Revision summary

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.

Controlled magnetic effects

A coil magnetises a suitable soft-iron core when current flows; low retention helps a lifting device release its load at switch-off. In the magnetic overcurrent-trip model, excessive current increases attraction, moves the armature and releases the latch. Contacts open, current stops, and the trip mechanism retains the open state until reset after the fault is addressed.

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.

Charged beams: a perpendicular field shifts an electron beam's fluorescent-screen spot; reversing the field reverses the shift. Conventional current follows positive-charge motion and opposes electron motion. With initial motion right and field into the page, positive charges initially deflect up and electrons down. Reversing field or the motion of the same charge type reverses the initial force; reversing both restores it. The local force changes direction as the path curves.

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.

More turns increase the combined turning effect when current, field, coil size and orientation remain comparable. Greater current increases the forces and turning effect with the other conditions fixed.

Sustained rotation in the d.c. motor

The split ring rotates with the coil while the brushes remain fixed. Every half-turn the segments exchange brushes, reversing current in each physical side of the coil. After A moves from left to right, its current changes from out of the page to into it, giving downward force and maintaining clockwise turning in the stated rightward field.

The field poles and external supply polarity stay fixed. Existing rotation carries the coil through the zero-turning/contact-gap position; a stationary single coil there may not self-start. Two complete slip rings do not provide this connection swap.

A soft-iron cylinder strengthens the field through the coil and increases its turning effect under comparable conditions. It has a different role from the commutator.