Magnetism and electromagnetism — GCSE Physics Revision
Everything you need to revise magnetism and electromagnetism for GCSE Physics: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
Magnetic Fields and Permanent Magnets
Permanent magnets produce their own magnetic field. This field is strongest at the poles (North and South). Field lines are used to represent magnetic fields; they always go from North to South outside the magnet and never cross. We can map these fields using a compass, which aligns with the field lines, or by sprinkling iron filings, which show the pattern. The Earth itself acts like a giant magnet, which is why a compass points North.
Electromagnetism: Current's Magnetic Effect
When an electric current flows through a wire, it creates a magnetic field around it. This phenomenon is called electromagnetism. The direction of the magnetic field around a straight wire can be determined using the right-hand grip rule. The strength of this field depends on the current and the distance from the wire. Coiling the wire into a solenoid significantly strengthens and concentrates this magnetic field, making it resemble a bar magnet.
Electromagnets: Uses and Factors Affecting Strength
An electromagnet is a temporary magnet created by passing current through a coil of wire, often with an iron core. They are incredibly useful because their magnetism can be turned on and off. Their strength can be increased by increasing the current, increasing the number of turns in the coil, or inserting a soft iron core. This adjustability makes them essential in cranes, relays, and circuit breakers.
The Motor Effect and Fleming's Left-Hand Rule
The motor effect describes the force experienced by a current-carrying conductor placed in a magnetic field. This force is perpendicular to both the current direction and the magnetic field direction. Fleming's Left-Hand Rule helps to determine the direction of this force: Thumb = Force, Forefinger = Field (North to South), Middle Finger = Current (Positive to Negative). This principle is fundamental to how electric motors operate.
Key facts
- Magnetic field lines go from North to South.|A current-carrying wire produces a magnetic field.|The strength of an electromagnet can be varied.|The motor effect is a force on a current in a B-field.|Fleming's Left-Hand Rule predicts motor effect direction.|Permanent magnets have a constant magnetic field.
Key terms
- Magnetic Field::A region around a magnet or a current-carrying conductor where magnetic forces can be detected.|Electromagnetism::The production of a magnetic field by an electric current.|Solenoid::A coil of wire that produces a strong and uniform magnetic field when current passes through it.|Permanent Magnet::A material that produces its own persistent magnetic field without external influence.|Motor Effect::The force experienced by a current-carrying conductor when placed in an external magnetic field.|Fleming's Left-Hand Rule::A mnemonic to determine the direction of force, magnetic field, and current in the motor effect.
Common mistakes
- Confusing the direction of magnetic field lines inside and outside a magnet (inside: South to North).|Incorrectly applying Fleming's Left-Hand Rule, mixing up the fingers for force, field, and current.|Believing permanent magnets can have their strength easily varied like electromagnets.|Thinking that a magnetic field only exists around electromagnets, not permanent magnets.
Exam tips
- Practice drawing magnetic field patterns for various situations (bar magnet, solenoid, current-carrying wire).|Memorise and practice using Fleming's Left-Hand Rule for determining direction of force, field, or current.|Understand the factors affecting the strength of an electromagnet and be able to explain how to increase/decrease it.|Clearly distinguish between permanent magnets (constant field) and electromagnets (variable field).
Quick quiz
1. Which of the following would NOT increase the strength of an electromagnet?
- Increasing the current flowing through the coil
- Inserting a soft iron core into the coil
- Increasing the number of turns in the coil
- Increasing the resistance of the coil
Show answer
Increasing the resistance of the coil — Increasing resistance would decrease the current for a given voltage, thereby weakening the electromagnet. The other options all increase its strength.
2. Magnetic field lines around a bar magnet are conventionally shown to go from:
- South pole to North pole outside the magnet
- North pole to South pole outside the magnet
- North pole to North pole outside the magnet
- South pole to South pole outside the magnet
Show answer
North pole to South pole outside the magnet — By convention, magnetic field lines always originate from the North pole and terminate at the South pole outside a magnet.
3. What rule is used to determine the direction of the magnetic field around a straight current-carrying wire?
- Fleming's Left-Hand Rule
- Right-Hand Grip Rule
- Fleming's Right-Hand Rule
- Lenz's Law
Show answer
Right-Hand Grip Rule — The Right-Hand Grip Rule (or Right-Hand Thumb Rule) is used to find the direction of the magnetic field around a straight current-carrying wire.
4. Which component of an electric motor directly experiences a force due to the motor effect?
- The power supply
- The commutator
- The coil carrying current
- The permanent magnets
Show answer
The coil carrying current — The coil carrying current is placed within the magnetic field of the permanent magnets, and it is this coil that experiences the force of the motor effect, causing it to rotate.
5. A compass needle points North because:
- It's attracted to the Sun
- The Earth acts as a giant electromagnet
- The Earth has a strong gravitational field
- The Earth has a magnetic field
Show answer
The Earth has a magnetic field — A compass needle is a small magnet that aligns itself with the Earth's natural magnetic field, causing its North pole to point towards the Earth's magnetic North pole.
Exam-style questions
Describe how to determine the direction of the magnetic field around a straight current-carrying wire. [2 marks]
Show mark scheme
- State the Right-Hand Grip Rule.
- Explain how to apply it (thumb in current direction, fingers show field).
Explain two ways to increase the strength of an electromagnet. [2 marks]
Show mark scheme
- Increase the current flowing through the coil.
- Increase the number of turns in the coil / windings.
- Insert a soft iron core into the coil (allow 'increase turns density').
A student uses Fleming's Left-Hand Rule. Her middle finger points right, and her forefinger points upwards. In which direction is the force on the conductor? [3 marks]
Show mark scheme
- Identify middle finger as current (right).
- Identify forefinger as field (upwards).
- Deduce that the thumb (force) points out of the page.
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