P4: Magnetism and Magnetic Effects

What you'll learn

This topic covers permanent magnets, magnetic fields, electromagnets, and their applications in motors and other devices. You'll explore how magnetic fields interact with current-carrying conductors and understand the principles behind electromagnetic devices commonly used in everyday life.

Key terms and definitions

Magnetic field — the region around a magnet where a magnetic force acts on magnetic materials or other magnets

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

Electromagnet — a coil of wire that produces a magnetic field when an electric current flows through it

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

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

Fleming's left-hand rule — a method to determine the direction of force on a current-carrying conductor in a magnetic field

Induced potential difference — the voltage generated across a conductor when it moves through a magnetic field or when the magnetic field through it changes

Electromagnetic induction — the process of generating a potential difference (and current if a circuit is complete) by changing the magnetic field through a conductor

Core concepts

Permanent magnets and magnetic fields

All magnets have two poles: a north pole and a south pole. Like poles repel each other, while opposite poles attract. This is a non-contact force acting at a distance.

Magnetic field lines show the direction and strength of a magnetic field:

• Field lines point from the north pole to the south pole
• The closer together the lines, the stronger the magnetic field
• Field lines never cross each other
• At any point, the direction of the field is shown by the direction of the field line

Magnetic materials include iron, steel, cobalt, and nickel. These materials can be magnetised or are attracted to magnets.

The difference between hard and soft magnetic materials:

• Hard magnetic materials (e.g., steel) are difficult to magnetise but retain their magnetism — used for permanent magnets
• Soft magnetic materials (e.g., iron) are easy to magnetise but lose their magnetism quickly — used for electromagnet cores

Induced magnetism occurs when a magnetic material is placed in a magnetic field and becomes temporarily magnetised. The induced pole closest to the magnet is always the opposite pole, causing attraction. This explains why magnets always attract magnetic materials.

Electromagnets and solenoids

When electric current flows through a wire, it produces a magnetic field around the wire. The field forms concentric circles centred on the wire. The direction of the field can be found using the right-hand thumb rule: point your thumb in the direction of conventional current, and your fingers curl in the direction of the magnetic field.

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

• The magnetic field inside is strong and uniform
• The field pattern outside resembles that of a bar magnet
• One end acts as a north pole, the other as a south pole
• The poles can be reversed by reversing the current direction

Strength of an electromagnet can be increased by:

• Increasing the current through the coil
• Increasing the number of turns in the coil
• Adding a soft iron core inside the solenoid

Applications of electromagnets:

• Electric bells — electromagnet repeatedly attracts and releases an arm
• Circuit breakers — electromagnet pulls a switch open when current is too high
• Scrap metal sorting — electromagnets lift magnetic metals and can be switched off to release them
• Relay switches — low-power circuit controls a high-power circuit safely

The motor effect

When a current-carrying conductor is placed in a magnetic field, it experiences a force. This is called the motor effect. The force is greatest when the conductor is perpendicular to the magnetic field, and zero when parallel.

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

• First finger = Field direction (north to south)
• Second finger = Current direction (positive to negative)
• Thumb = Motion/force direction

The size of the force depends on:

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

The equation linking these factors is:

F = B × I × L

Where:

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

This equation is only valid when the conductor is perpendicular to the magnetic field.

Electric motors

A simple d.c. motor uses the motor effect to rotate a coil. The basic components are:

• A rectangular coil of wire
• A permanent magnet or electromagnet providing the magnetic field
• A split-ring commutator
• Brushes making electrical contact with the commutator
• A d.c. power supply

How a d.c. motor works:

1. Current flows through the coil in a magnetic field
2. Forces act on opposite sides of the coil in opposite directions (using Fleming's left-hand rule)
3. This creates a turning effect (moment) that rotates the coil
4. The split-ring commutator reverses the current direction every half turn
5. This keeps the coil rotating in the same direction continuously

Increasing motor speed:

• Increase the current
• Increase the magnetic flux density (stronger magnets)
• Increase the number of turns on the coil

Electromagnetic induction

When a conductor moves through a magnetic field, or when the magnetic field through a conductor changes, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current flows. This is electromagnetic induction.

Ways to induce a potential difference:

• Move a conductor through a magnetic field
• Move a magnet near a conductor
• Change the current in a nearby coil (changing its magnetic field)

The size of the induced potential difference depends on:

• The speed of movement — faster movement gives larger p.d.
• The strength of the magnetic field — stronger field gives larger p.d.
• The number of turns on the coil — more turns give larger p.d.
• The area of the coil — larger area gives larger p.d.

The direction of the induced potential difference (and current) always opposes the change that caused it. This is summarised by Lenz's law.

Applications:

• Moving a magnet in and out of a coil induces alternating current
• Rotating a coil in a magnetic field (or rotating a magnet inside a coil) continuously induces current
• The induced current reverses direction every half turn, producing alternating current (a.c.)

Generators and transformers

A.C. generators convert kinetic energy into electrical energy using electromagnetic induction:

• A coil rotates in a magnetic field (or magnets rotate around a stationary coil)
• The magnetic field through the coil continuously changes
• This induces an alternating potential difference
• Slip rings and brushes maintain continuous contact while allowing rotation
• The output is alternating current

Practical generators (e.g., in power stations):

• Turbines rotate electromagnets inside stationary coils
• Steam, water, or wind provides the kinetic energy to turn turbines
• Large-scale generators produce three-phase a.c. at 25,000 V

The frequency of a.c. output depends on:

• The speed of rotation — faster rotation gives higher frequency
• The number of poles — more poles give higher frequency for the same rotation speed

Microphones use electromagnetic induction:

• Sound waves cause a diaphragm to vibrate
• A coil attached to the diaphragm moves in a magnetic field
• This induces a varying potential difference matching the sound wave pattern
• The electrical signal can be amplified and transmitted

Loudspeakers work in reverse using the motor effect:

• Varying current (representing sound) flows through a coil
• The coil is in a magnetic field
• The motor effect causes the coil to vibrate
• A cone attached to the coil vibrates, creating sound waves

Worked examples

Example 1: Calculating force on a conductor

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

Solution:

State the equation: F = B × I × L (1 mark)

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

Calculate: F = 0.48 N (1 mark)

Note: Always include units in your final answer. Check values are perpendicular before using this equation.

Example 2: Electromagnet applications

Question: Explain why soft iron is used as the core of an electromagnet in an electric bell, rather than steel. (2 marks)

Solution:

Soft iron magnetises easily when current flows / loses magnetism quickly when current stops (1 mark)

This allows the electromagnet to turn on and off rapidly, making the bell ring repeatedly (1 mark)

Alternative mark: Steel would retain magnetism and the arm wouldn't spring back (1 mark)

Example 3: Induced potential difference

Question: A student moves a bar magnet quickly into a coil connected to a sensitive voltmeter. The voltmeter shows a reading of 0.3 V.

(a) State what happens to the voltmeter reading when the magnet is held stationary inside the coil. (1 mark)

(b) The student now removes the magnet quickly from the coil. Describe how the voltmeter reading changes compared to when the magnet was being inserted. (2 marks)

Solution:

(a) The reading becomes zero / no p.d. is induced (1 mark)

Explanation: There must be relative movement or a changing field to induce p.d.

(b) The voltmeter shows a reading in the opposite direction / negative value (1 mark)

The magnitude may be similar if the speed is the same (1 mark)

Explanation: The change is reversed (field decreasing rather than increasing), so induced p.d. reverses (Lenz's law).

Common mistakes and how to avoid them

• Confusing magnetic field direction with force direction — Use Fleming's left-hand rule correctly. Field goes north to south, not south to north. Check you're using your left hand, not right hand.

• Forgetting that F = BIL only applies when perpendicular — This equation requires the conductor to be at 90° to the magnetic field. Always check the question states this.

• Thinking magnets repel all magnetic materials — Magnets always attract magnetic materials through induced magnetism. Only magnets can repel each other (like poles).

• Confusing motor effect with electromagnetic induction — Motor effect: current in field → force/motion. Electromagnetic induction: motion in field → induced current/p.d. These are opposite processes.

• Not including units or using wrong units — Force must be in newtons (N), flux density in tesla (T), current in amperes (A), length in metres (m). Convert cm to m before calculating.

• Reversing the motor/generator distinction — Motors convert electrical energy to kinetic energy. Generators convert kinetic energy to electrical energy. Learn which way each device works.

Exam technique for "P4: Magnetism and Magnetic Effects"

• "Explain" questions require reasons, not just descriptions — For example, don't just state "the force increases"; explain "the force increases because there are more coil turns, so more current-carrying conductors experience the motor effect."

• Diagram questions testing Fleming's left-hand rule appear frequently — Practice applying the rule in different orientations. Mark on your diagram which direction is which if it helps, but ensure your final answer is clear.

• Calculation questions require working — Even if you can do F = BIL in your head, write out the equation, substitution, and answer. Each step typically earns a mark. Show units throughout.

• Application questions link physics to real devices — Understand how electric bells, motors, generators, and loudspeakers work. Exam questions often ask you to apply principles to unfamiliar contexts, so focus on understanding principles, not just memorising facts.

Quick revision summary

Magnets produce magnetic fields shown by field lines from north to south. Electromagnets use current in a coil; strength increases with more current, more turns, or an iron core. The motor effect produces force on current-carrying conductors in fields (F = BIL). Motors use this with commutators for continuous rotation. Electromagnetic induction generates p.d. when conductors move in fields or fields change. Generators convert kinetic to electrical energy; motors do the reverse. Fleming's left-hand rule determines force direction.