Magnetism and the Motor Effect

What you'll learn

This topic examines how magnetic fields interact with electric currents to produce forces and motion. You'll study permanent magnets, electromagnets, and how conductors carrying current experience forces in magnetic fields—the principle behind electric motors. These concepts appear regularly in Paper 2 and typically account for 8-12 marks across multiple question types.

Key terms and definitions

Magnetic field — the region around a magnet where a magnetic force acts on magnetic materials or other magnets. Represented by field lines that flow from north to south poles.

Magnetic flux density — the strength of a magnetic field, measured in tesla (T). A stronger field has more closely packed field lines.

Electromagnet — a coil of wire that produces a magnetic field when an electric current flows through it. The field disappears when the current stops.

Motor effect — when a current-carrying conductor is placed in a magnetic field, it experiences a force. This force can cause movement and is the basis of electric motors.

Fleming's left-hand rule — a method to determine the direction of force on a current-carrying conductor in a magnetic field, using the thumb (force/motion), first finger (field) and second finger (current).

Solenoid — a long coil of wire that produces a uniform magnetic field inside when current flows through it. The field pattern resembles that of a bar magnet.

Induced magnetism — the process by which magnetic materials become temporarily magnetised when placed in a magnetic field.

Uniform magnetic field — a field where the magnetic flux density is the same at all points, shown by equally spaced parallel field lines.

Core concepts

Magnetic fields and field lines

Magnetic fields surround all magnets and current-carrying conductors. Field lines provide a visual representation:

• Field lines always point from the north pole to the south pole outside the magnet
• The closer together the lines, the stronger the magnetic field
• Field lines never cross each other
• At the poles, fields are strongest (lines most concentrated)

For a bar magnet, field lines curve from north to south externally. When two magnets interact:

• Like poles repel — field lines push away from each other
• Unlike poles attract — field lines connect between the poles

Magnetic materials (iron, steel, nickel, cobalt) experience forces in magnetic fields. Soft magnetic materials like iron lose their magnetism quickly; hard magnetic materials like steel retain magnetism and make good permanent magnets.

Plotting magnetic fields

You can map magnetic field patterns using:

Iron filings method:

• Sprinkle iron filings around a magnet on paper
• Tap gently—filings align along field lines
• Shows overall field pattern clearly

Plotting compass method:

• Place a plotting compass near the magnet
• Mark the direction the compass needle points
• Move compass to a new position and repeat
• Join the marks to create field lines
• More accurate for direction than iron filings

Electromagnets and solenoids

When electric current flows through a wire, a magnetic field forms around it. The field pattern is concentric circles centred on the wire. The right-hand thumb rule determines direction: thumb points in current direction, fingers curl in field direction.

A solenoid amplifies this effect by coiling the wire. Inside a solenoid carrying current:

• The magnetic field is strong and uniform
• Field lines run parallel from one end to the other
• One end acts as a north pole, the other as south
• Outside the solenoid, the field pattern resembles a bar magnet

Factors affecting electromagnet strength:

1. Current magnitude — increasing current strengthens the field proportionally
2. Number of turns — more coils create a stronger field
3. Core material — adding an iron core dramatically increases field strength because iron is easily magnetised
4. Coil spacing — tighter coils produce stronger fields

Electromagnets have crucial advantages over permanent magnets:

• Can be switched on and off
• Strength can be varied by changing current
• Magnetic poles can be reversed by reversing current direction

Common applications:

• Electric bells and buzzers
• Scrapyard cranes for lifting magnetic materials
• Circuit breakers and relays
• MRI scanners in hospitals
• Magnetic door locks

The motor effect

When a conductor carrying electric current is placed in a magnetic field perpendicular to the current, it experiences a force. This is the motor effect—the fundamental principle of electric motors.

The magnitude of the force depends on three factors:

1. Magnetic flux density (B) — measured in tesla (T)
2. Current (I) — measured in amperes (A)
3. Length of conductor in the field (L) — measured in metres (m)

The relationship is given by:

F = B × I × L

Where F is force in newtons (N).

This equation applies when the conductor is perpendicular to the magnetic field. If the conductor is parallel to the field, no force acts on it. At other angles, the force is reduced.

Fleming's left-hand rule

To determine the direction of the force, use Fleming's left-hand rule:

1. First finger — points in direction of the magnetic Field (north to south)
2. SeCond finger — points in direction of the Current (positive to negative)
3. ThuMb — points in direction of Motion/force on the conductor

Hold your left hand with these three fingers mutually perpendicular. Align the first finger with field direction and second finger with current direction—your thumb shows the force direction.

Key examination points:

• Always use the LEFT hand (right hand is for generators, not motors)
• Ensure fingers are at right angles to each other
• Current direction is conventional current (positive to negative), not electron flow
• If current or field direction reverses, force direction reverses
• If both reverse, force direction stays the same

Simple electric motors

A direct current (DC) motor converts electrical energy into kinetic energy using the motor effect. Essential components:

Basic structure:

• Rectangular coil of wire (the armature)
• Permanent magnets or electromagnets providing the field
• Split-ring commutator—a metal ring split into two halves
• Carbon brushes maintaining electrical contact with the commutator
• DC power supply

How it works:

1. Current flows through the coil via brushes and commutator
2. The motor effect produces forces on both sides of the coil
3. Using Fleming's left-hand rule, forces act in opposite directions on opposite sides
4. This creates a turning effect (moment) around the central axis
5. The coil rotates through 180°
6. The split-ring commutator reverses current direction every half turn
7. This reverses the forces, maintaining rotation in the same direction
8. Continuous rotation results

Increasing motor speed:

• Increase the current (larger force)
• Increase magnetic field strength (use stronger magnets)
• Increase the number of turns on the coil
• Add an iron core to the coil
• Decrease friction at the axle

The split-ring commutator is essential—without it, the coil would oscillate back and forth rather than rotate continuously.

Loudspeakers and the motor effect

Loudspeakers use the motor effect to convert electrical signals into sound waves:

• An alternating current (AC) passes through a coil attached to a paper cone
• The coil sits in a permanent magnetic field
• As current direction alternates, the force direction alternates
• The coil and cone vibrate back and forth
• These vibrations create sound waves in the air
• Signal frequency determines sound pitch; signal amplitude determines volume

Worked examples

Example 1: Calculating force on a conductor

Question: A straight wire of length 0.15 m carries a current of 4.0 A perpendicular to a uniform magnetic field of flux density 0.80 T. Calculate the force acting on the wire. (3 marks)

Solution:

Write down the formula: F = B × I × L (1 mark)

Substitute values: F = 0.80 × 4.0 × 0.15 (1 mark)

Calculate: F = 0.48 N (1 mark)

Exam tip: Always write the formula first, then substitute with units, then calculate. This ensures method marks even if the final answer is incorrect.

Example 2: Applying Fleming's left-hand rule

Question: A horizontal wire carries a current from west to east. It is placed in a uniform magnetic field directed vertically downwards. Use Fleming's left-hand rule to determine the direction of the force on the wire. (2 marks)

Solution:

Apply Fleming's left-hand rule:

• First finger (field): points downwards (1 mark)
• Second finger (current): points from west to east
• Thumb (force): points towards the south (1 mark)

Alternative acceptable answer: The force acts horizontally towards the south / perpendicular to both current and field direction.

Example 3: Electromagnet strength

Question: A student investigates electromagnets by wrapping wire around an iron nail and connecting it to a power supply. The electromagnet picks up 8 paper clips when the current is 0.5 A.

(a) Suggest two ways the student could increase the number of paper clips picked up. (2 marks)

(b) Explain why adding an iron core increases the strength of the electromagnet. (2 marks)

Solution:

(a) Any two from:

• Increase the current through the coil (1 mark)
• Increase the number of turns of wire around the nail (1 mark)
• Use a power supply with higher voltage (1 mark)
• Wind the coils closer together (1 mark)

(Accept any two valid methods)

(b) Iron is a magnetic material / soft magnetic material (1 mark) It becomes magnetised when the electromagnet is switched on / the magnetic field lines become more concentrated through the iron, making the overall field stronger (1 mark)

Common mistakes and how to avoid them

Mistake: Using the right hand for Fleming's left-hand rule. Correction: Always use the LEFT hand for motors and the motor effect. The right-hand rule applies to generators and electromagnetic induction, which is different content. Associate "left" with "motor" to remember.

Mistake: Confusing magnetic field direction with current direction when applying Fleming's left-hand rule. Correction: The field direction is always from north to south pole of the magnet providing the field. Current direction is from positive to negative terminal (conventional current). Keep first finger for field and second finger for current.

Mistake: Stating that magnetic field lines go from south to north. Correction: Magnetic field lines always flow from north poles to south poles outside the magnet. Inside a magnet, they continue from south to north to form closed loops, but exam questions focus on external fields.

Mistake: Forgetting units in motor effect calculations or using incorrect units. Correction: F = BIL requires B in tesla (T), I in amperes (A), and L in metres (m) to give F in newtons (N). Convert all measurements before calculating (e.g., cm to m, mA to A).

Mistake: Thinking the split-ring commutator provides power to the motor. Correction: The commutator's role is to reverse the current direction every half rotation, ensuring continuous rotation in one direction. The brushes provide the electrical connection; the commutator switches the direction.

Mistake: Believing that no force acts on a conductor when it's at an angle to the field. Correction: Maximum force occurs when the conductor is perpendicular (90°) to the field. At other angles, force is reduced but still present. Only when the conductor is parallel (0°) to the field is force zero. The formula F = BIL applies only at 90°.

Exam technique for Magnetism and the Motor Effect

Command word recognition: "Describe" questions require you to state features or characteristics (e.g., describing how field lines show field strength). "Explain" questions need reasons and mechanisms (e.g., why electromagnets can be switched on/off). "Calculate" questions require formula, substitution, and answer with unit—typically worth 3 marks.

Diagram expectations: When asked to draw or complete magnetic field patterns, use ruled lines with arrows showing direction from north to south. Make lines evenly spaced for uniform fields, closer together for stronger regions. Never let field lines cross. Plotting compass diagrams should show needle positions at multiple points.

Fleming's left-hand rule questions: Write out which finger represents which quantity before giving your answer. State all three directions clearly (field, current, force). Drawing a quick diagram of the situation helps—mark the directions on it. Remember that reversing either current OR field reverses force, but reversing both keeps force in the same direction.

Calculation structure: For F = BIL questions, write the formula first (1 mark), substitute values with units (1 mark), then give the answer with correct unit (1 mark). Show all working even if using a calculator. If values are given in wrong units (mA, cm, mT), convert them first and show this working—examiners commonly test unit conversion.

Quick revision summary

Magnetic fields surround magnets and current-carrying conductors, shown by field lines running north to south. Electromagnets are solenoids producing controllable magnetic fields, strengthened by increasing current, adding turns, or using iron cores. The motor effect occurs when a conductor carrying current perpendicular to a magnetic field experiences a force given by F = BIL. Fleming's left-hand rule determines force direction: first finger for field, second for current, thumb for motion. DC motors use the split-ring commutator to reverse current every half turn, maintaining continuous rotation. These principles underpin motors, loudspeakers, and electromagnetic devices.