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Magnetism: Fields, Forces and Compasses
Electromagnetism
Magnetism is the force that makes fridge magnets stick and compass needles point north. It comes from moving charges, and it is closely linked to electricity.
What a magnetic field is
A magnetic field is a region where a magnetic material or a moving charge experiences a force. Every magnet has a north pole and a south pole, and opposite poles attract while like poles repel. Unlike electric charge, a magnetic pole cannot be isolated: cutting a magnet in half produces two smaller magnets, each with its own north and south pole. Magnetic field lines run from north to south outside a magnet, never cross, and are closest together where the field is strongest. The strength of a magnetic field is called magnetic flux density, symbol B, measured in teslas. Earth itself acts like a giant magnet, which is why a compass needle lines up with the planet's field.
Forces on moving charges and wires
A charge moving through a magnetic field feels a force that is perpendicular both to its velocity and to the field direction. The force is largest when the charge moves at right angles to the field and zero when it moves parallel to it. The same idea applies to a current-carrying wire, because a current is a flow of moving charges: the wire feels a force perpendicular to both the current and the field. This is the principle behind electric motors, where a coil in a magnetic field is pushed round, and loudspeakers, where a coil moves a cone back and forth. Fleming's left-hand rule gives the direction of the force on a wire, with thumb for force, first finger for field and second finger for current.
Where you see it
Compasses use Earth's magnetic field for navigation, and the field also shields the planet by deflecting charged particles from the Sun, producing the aurorae near the poles. Hard drives, credit-card strips and fridge magnets store information or hold objects with magnetism. Motors, generators, transformers and MRI scanners all depend on magnetic fields. Electromagnets, made by winding wire into a coil around an iron core, can be switched on and off and are used in scrapyard cranes, door locks and relays. Even the magnets in headphones and electric-vehicle motors are part of everyday life.
Worked example: force on a wire in a magnetic field
A straight wire of length 0.30 m carries a current of 2.0 A. It sits at right angles to a uniform magnetic field of 0.50 T. Use F = B I L sin(theta), with theta = 90 degrees, so sin(theta) = 1. Substitute: F = 0.50 x 2.0 x 0.30 x 1. F = 0.30 N. The force is 0.30 N, perpendicular to both the wire and the field. If the wire were parallel to the field, theta = 0, sin(theta) = 0, and the force would be zero.
Common mistakes
- Expecting a force on a stationary charge: a magnetic field exerts no force on a charge at rest; the charge must be moving.
- Forgetting the angle: the force is largest at 90 degrees and zero when the motion or current is parallel to the field.
- Confusing north-seeking poles: the north pole of a compass points toward Earth's magnetic south pole, which sits near the geographic North Pole.
Practice questions
- A wire 0.40 m long carries 3.0 A at right angles to a 0.20 T field. What is the force?
Show answer
F = BIL = 0.20 x 3.0 x 0.40 = 0.24 N.
- An electron moves parallel to a magnetic field. What force does it feel?
Show answer
Zero, because sin(0) = 0 in F = qvB sin(theta).
- Why can a magnetic pole never be isolated?
Show answer
Cutting a magnet always creates new north and south poles on each piece; no magnetic monopole has been observed.
Frequently asked questions
What is the difference between a permanent magnet and an electromagnet?
A permanent magnet keeps its magnetism, while an electromagnet only produces a field when current flows through its coil.
Why does a compass point north?
Its needle aligns with Earth's magnetic field, with its north-seeking end pointing toward Earth's magnetic south pole near the geographic north.