20.4 Magnetic fields due to currents

A long straight current gives concentric circular fields; a flat current sheet gives parallel fields; a long solenoid gives an approximately uniform internal field.

Use the right-hand grip rule and distinguish ideal interior uniformity from edge fringing. Field strength depends on current and geometry.

Reversing current in a solenoid reverses its north and south poles and the internal field direction.

Field lines around a straight wire are not parallel, and a real solenoid’s field is not perfectly uniform outside its ends.

For a long solenoid, field strength is approximately proportional to current and turns per unit length; a high-permeability core can increase it further.

Use the right-hand grip rule for polarity and distinguish ideal uniform interior field from fringing at the ends.

Increasing turns per metre or current strengthens the field, while reversing current reverses the poles.

A solenoid is not a permanent magnet by default; its field depends on current and core conditions.

Each current creates a magnetic field that acts on the other conductor, producing attraction for currents in the same direction and repulsion for opposite directions.

Use the field of one wire and F=BIL on the other; the force per length falls as separation increases.

Two long parallel wires carrying equal currents in the same direction pull toward each other.

The force is not an action of one current on itself; it is an interaction between the fields and the other conductor’s current.