Magnetism covers what a magnet is, which materials it acts on, how to prove an object is a magnet, how magnets are made and destroyed, and how to plot and draw magnetic field patterns. It is the foundation for Electromagnetism in Sec 4: the motor effect, electromagnets, relays and electromagnetic induction all build on it.
The topic is almost entirely qualitative, with no equations to rearrange. Marks come from precise definitions, such as magnetic field, induced magnetism and the repulsion test, and from accurate, tidy field diagrams with correctly drawn arrows. These appear regularly in MCQ and short structured questions.
What these notes cover
- •Properties of magnets and poles
- •Magnetic and non-magnetic materials
- •The law of magnetism
- •Repulsion as the test for a magnet
- •Freely suspended magnets and the Earth's magnetic poles
- •Induced magnetism and the chain of paper clips
- •Magnetisation by stroking and by d.c. in a solenoid
- •Demagnetisation: heating, hammering and a.c. in a solenoid
- •Temporary vs permanent magnets: iron and steel
- •Plotting fields with a compass and with iron filings
- •Field patterns, the uniform field and the neutral point
- •Exam-style worked examples
Key definitions
- Magnet
- An object that produces a magnetic field around itself and can exert a magnetic force on other magnets and on magnetic materials without touching them. It has two poles, N and S, which always come in pairs.
- Magnetic material
- A material that can be attracted by a magnet and can itself be magnetised: iron, steel, nickel and cobalt.
- Induced magnetism
- When an unmagnetised magnetic material is brought near to, or touches, a pole of a strong magnet, it becomes a magnet itself. The near end acquires the opposite pole to the magnet and the far end acquires a like pole.
- Magnetic field
- The region around a magnet in which a magnetic material, or another magnet, experiences a magnetic force.
- Neutral point
- A point where two magnetic fields are equal in strength and opposite in direction, so the resultant field is zero. A plotting compass placed there shows no preferred direction.
Worked example
Three identical-looking grey rods P, Q and R are tested one at a time against the N-pole of a bar magnet. Rod P is attracted at both ends. Rod Q is repelled at one end and attracted at the other. Rod R shows no effect. Identify each rod.
- 1Rod Q is repelled at one end, and repulsion can only occur between two magnets, so Q is a magnet.
- 2Like poles repel, so the end of Q that was repelled is its north pole, and the attracted end is its south pole.
- 3Rod P is attracted at both ends but never repelled, and an unmagnetised magnetic material is attracted at either end by induced magnetism.
- 4A magnet would have shown repulsion at one end, so P is an unmagnetised magnetic material (for example iron).
- 5Rod R experiences no force at either end, so it is a non-magnetic material (for example aluminium).
Watch out: When you conclude that P is not a magnet, you must add the reason: because neither end is repelled. The reasoning carries the mark, not just the label.
Common mistakes
Using attraction as the test for a magnet
Wrong: The paper clip is attracted to the magnet, so the paper clip is a magnet.
Right: A magnet attracts any magnetic material, magnetised or not, through induced magnetism. Only repulsion proves that an object is a magnet.
Giving only half of the iron explanation
Wrong: Iron is used for the electromagnet core because it is easily magnetised.
Right: Iron is easily magnetised and easily demagnetised, so the core loses its magnetism as soon as the current is switched off. A steel core would retain its magnetism.
Claiming iron filings show field direction
Wrong: The iron filings show that the field points to the right.
Right: Iron filings show the pattern only, because a filing has no labelled N end. Use a plotting compass instead: the field points the way its N end points, from N to S outside the magnet.
Questions students ask
What is the test for a magnet in physics?
Repulsion is the only conclusive test for a magnet. Bring each end of the object in turn to a known pole: if either end is repelled, it is a magnet; if both ends are attracted, it is an unmagnetised magnetic material; if there is no effect, it is non-magnetic. Attraction proves nothing, because any magnetic material is attracted through induced magnetism.
Why is soft iron used for an electromagnet core instead of steel?
Iron is easily magnetised and easily demagnetised. The core is strongly magnetised when current flows and loses its magnetism as soon as the current is switched off, so the electromagnet can release its load, whereas a steel core would retain its magnetism. An answer that says only “easily magnetised” typically earns half the marks at most.
How do you demagnetise a magnet?
Heat it strongly and let it cool lying east-west, hammer or drop it while it lies east-west, or place it in a solenoid carrying a.c. and slowly withdraw it with the current still on. If the a.c. is switched off with the magnet still inside, the magnet may be left magnetised in the direction the current had at that instant.
Does a compass point to the Earth's magnetic north pole?
No. The north-seeking pole of a compass points to geographic north, where the Earth’s magnetic south pole lies, because unlike poles attract. The Earth’s magnetic north pole lies near the geographic south pole.
In the full PDF
- •Labelled field-pattern diagrams for a single bar magnet, unlike poles facing and like poles facing with the neutral point
- •Step-by-step plotting-compass method and a comparison table of plotting compass versus iron filings
- •Further worked examples on stroking polarity, the right-hand grip rule, relays, iron versus steel and locating the neutral point
- •The full iron versus steel comparison table with uses, plus the paper-clip experiment explained
- •The remaining common mistakes on drawing field lines and describing experiments
- •Section summaries and a one-page table of every magnetising and demagnetising process