Understanding Magnetism and electromagnetism: Edexcel IGCSE Physics Explained

Magnetism and electromagnetism link two of the most fundamental forces in physics. Mastering this section means understanding how magnets work, how electricity creates magnetic fields, and how those fields generate electricity in return.

The magnetism and electromagnetism edexcel igcse section covers four topics: units, magnetism, electromagnetism, and electromagnetic induction. The content builds logically: permanent magnets first, then the magnetic effects of electric currents, then motors and generators, and finally transformers. Each layer depends on the one before it, so working through them in order is important.

These edexcel igcse physics revision notes give you every concept, equation and worked example you need for the exam. The edexcel igcse physics magnetism and electromagnetism content is tested regularly, with transformer and motor questions being particularly common. Approach this section systematically and the marks will follow.

Units for magnetism and electromagnetism

Magnetism

Magnetic materials and poles

Magnets attract magnetic materials (iron, steel, nickel, cobalt). Every magnet has a north pole and a south pole. Like poles repel; unlike poles attract. A freely suspended magnet aligns with the Earth's magnetic field, which is why a compass needle points north.

Hard and soft magnetic materials

• Magnetically hard materials (e.g. steel) are difficult to magnetise but retain their magnetism once magnetised. They are used for permanent magnets.
• Magnetically soft materials (e.g. iron) are easy to magnetise but lose their magnetism quickly. They are used for temporary magnets and electromagnet cores.

Magnetic field lines

A magnetic field is the region around a magnet where a magnetic material experiences a force. Field lines go from north to south outside the magnet. They never cross. The closer the field lines are together, the stronger the field. Between two unlike poles, the field lines run from one pole to the other, creating a fairly uniform field in the central region. Between two like poles, the field lines curve away from each other, showing repulsion.

Induced magnetism occurs when a magnetic material is placed in a magnetic field. The material becomes a temporary magnet and is attracted to the permanent magnet.

Uniform magnetic field

Two permanent magnets placed with unlike poles facing each other and a small gap between them produce a uniform field in the gap. The field lines are parallel and equally spaced. This arrangement is used in practical applications such as electric motors.

Electromagnetism

Magnetic field around a current-carrying conductor

An electric current in a conductor produces a magnetic field around it. The shape of the field depends on the shape of the conductor:

• Straight wire: Concentric circles centred on the wire. The direction of the field is found using the right-hand grip rule: grip the wire with your right hand so your thumb points in the direction of current flow; your fingers curl in the direction of the field lines.
• Flat circular coil: Similar to a bar magnet's field pattern. The field lines come out of one face and go into the other.
• Solenoid: The field inside is strong and nearly uniform (parallel lines). Outside, it looks like a bar magnet's field. One end acts as a north pole, the other as south. Increasing the current or the number of turns increases the field strength. Adding an iron core strengthens it further, creating an electromagnet.

Force on a current-carrying conductor

When a current-carrying wire is placed in a magnetic field (and the current is not parallel to the field), the wire experiences a force. This is the motor effect. The direction of the force is found using Fleming's left-hand rule:

• First finger: direction of the magnetic Field (north to south)
• Second finger: direction of the Current (conventional current, positive to negative)
• Thumb: direction of the Thrust (force/motion)

Hold your left hand with these three fingers at right angles to each other. The force increases if the current increases or the magnetic field strength increases. Reversing either the current or the field direction reverses the force.

The d.c. motor

A simple d.c. motor has a coil of wire between the poles of a magnet. Current flows through the coil, and the motor effect produces a force on each side of the coil in opposite directions (one up, one down), causing the coil to rotate. A split-ring commutator reverses the current direction every half turn so that the coil keeps rotating in the same direction.

Loudspeakers

A loudspeaker uses the motor effect. A coil of wire attached to a paper cone sits in the field of a permanent magnet. An alternating current in the coil makes it move back and forth, vibrating the cone and producing sound waves.

Force on a charged particle

A charged particle moving through a magnetic field experiences a force, as long as its motion is not parallel to the field. This is the same principle as the motor effect but applied to individual charges rather than a current in a wire.

Electromagnetic induction

Inducing a voltage

A voltage is induced in a conductor when it moves through a magnetic field, or when the magnetic field through a coil changes. This is electromagnetic induction. The induced voltage (and therefore current, if the circuit is complete) can be increased by:

• Moving the conductor faster
• Using a stronger magnet
• Increasing the number of turns on the coil

Generators

A generator works by rotating a coil of wire inside a magnetic field (or rotating a magnet inside a coil). The changing magnetic field through the coil induces a voltage. A simple a.c. generator produces alternating current because the direction of the induced voltage reverses every half turn.

Transformers

A transformer consists of two coils of wire (primary and secondary) wound around an iron core. An alternating voltage in the primary coil creates a changing magnetic field in the core, which induces a voltage in the secondary coil.

The transformer equation relates the voltages to the number of turns:

V_p / V_s = N_p / N_s

where V_p and V_s are the primary and secondary voltages, and N_p and N_s are the number of turns on the primary and secondary coils.

• Step-up transformer: More turns on the secondary than the primary. Increases voltage, decreases current.
• Step-down transformer: Fewer turns on the secondary than the primary. Decreases voltage, increases current.

For a 100% efficient transformer: V_p x I_p = V_s x I_s (input power = output power).

Power transmission

Electricity is transmitted across the national grid at high voltage and low current. This reduces energy losses in the cables (since power lost = I² x R, and lower current means less power wasted as heat). Step-up transformers at the power station increase the voltage for transmission; step-down transformers near homes reduce it to a safe level for domestic use.

Common mistakes in magnetism and electromagnetism

1. Confusing Fleming's left-hand rule and the right-hand grip rule. The left-hand rule is for the motor effect (force on a current in a field). The right-hand grip rule is for finding the field direction around a current-carrying wire. Using the wrong hand or the wrong rule gives the wrong direction.
2. Forgetting that transformers only work with a.c. A transformer requires a changing magnetic field to induce a voltage in the secondary coil. Direct current produces a constant field, so no voltage is induced.
3. Getting the turns ratio upside down. V_p / V_s = N_p / N_s. Primary goes with primary, secondary with secondary. Cross-multiplying incorrectly is a common error in the edexcel exam.
4. Saying the iron core "conducts electricity" in a transformer. The iron core carries the magnetic field from the primary to the secondary coil. It is laminated (layered with insulation) to reduce eddy currents and energy losses.
5. Drawing field lines that cross. Magnetic field lines never cross. If your diagram shows crossing lines, redraw it.

Self-check: edexcel igcse physics practice questions

1. A transformer has 500 primary turns and 50 secondary turns. The input voltage is 230 V. Calculate the output voltage.
2. The output current of the transformer in question 1 is 10 A. Assuming 100% efficiency, calculate the input current.
3. Describe how you would increase the strength of an electromagnet (give three ways).
4. Explain why a transformer does not work with d.c.
5. State Fleming's left-hand rule and use it to predict the direction of force on a wire carrying current to the right in a field directed into the page.

Answers: (1) V_s = 230 x (50 / 500) = 23 V; (2) V_p x I_p = V_s x I_s, 230 x I_p = 23 x 10, I_p = 1 A; (3) Increase the current, increase the number of turns, add a soft iron core; (4) A transformer needs a changing magnetic field to induce a voltage in the secondary coil; d.c. produces a constant field, so no voltage is induced; (5) First finger: field (into page), second finger: current (right), thumb: force (upward).

These edexcel igcse physics notes cover every part of the igcse 4ph1 magnetism and electromagnetism specification. The edexcel igcse physics explained approach here gives you the mechanism behind each phenomenon, not just the label. Work through the examples, draw the diagrams, and practise the practice questions until the principles feel instinctive.