Magnetism and Electromagnetism

What you'll learn

This guide covers the essential principles of magnetism and electromagnetism tested in Pearson Edexcel International IGCSE Physics examinations. You will explore permanent magnets, magnetic fields, electromagnets, the motor effect, electromagnetic induction and transformers. Understanding these concepts is crucial for both Paper 1 and Paper 2, typically contributing 8-12% of your final marks.

Key terms and definitions

Magnetic field — region around a magnet where a magnetic force acts on another magnet or magnetic material

Magnetic flux density — measure of the strength of a magnetic field, measured in tesla (T)

Electromagnet — temporary magnet created by passing electric current through a coil of wire wrapped around an iron core

Motor effect — force experienced by a current-carrying conductor placed in a magnetic field

Electromagnetic induction — production of a potential difference (voltage) across a conductor when it experiences a changing magnetic field

Transformer — device that uses electromagnetic induction to change the voltage of an alternating current

Solenoid — long coil of wire which produces a uniform magnetic field inside when current flows through it

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

Core concepts

Permanent magnets and magnetic materials

All magnets have two poles: a north pole and a south pole. Like poles repel; unlike poles attract. This is a non-contact force.

Magnetic materials are attracted to magnets. The main magnetic materials are:

• Iron
• Steel
• Cobalt
• Nickel

Hard magnetic materials (like steel) are difficult to magnetise but retain their magnetism permanently. Soft magnetic materials (like iron) are easily magnetised but lose magnetism quickly.

Magnets can be made by:

• Stroking a magnetic material with a permanent magnet in one direction
• Placing the material inside a solenoid and passing direct current through the coil

Magnets can be demagnetised by:

• Hammering them
• Heating them above their Curie temperature
• Placing them inside a solenoid and passing alternating current through the coil

Magnetic fields and field lines

Magnetic field lines show the direction of the force on a north pole. They have specific properties:

• Run from north to south poles outside the magnet
• Never cross each other
• Are closer together where the field is stronger
• Form closed loops

A uniform magnetic field has parallel, equally-spaced field lines. This occurs between opposite poles of two bar magnets placed close together.

You can plot magnetic field patterns using:

• Plotting compass — small compass placed at various points around the magnet
• Iron filings — sprinkled on paper above a magnet, they align with field lines

Electromagnets and the magnetic effect of a current

When current flows through a wire, a circular magnetic field is produced around the wire. The direction can be found using the right-hand grip rule: thumb points in the direction of conventional current, fingers curl in the direction of the magnetic field.

A solenoid is a long coil of wire. When current passes through it:

• The magnetic field inside is strong and uniform
• The field pattern outside resembles that of a bar magnet
• One end becomes a north pole, the other a south pole

The strength of an electromagnet depends on:

• Current — increasing current increases field strength
• Number of turns — more coils produce a stronger field
• Core material — iron core concentrates the magnetic field

Electromagnets have many applications:

• Electric bells
• Circuit breakers
• Relay switches
• Scrapyard cranes
• MRI scanners

The motor effect and electric motors

When a current-carrying conductor is placed in a magnetic field, it experiences a force (the motor effect). This occurs because the magnetic field around the conductor interacts with the external field.

The force is:

• Maximum when the conductor is perpendicular to the field
• Zero when the conductor is parallel to the field
• Reversed if either the current direction or field direction is reversed

Fleming's left-hand rule determines the direction of the force:

• First finger = Field direction (N to S)
• SeCond finger = Current direction (+ to −)
• ThuMb = Motion/force direction

The force on a conductor can be calculated using:

F = BIL

Where:

• F = force (newtons, N)
• B = magnetic flux density (tesla, T)
• I = current (amperes, A)
• L = length of conductor in the field (metres, m)

A simple d.c. motor consists of:

• Rectangular coil of wire (armature)
• Permanent magnet providing the magnetic field
• Split-ring commutator — reverses current direction every half turn
• Carbon brushes maintaining electrical contact with the commutator

The coil rotates because:

1. Current flows through opposite sides of the coil in opposite directions
2. Forces act in opposite directions on each side (Fleming's left-hand rule)
3. This creates a couple (turning effect)
4. The commutator reverses current every half turn, maintaining rotation in the same direction

Motor speed can be increased by:

• Increasing current
• Increasing number of turns on the coil
• Using a stronger magnet
• Adding an iron core to the coil

Electromagnetic induction

Electromagnetic induction is the production of a potential difference across a conductor experiencing a changing magnetic field. This was discovered by Michael Faraday.

Ways to induce a voltage:

• Moving a magnet into or out of a coil
• Moving a coil in a magnetic field
• Changing the current in a nearby coil

The induced voltage is increased by:

• Moving the magnet/coil faster
• Using a stronger magnet
• Increasing the number of turns on the coil
• Using a coil with larger cross-sectional area

The direction of the induced current opposes the change causing it (Lenz's law). This ensures energy conservation.

Fleming's right-hand rule determines the direction of induced current:

• First finger = Field direction
• ThuMb = Motion direction
• SeCond finger = Induced Current direction

Applications of electromagnetic induction:

• Generators (a.c. and d.c.)
• Transformers
• Induction cookers
• Metal detectors

An a.c. generator contains:

• Coil rotating in a magnetic field
• Slip rings and brushes maintaining continuous electrical contact
• As the coil rotates, the induced voltage alternates producing alternating current

A d.c. generator is similar but uses a split-ring commutator instead of slip rings, producing direct current (though pulsating).

Transformers

A transformer changes the voltage of an alternating current. It consists of:

• Primary coil — input coil connected to a.c. supply
• Secondary coil — output coil
• Iron core — links the magnetic field between coils

How transformers work:

1. Alternating current in primary coil creates changing magnetic field
2. Iron core channels this changing field through secondary coil
3. Changing field induces alternating voltage in secondary coil

Step-up transformer: more turns on secondary than primary; increases voltage

Step-down transformer: fewer turns on secondary than primary; decreases voltage

The transformer equation relates voltages and turns:

Vₚ / Vₛ = Nₚ / Nₛ

Where:

• Vₚ = primary voltage (V)
• Vₛ = secondary voltage (V)
• Nₚ = number of turns on primary coil
• Nₛ = number of turns on secondary coil

For an ideal (100% efficient) transformer:

Vₚ Iₚ = Vₛ Iₛ

Where:

• Iₚ = primary current (A)
• Iₛ = secondary current (A)

In reality, transformers have energy losses due to:

• Resistance in coils — causes heating (minimised using thick copper wire)
• Eddy currents in core — circular currents causing heating (minimised using laminated iron core)
• Magnetic field not fully contained — some flux leakage (minimised using complete iron core)

Efficiency is calculated:

Efficiency = (power output / power input) × 100%

Transformers are essential for the National Grid power transmission system:

• Step-up transformers increase voltage to 400,000 V for transmission
• High voltage reduces current for the same power (P = IV)
• Lower current reduces energy loss in cables (P = I²R)
• Step-down transformers reduce voltage to 230 V for homes

Worked examples

Example 1: Motor effect force calculation

Question: A wire of length 8.0 cm carrying a current of 3.5 A is placed perpendicular to a uniform magnetic field of flux density 0.25 T. Calculate the force on the wire. [3 marks]

Solution:

Convert length to metres: L = 8.0 cm = 0.080 m [1 mark]

Use F = BIL [1 mark]

F = 0.25 × 3.5 × 0.080 = 0.070 N [1 mark]

Mark scheme notes: Must show unit conversion, correct formula selection, and final answer to 2 significant figures.

Example 2: Transformer calculation

Question: A transformer is used to reduce the mains voltage of 230 V to 12 V for a laptop charger. The primary coil has 2300 turns.

(a) Calculate the number of turns on the secondary coil. [2 marks]

(b) The laptop charger supplies a current of 4.0 A. Calculate the current in the primary coil, assuming the transformer is 100% efficient. [3 marks]

Solution:

(a) Using Vₚ / Vₛ = Nₚ / Nₛ [1 mark]

230 / 12 = 2300 / Nₛ

Nₛ = (2300 × 12) / 230 = 120 turns [1 mark]

(b) For 100% efficiency: Vₚ Iₚ = Vₛ Iₛ [1 mark]

230 × Iₚ = 12 × 4.0 [1 mark]

Iₚ = (12 × 4.0) / 230 = 0.21 A [1 mark]

Example 3: Electromagnetic induction

Question: Describe and explain how you would increase the size of the voltage induced in a coil when a bar magnet is moved through it. [4 marks]

Solution:

Move the magnet faster [1 mark] — this increases the rate of change of magnetic flux through the coil [1 mark]

Use a stronger magnet [1 mark] — this increases the magnetic flux density [1 mark]

Also acceptable: increase number of turns on coil; use coil with larger cross-sectional area (with appropriate explanation)

Mark scheme notes: Must state the change AND explain why it increases induced voltage. Maximum 4 marks for any two valid methods with explanations.

Common mistakes and how to avoid them

• Confusing magnetic and non-magnetic materials — Remember only iron, steel, cobalt and nickel are magnetic. Aluminium and copper are NOT magnetic despite being metals.

• Drawing magnetic field lines incorrectly — Field lines must never cross and always form closed loops. Outside the magnet they go from N to S; inside from S to N.

• Mixing up Fleming's left-hand and right-hand rules — Left hand for motors (force on current-carrying conductor); right hand for generators (induced current from motion).

• Forgetting unit conversions — Always convert cm to m and mA to A before using equations. Length must be in metres for F = BIL.

• Misunderstanding transformer equations — More turns means higher voltage. If voltage increases, current decreases (for same power). Step-up increases voltage but decreases current.

• Thinking transformers work with d.c. — Transformers only work with alternating current because a changing magnetic field is essential for induction. Direct current produces a constant field.

Exam technique for "Magnetism and Electromagnetism"

• Command words matter: "Describe" requires stating what happens; "Explain" requires reasons using physics principles. For induced voltage questions, "explain how to increase" needs both the method and the physics reason (e.g., "move faster" AND "increases rate of change of flux").

• Drawing field patterns accurately: Use a ruler for uniform fields. Show field direction with arrows. Make spacing consistent for uniform fields, closer together for stronger regions. Label poles clearly.

• Calculations require structure: Write the formula, substitute values with units, show working, give answer to appropriate significant figures (usually 2 or 3). For transformer questions, identify whether it's step-up or step-down first.

• Application questions are common: Be prepared to explain how electromagnets work in devices (relays, circuit breakers, bells) or why the National Grid uses high voltages. Link your answer to the physics principles—don't just describe the device.

Quick revision summary

Magnetic fields surround magnets and current-carrying conductors; field lines show force direction on a north pole. The motor effect produces force on current-carrying conductors in magnetic fields (F = BIL, Fleming's left-hand rule). Electromagnetic induction generates voltage when conductors experience changing magnetic fields (generators, Fleming's right-hand rule). Transformers use mutual induction to change a.c. voltages (Vₚ/Vₛ = Nₚ/Nₛ); step-up transformers enable efficient National Grid transmission by reducing current and minimising power loss in cables.