Magnetism and electromagnetism describe how permanent magnets, electric currents and magnetic fields interact. The topic is the qualitative foundation for motors, relays, circuit breakers and loudspeakers, and it regularly earns structured-question marks in O-level Physics Paper 2.
The two key skills are using the right-hand grip rule to find the magnetic field produced by a current, and using Fleming's Left-Hand Rule to find the force on a current in an external magnetic field.
What these notes cover
- •Magnets and the Law of Magnetism
- •Hard and Soft Magnetic Materials
- •Magnetising and Demagnetising
- •Induced Magnetism
- •Magnetic Fields and Field Line Rules
- •Magnetic Effect of a Current-Carrying Conductor
- •Right-Hand Grip Rule for a Straight Wire and a Solenoid
- •Applications of Electromagnets: Bell, Relay and Circuit Breaker
- •The Motor Effect
- •Fleming's Left-Hand Rule
- •Force on a Beam of Charged Particles
- •Common Mistakes
Key definitions
- Magnetic field
- A region of space in which a magnet (or a moving charge or current) experiences a magnetic force.
- Hard magnetic material
- A material, such as steel, that is difficult to magnetise and demagnetise. It is used to make permanent magnets.
- Soft magnetic material
- A material, such as iron, that is easy to magnetise and demagnetise. It is used to make temporary magnets such as electromagnets and transformer cores.
- Induced magnetism
- Magnetism gained by an unmagnetised magnetic material when it is brought near or touches a magnet's pole. The near end takes the opposite polarity to that pole, and the effect weakens with distance from the magnet.
- Motor effect
- The force on a current-carrying conductor in an external magnetic field. The conductor's own field reinforces the external field on one side and weakens it on the other, which pushes the wire from the strong side towards the weak side.
Worked example
A horizontal wire carries current from left to right through a magnetic field that points into the page. (a) Use Fleming's Left-Hand Rule to find the direction of the force on the wire. (b) A beam of electrons travels through the same field, moving from left to right. State the direction of the force on the beam, and describe the resulting path.
- 1(a) Point the second finger (current) from left to right and the first finger (field) into the page.
- 2The thumb (force) then points vertically upward, so the wire is pushed up.
- 3(b) Electrons are negative, so the ‘current direction’ for the rule is opposite to their motion, i.e. from right to left.
- 4With the field still into the page, the rule gives a force vertically downward on the beam at that instant.
- 5The force stays perpendicular to the constantly changing velocity, so the electrons follow a circular arc that curves downward.
Watch out: For a beam of electrons, point the second finger opposite to the direction of motion, and always use the left hand for Fleming's rule.
Common mistakes
Mixing up the right-hand grip rule and Fleming's Left-Hand Rule
Wrong: Using Fleming's Left-Hand Rule to find the field produced by a current, or the grip rule to find the force on a conductor.
Right: Use the right-hand grip rule to find the field produced by a current in a straight wire or solenoid, with no external field involved. Use Fleming's Left-Hand Rule to find the force on a current-carrying conductor inside an external magnetic field.
Using the right hand for force questions
Wrong: Reaching for the right hand out of habit when the question asks about the force on a conductor (the motor effect).
Right: Always use the left hand for Fleming's rule.
Treating attraction as proof of a magnet
Wrong: Concluding that an object is a magnet because it is attracted to a magnet.
Right: Only repulsion between two objects proves that both are magnets. Attraction could just be induced magnetism in an unmagnetised piece of magnetic material such as iron, steel or nickel.
Questions students ask
How do you test if an object is a magnet?
Bring it near a known magnet and look for repulsion. Only two magnets can repel each other. An unmagnetised magnetic material is only ever attracted, because the magnet induces magnetism in it.
When do you use the right-hand grip rule instead of Fleming's left-hand rule?
Use the right-hand grip rule to find the magnetic field produced by a current in a straight wire or solenoid, with no external field involved. Use Fleming's Left-Hand Rule to find the force on a current-carrying conductor placed in an external magnetic field.
How can you make an electromagnet stronger?
Increase the current, increase the number of turns per unit length of the solenoid, or insert a soft iron core to concentrate the field. At this level you only need to reason about these three factors qualitatively. You do not need to calculate the field strength.
How do you demagnetise a magnet?
There are three methods: place the magnet in a solenoid carrying alternating current and slowly withdraw it, hammer it repeatedly while it is aligned east-west, or heat it strongly. Soft magnetic materials such as iron lose their magnetism easily, while hard magnetic materials such as steel are difficult to demagnetise.
In the full PDF
- •Labelled diagrams: field lines around a bar magnet, field patterns around a straight wire, and Fleming's Left-Hand Rule
- •Two further worked examples: induced magnetism along a nail-and-pin chain, and whether two solenoids attract or repel
- •Step-by-step explanations of how the electric bell, magnetic relay and circuit breaker work
- •Methods of magnetising by stroking and by direct current, plus the dot-and-cross convention
- •End-of-topic summary of the key points