Revision summary

Magnet properties and observations

Like poles repel and unlike poles attract. An ordinary magnet has N and S poles; dividing it produces smaller magnets with both poles. A freely suspended magnet turns approximately north-south when nearby disturbances are small.

A magnetic material can become magnetised. An end repelling a known N pole is N in the simple end-to-end test. Attraction alone may involve an initially unmagnetised material, so it does not prove that the object was already a permanent magnet.

Induced magnetism and material choice

Near a strong magnet, the nearer end of an initially unmagnetised rod develops the opposite pole to the inducing pole. Inside a current-carrying solenoid, a suitable core becomes magnetised in the coil's field and strengthens the magnetic effect.

Soft iron is readily magnetised and has low retention, useful for a switched lifting magnet. Suitable steel is harder to magnetise but retains magnetism, useful for a permanent magnet or compass needle. Compare retention after removing the inducing field, using comparable specimens and a consistent test.

Find direction with a compass

The N end of a settled compass gives the local field direction. Fix the magnet and paper, mark the needle ends and N-end direction, move S to the previous N mark, and repeat. Join the marks smoothly with arrows and start again elsewhere.

Nearby magnetic objects, a stuck needle and Earth's field can affect the map. Iron filings reveal a rough pattern but do not supply the arrow directions.

Magnetic field patterns

• Outside a bar magnet, lines leave N and enter S; they return through it from S to N.
• Unlike facing poles have connecting lines across the gap from N to S.
• Like facing poles have lines bending away from the central gap region. Do not join N to N or S to S. Reverse the arrows for the corresponding S/S rather than N/N pattern.
• Lines do not cross. A compass is tangent to a local line, and closer spacing indicates a stronger field within a consistently drawn pattern.

Fields from currents

Straight wire

Field lines are circles centred on the wire. Right thumb follows conventional current; curled fingers give field direction. In a fixed end view, current out of the page gives anticlockwise field and current into it gives clockwise field.

Solenoid

Right-hand fingers follow current around the turns and the thumb points towards N. Anticlockwise current viewed from an end makes that end N. Field is approximately uniform in the central region of a long coil, returning inside from S to N and outside from N to S.

Change the current

Greater magnitude gives a stronger field with geometry and core fixed. Reversal reverses the wire's field and swaps a solenoid's poles. Do not confuse strength with direction.

In a compass investigation, fix viewpoint and positions, wait for settling, and compare one change at a time. Record actual current and account for background fields. Compass direction alone is not a calibrated numerical field-strength reading.

Force direction and controlled reversals

For mutually perpendicular directions, Fleming's left hand gives first finger = field, second finger = conventional current, thumb = magnetic force. Any two known directions determine the third.

• Field right, current out of the page: force up.
• Field right, current into the page: force down.
• Field left, current out of the page: force down.
• Field left, current into the page: force up.

In the movable-wire experiment, reverse current alone, then restore it and reverse field alone. Either reverses the magnetic force; reversing both restores it. Keep geometry and supports comparable and limit heating. Current off removes this current-dependent magnetic force, not gravity or support forces. Parallel current and field give no magnetic force in the model.

The coil's turning effect

Viewed along the axle with field right, the left active side carrying current out of the page is forced up; the right side carrying current into the page is forced down. Their separated lines of action give clockwise turning. Equal opposite forces can have zero resultant force and a nonzero combined moment.

Identify both force directions and their distances from the axle. If their lines pass through the axle, their moments about it are zero.