20.4 Magnetic fields due to currents
- Syllabus
- 9702–2028–2029
- Topic
- 20.4
- Level
- A2
Learning objectives
20.4.1Magnetic field patterns due to the currents in a long straight wire, a flat• sketch magnetic field patterns due to the currents in a long straight wire, a flat circular coil and a long solenoid20.4.2The magnetic field due to the current in a solenoid is increased by a• understand that the magnetic field due to the current in a solenoid is increased by a ferrous core20.4.3The origin of the forces between current-carrying conductors and determine• explain the origin of the forces between current-carrying conductors and determine the direction of the forces
Sketch magnetic fields from a straight wire, circular coil and long solenoid
| Current source | Required field-line pattern | Strength cue |
|---|---|---|
| long straight wire | concentric circles centred on wire | circles spread farther apart with distance |
| flat circular coil | lines pass nearly parallel through centre, perpendicular to coil plane, then curve around outside in closed loops like a bar magnet | closest near centre |
| long solenoid | straight, parallel, equally spaced lines inside along axis; curved closed loops outside like a bar magnet | strong/uniform inside, weak outside |
For a straight wire, point the right thumb along conventional current; curled fingers give circular field direction. Current into the page gives clockwise arrows; current out gives anticlockwise arrows.
For a circular coil or solenoid, curl right-hand fingers with conventional current around the turns; the thumb points through the coil toward its magnetic north end and gives the internal field direction.
Reversing current reverses every field arrow and swaps the coil/solenoid north and south poles, while leaving the field-line shape unchanged.
The syllabus requires a flat circular coil, not a flat current sheet. Do not draw straight-wire lines parallel to the wire or show a real solenoid as perfectly uniform outside its ends.
A solenoid’s internal magnetic field increases with turns per length and current
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.
Parallel currents exert forces through their magnetic fields
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.