Physics · Lesson 22

Magnets, magnetic fields & force on a conductor

JAMBWAECNECO

What the syllabus asks

JAMB: magnets, magnetic fields, Earth’s field, electromagnetism, force on a conductor, motors

WAEC: magnets and magnetic materials; properties and laws of magnets; magnetisation and demagnetisation; domain theory; magnetic fields and field lines; Earth’s magnetic field (declination, dip, horizontal component); magnetic field of a current in a straight wire and a solenoid; electromagnets and uses; force on a current-carrying conductor in a magnetic field (F = BIL sin θ), Fleming’s left-hand…

NECO: magnets and magnetic materials; properties and laws of magnets; magnetisation and demagnetisation; domain theory; magnetic fields and field lines; Earth’s magnetic field (declination, dip, horizontal component); magnetic field of a current in a straight wire and a solenoid; electromagnets and uses; force on a current-carrying conductor in a magnetic field (F = BIL sin θ), Fleming’s left-hand…

What you will learn

State the properties and laws of magnets and distinguish magnetic from non-magnetic materials.
Describe methods of magnetising and demagnetising and explain them with domain theory.
Draw field patterns of magnets and locate neutral points.
Describe the Earth’s magnetic field, declination and dip.
Describe and draw the fields of a straight wire and a solenoid and give uses of electromagnets.
Use F = BIL sin θ and Fleming’s left-hand rule, and explain the force between parallel currents.
Explain the d.c. motor and moving-coil galvanometer, and use F = qvB.

Key points

Like poles repel, unlike attract; iron, nickel, cobalt and steel are magnetic; repulsion is the sure test.
Magnetise with d.c. in a solenoid (or stroking); demagnetise with a.c., heating or hammering.
Domain theory: domains random when unmagnetised, aligned when magnetised; saturation when all aligned.
Soft iron for temporary magnets; steel for permanent magnets.
Field lines N to S, never cross; neutral points where fields cancel.
Earth’s field: declination (true vs magnetic north) and dip (angle below horizontal); tan δ = B_V/B_H.
Straight wire: circular field (right-hand grip); solenoid: bar-magnet field, anticlockwise end N.
Electromagnets: soft-iron core; bell, relay, crane, loudspeaker.
Motor effect F = BIL sin θ, Fleming’s left-hand rule; parallel currents in the same direction attract.
D.c. motor uses a split-ring commutator; galvanometer uses a radial field for a linear scale; moving charge F = qvB, r = mv/(qB).

Start here: Magnets and magnetic materials

Fridge magnets, the needle of a compass, the speaker in your phone and the motor in a ceiling fan all depend on magnetism.

A magnet attracts iron and similar materials and, when freely suspended, comes to rest pointing roughly north–south. The end pointing north is the north-seeking pole (N); the other is the south-seeking pole (S).

Magnetic materials (ferromagnetic) are strongly attracted: iron, nickel, cobalt and their alloys, such as steel. Non-magnetic materials, such as copper, aluminium, brass, wood, plastic, glass and paper, are not attracted. (Scrap dealers use this difference to separate iron from aluminium cans.)

Properties of magnets

Magnetic effect is strongest at the poles; iron filings cluster at the ends of a bar magnet.
A freely suspended magnet sets in the north–south direction.
Poles always occur in pairs: cut a magnet in half and you get two smaller magnets, each with N and S. There is no isolated pole.

Law of magnetism: like poles repel; unlike poles attract.

Worked example 22.1 (testing for a magnet)

You have two identical-looking iron bars, A and B. One is a magnet. How do you find which, using only the two bars?

Simply bringing them together does not help: a magnet attracts unmagnetised iron, so they will attract whichever is the magnet.
Use the fact that a magnet’s effect is strongest at its poles and almost zero at its middle. Touch one end of A to the middle of B.
If they attract strongly, A is the magnet: its pole attracts the iron bar B at any point.
If there is little or no attraction, B is the magnet: the middle of a magnet has almost no magnetic effect, and A (plain iron) has no pole of its own.

End of one bar to the middle of the other

💡 Exam tip: "Repulsion is the only sure test for a magnet." Attraction could be between a magnet and plain iron.

Try it yourself. Which of these is attracted by a magnet: brass, nickel, aluminium, copper?

Show solution

Nickel.

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Practice questions

1. Name three magnetic and three non-magnetic materials.

Show worked answer

Iron, nickel, cobalt; copper, aluminium, wood (for example).

2. Explain, using domain theory, why heating a magnet weakens it.

Show worked answer

Heat makes the atoms vibrate more, disturbing the alignment of the domains.

3. At a place B = 4.0 × 10⁻⁵ T and dip = 30°. Find the horizontal component.

Show worked answer

4.0 × 10⁻⁵ × cos 30° ≈ 3.5 × 10⁻⁵ T.

4. State three ways to increase the strength of an electromagnet.

Show worked answer

More current, more turns, soft-iron core.

5. A 0.25 m wire carries 8 A at 90° to a 0.3 T field. Find the force.

Show worked answer

0.3 × 8 × 0.25 = 0.6 N.

Key words

Magnet: An object that attracts magnetic materials and sets in a north–south direction when freely suspended.
Magnetic material: A material attracted by a magnet: iron, nickel, cobalt and their alloys.
Pole: The region near the end of a magnet where its effect is strongest.
Domain: A small region in which all the atomic magnets point the same way.
Magnetic field: A region where a magnetic force is felt.
Field line: A line showing the direction a free north pole would move.
Neutral point: A point where the resultant magnetic field is zero.
Declination: The angle between the geographic and magnetic meridians.
Dip (inclination): The angle the Earth’s field makes with the horizontal.
Solenoid: A long coil of wire carrying current.
Electromagnet: A soft-iron core magnetised by a current in a surrounding coil.
Tesla (T): The unit of magnetic flux density B.

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