4.5.3 Magnetic effect of a current
- Syllabus
- 0625–2026–2027
- Topic
- 4.5.3
- Level
- —
Learning objectives
4.5.3.1Pattern and direction of the magnetic• Describe the pattern and direction of the magnetic field due to currents in straight wires and in solenoids4.5.3.2An experiment to identify the pattern• Describe an experiment to identify the pattern of the magnetic field (including direction) due to currents in straight wires and in solenoids4.5.3.3Magnetic effect of a current is used• Describe how the magnetic effect of a current is used in relays and loudspeakers and give examples of their application4.5.3.4Qualitative variation of the strength• State the qualitative variation of the strength of the magnetic field around straight wires and solenoids4.5.3.5Effect on the magnetic field around• Describe the effect on the magnetic field around straight wires and solenoids of changing the magnitude and direction of the current
Magnetic fields from wires and solenoids
| Current-carrying conductor | Magnetic-field pattern |
|---|---|
| long straight wire | concentric circles centred on the wire, in planes perpendicular to it |
| solenoid | nearly straight, parallel lines along the axis inside; curved lines return around the outside like a bar magnet |
For a straight wire, use the right-hand grip rule: point the right thumb in the conventional-current direction; the curled fingers show the circular field direction. Current out of the page (•) gives an anticlockwise field; current into the page (×) gives a clockwise field.
For a solenoid, curl the right-hand fingers in the conventional-current direction around its turns. The thumb points along the field inside the solenoid and towards its north pole. Field lines continue outside from north to south and return inside from south to north.
Magnetic field lines form continuous loops and never cross. Around a straight wire they are circles, not radial spokes; inside a solenoid they run along the axis, not around each turn separately.
Map a current's magnetic field experimentally
| Arrangement | Reveal the pattern | Identify direction |
|---|---|---|
| straight wire through a horizontal card | sprinkle iron filings and tap the card gently; filings align in concentric circles | place a plotting compass at several points and mark the direction of its north pole |
| current-carrying solenoid | place plotting compasses inside, near both ends and around the outside; trace the smooth route through their directions | follow each compass north pole to add arrows and identify the solenoid's north end |
Use a d.c. supply so the direction is steady. Switch on only while taking observations, repeat at enough positions to trace smooth non-crossing field lines, and compare with the current direction using the right-hand grip rule.
Reverse the supply: the pattern keeps the same shape but every compass direction reverses. Move a compass farther from a straight wire, or from inside to outside a solenoid: the current's effect becomes weaker and Earth's field may influence the reading more.
Iron filings show the shape and relative strength but do not reliably show direction. A plotting compass is needed for arrows; one compass position alone is not enough to map the full pattern.
How relays and loudspeakers use current
| Device | Magnetic-effect chain | Typical use |
|---|---|---|
| relay | a small current in a coil magnetises a soft-iron core; it attracts an armature, moving contacts that open or close a separate circuit | a sensor or low-power control switches a motor, lamp, bell or other larger-current load |
| loudspeaker | audio current in a voice coil creates a magnetic field that interacts with a permanent magnet; the coil and attached cone experience a force | electrical audio signal is converted into sound |
When the relay-coil current stops, the soft iron rapidly loses its magnetism and a spring returns the armature and contacts. The control circuit and load circuit are electrically separate but mechanically linked by the moving contacts.
An alternating audio current repeatedly reverses the magnetic effect and therefore the force on the voice coil. The cone moves backwards and forwards, making surrounding air vibrate; the changing current waveform sets the sound vibration.
A relay is a magnetically operated switch; it does not itself increase voltage or current. A loudspeaker does not use electromagnetic induction to generate its input—it uses the magnetic force produced by its input current.
Where a current's magnetic field is strongest
| Region | Qualitative field strength | Evidence in a field-line diagram |
|---|---|---|
| close to a straight wire | strongest | circular lines are closest together |
| farther from a straight wire | weaker | circular lines are more widely spaced |
| inside a long solenoid, especially near its centre | strong and approximately uniform | straight parallel lines are close and evenly spaced |
| near the ends and outside a solenoid | weaker and non-uniform | lines spread and curve around |
Field-line density represents strength: closer spacing means a stronger field. Compare spacing in the same diagram; field lines are a model, not physical strands whose individual thickness sets strength.
A solenoid's field is not equally strong everywhere. The nearly uniform region is inside the central part; the external return field and end regions are weaker.
Change a current, change its magnetic field
| Change to current | Field strength | Field direction / pattern |
|---|---|---|
| increase current magnitude | increases | same pattern and direction; draw lines closer together |
| decrease current magnitude | decreases | same pattern and direction; draw lines farther apart |
| reverse current, same magnitude | unchanged | every arrow reverses; a solenoid's north and south ends swap |
| increase magnitude and reverse current | stronger | reversed direction |
The current creates the field, so its magnitude controls field strength and its direction controls field direction. These rules apply to both a straight wire and a solenoid.
With alternating current, the current magnitude and direction change repeatedly, so the magnetic field strength varies and its direction repeatedly reverses.
Reversing current does not weaken the field when the magnitude stays the same. Increasing current changes line density, not the basic circular or solenoidal shape.