Magnetism and Electromagnetism

This guide covers Topic 6, Magnetism and electromagnetism, in full — sub-topics (a) Units, (b) Magnetism, (c) Electromagnetism and (d) Electromagnetic induction — from the Pearson Edexcel International GCSE in Physics (4PH1), Issue 4 specification. Statements marked “P” are Physics-only content.

Before studying this

This resource assumes current and circuits from Electricity.

Syllabus coverage

PEARSON EDEXCEL INTERNATIONAL GCSE PHYSICS (4PH1) — Topic 6

(a) Units — using ampere (A), volt (V) and watt (W).

(b) Magnetism — knowing magnets repel and attract other magnets and attract magnetic substances; describing magnetically hard and soft materials; understanding magnetic field lines; knowing magnetism can be induced; investigating magnetic field patterns practically; describing producing a uniform field with two permanent magnets.

(c) Electromagnetism — knowing a current-carrying conductor produces a magnetic field around it; [P] describing electromagnet construction; [P] drawing field patterns for a straight wire, flat coil and solenoid; [P] knowing a force acts on a moving charged particle in a magnetic field unless motion is parallel to the field; understanding the force on a current-carrying wire in a field and its use in motors and loudspeakers; using the left-hand rule; describing how force varies with field and current magnitude and direction.

(d) Electromagnetic induction — knowing a voltage is induced in a conductor/coil moving through, or experiencing a changing, magnetic field, and the factors affecting its size; describing electricity generation by rotating a magnet in a coil or vice versa; [P] describing transformer structure and voltage change via turns ratio; [P] explaining step-up/down transformer use in transmission; [P] using the turns ratio relationship; [P] using input power = output power for 100% efficiency.

Magnetism

Magnets attract or repel other magnets, and always attract magnetic substances (e.g. iron, nickel). Magnetically hard materials retain their magnetism once magnetised (used for permanent magnets); magnetically soft materials lose their magnetism easily (useful in electromagnets that need to switch on and off). Magnetic field lines show the direction of the field, from north to south pole outside a magnet. Magnetism can be induced in some materials placed in a magnetic field. Two permanent magnets, correctly arranged, can produce a region of uniform field between their opposite poles.

Electromagnetism

Any current-carrying conductor produces a magnetic field around it. An electromagnet uses this to create a controllable magnetic field (switchable via current), typically wound as a coil around a soft-iron core. Field patterns differ by conductor shape: a straight wire gives concentric circular field lines; a flat coil gives a field resembling a short bar magnet; a solenoid gives a strong, mostly uniform field inside it.

A charged particle moving through a magnetic field experiences a force, unless it moves parallel to the field. This underlies the motor effect: a current-carrying wire in a magnetic field experiences a force, predictable using the left-hand rule (relating field, current and force directions), used in electric motors and loudspeakers. This force increases with both the field strength and the current, and reverses direction if either the field or current direction reverses.

Electromagnetic induction

Electromagnetic induction occurs when a conductor or coil moves through a magnetic field, or experiences a changing magnetic field, inducing a voltage. The size of the induced voltage depends on factors such as the speed of relative motion, the strength of the field, and the number of turns of a coil. This is the principle behind generating electricity: rotating a magnet inside a coil (or a coil within a magnetic field) induces a voltage — the basis of generators.

Transformers (Physics only)

A transformer changes the size of an alternating voltage using two coils with different numbers of turns wound around a shared core. The ratio of voltages equals the ratio of turns:

input (primary) voltage / output (secondary) voltage = primary turns / secondary turns

For an ideal (100% efficient) transformer, input power equals output power:

Vₚ × Iₚ = Vₛ × Iₛ

Step-up transformers increase voltage (and correspondingly decrease current) for efficient long-distance electricity transmission, reducing energy losses in transmission cables; step-down transformers reduce voltage back to safer, usable levels for consumers.

Worked example. A transformer has 100 primary turns and 500 secondary turns, with a 12 V input. The output voltage:

Vₚ/Vₛ = Nₚ/Nₛ
12/Vₛ = 100/500
Vₛ = 60 V

Common mistakes

• Forgetting that a magnetic field alone does not force a charged particle unless it has a velocity component not parallel to the field.
• Mixing up field patterns for straight wires, flat coils and solenoids — each has a distinct shape.
• Using the turns-ratio equation with power instead of voltage — the turns ratio relates voltages (and turns), not power directly; power relates via VₚIₚ = VₛIₛ separately.
• Assuming real transformers are always 100% efficient — the input power = output power relationship is an idealisation; real transformers have some energy loss.

Quick revision checklist

• Magnetically hard vs soft materials; magnetic field lines and patterns
• The motor effect and the left-hand rule
• Field patterns for straight wires, flat coils and solenoids (Physics only)
• Electromagnetic induction and generator principle
• Turns ratio and Vₚ Iₚ = Vₛ Iₛ for transformers (Physics only)

Related resources

• Solids, Liquids and Gases — the previous topic
• Radioactivity and Particles — the next topic
• Pearson Edexcel International GCSE Physics hub

Written against the Pearson Edexcel International GCSE in Physics (4PH1) specification, Issue 4. Always check the current specification for your examination year.