• State which direction rule you are applying.
• Check that the conductor is perpendicular to B before using F = BIℓ.
• In a motor explanation, state the function of the split-ring commutator.

Exam-style questions

Five exam-style questions with worked answers on Magnetism.

Further required content

Additional required content for all students. This is not a separate Higher Level requirement.

• These forms apply when the conductor or charge moves perpendicular to the magnetic field.
• Use Fleming's left-hand rule for conventional current; reverse the predicted direction for an electron.

A perpendicular magnetic force is always at right angles to the velocity, so it changes direction rather than speed and can act as the centripetal force.

End-of-unit revision

Formulae, definitions, experiments and mixed revision questions for Magnetism.

• F = BIℓ when the conductor is perpendicular to the magnetic field
• F = qvB (force on a moving charge, v perpendicular to B; recap from the Electron and Photoelectric Effect topic)

• Magnetic field
• Solenoid
• Motor effect
• Magnetic flux density
• Split-ring commutator

• Field around a bar magnet, a straight wire, and a solenoid
• Motor effect force direction (Fleming's left-hand rule)
• Simple DC motor with split-ring commutator

• Mapping magnetic field patterns around magnets, wires and solenoids

• Confusing wire field (circles) with solenoid field (bar-magnet shape)
• Forgetting F=BIl needs perpendicular geometry for full force
• Mixing up the right-hand grip rule with Fleming's left-hand rule
• Thinking a DC motor works without a commutator

1. A current-carrying wire experiences a maximum force of 0.24 N in a field of flux density 0.6 T, over a length of 0.2 m. Calculate the current.

2. Explain why the force on a current-carrying conductor is zero when the conductor is parallel to the magnetic field.

3. Describe one everyday application of an electromagnet (temporary magnet).

4. Explain why reversing the current direction in a DC motor's coil at the right moment is essential for continuous rotation.

1. F = BIℓ → I = F/(Bl) = 0.24 ÷ (0.6 × 0.2) = 2 A.

2. The motor effect force depends on the component of current perpendicular to the field; when the conductor is parallel to the field, there is no perpendicular component of current relative to the field, so no force is produced.

3. Any reasonable example, e.g. an electromagnetic crane for lifting scrap metal (the field can be switched on to attract and off to release), or a relay switch, or an electric door lock — all rely on a solenoid's field being switched on/off with the current.

4. Without reversing the current at the right moment, the turning effect on the coil would reverse direction as it passes the vertical, causing it to oscillate rather than keep turning in the same direction; the split-ring commutator reverses the current at exactly this point, keeping the torque acting consistently to maintain continuous rotation.

The table below shows which equations from this unit appear in the official State Examinations Commission Formulae and Tables book, and where.

Magnetic-field patterns, magnetic-force relationships, Fleming's left-hand rule and the DC motor are for all students.