Describe how the magnetic effect of a current is used in relays and loudspeakers and give examples of their application

4.5.3 Magnetic Effect of a Current

1. Magnetic field produced by a current‑carrying conductor

• Straight wire – the magnetic field lines form concentric circles centred on the wire.
• Direction – given by the right‑hand rule: point the thumb in the direction of the conventional current; the curled fingers show the direction of the field lines.
• Solenoid (coil of many turns)

  • Inside the coil the field is almost uniform and parallel to the axis.
  • Outside the coil the field is much weaker and spreads out.

2. Fleming’s left‑hand rule (force rule)

For a current‑carrying conductor placed in a magnetic field:

• First finger → direction of magnetic field B (north → south).
• Second finger → direction of current I (conventional).
• Thumb → direction of the force F on the conductor.

3. Force on a current‑carrying conductor

The magnitude of the force is

\$F = B I L \sin\theta\$

• B – magnetic flux density (tesla, T).
• I – current (ampere, A).
• L – length of the conductor within the field (metre, m).
• θ – angle between the direction of the current and the magnetic field.

When the conductor is perpendicular to the field (θ = 90°), sin θ = 1 and the force is maximum: F = BIL.

4. Effect of changing the current

5. Practical investigations (AO3)

5.1 Mapping the magnetic field of a straight wire

1. Set up a vertical straight copper wire (≈15 cm) on an insulated stand.
2. Connect the wire to a low‑voltage DC supply (0–5 V) and a rheostat to vary the current.
3. Place a sheet of white paper underneath the wire and sprinkle fine iron filings evenly over the paper.
4. Switch on the current and observe the pattern formed by the filings – concentric circles centred on the wire.
5. Repeat with a small compass placed at several points around the wire to record the needle deflection; draw the direction of the field vectors.
6. Discuss how the observed pattern matches the right‑hand rule.

5.2 Mapping the field inside a solenoid

1. Wind a coil of insulated copper wire (≈200 turns) around a cylindrical former (≈5 cm long, 2 cm diameter).
2. Connect the coil to a DC supply and a current‑measuring ammeter.
3. Place a row of tiny compass needles along the axis of the solenoid, spaced 1 cm apart.
4. Switch on the current and note the direction each needle points; the needles should all align parallel to the axis, showing a uniform field.
5. Switch the current off and observe the needles return to random orientations.

5.3 Force on a current‑carrying conductor (the “deflection” experiment)

1. Mount a short insulated copper wire (≈10 cm) on a low‑friction wooden board so it can move freely.
2. Place a strong permanent magnet beneath the board with the field perpendicular to the wire.
3. Connect the wire to a variable DC supply and a galvanometer to monitor the current.
4. Increase the current stepwise, recording the lateral deflection of the wire (using a ruler or a light‑gate sensor).
5. Repeat for different angles between the wire and the magnetic field to verify the sin θ dependence.

Safety considerations (common to all experiments)

• Use insulated wires and avoid touching live terminals.
• Do not exceed the current rating of the coil; overheating can cause burns.
• Keep ferromagnetic objects away from strong permanent magnets to prevent sudden attraction.
• When mains voltages are involved (e.g., relay demonstrations), ensure the circuit is switched off before making connections.

Relays – Electromagnetic Switches

Principle of operation

A relay uses the magnetic effect of a current to move an armature and therefore open or close a set of contacts. A small control current produces a magnetic field that can switch a much larger load current.

Construction (key components)

Operation steps

1. Apply the control voltage to the coil.
2. Current I flows, producing a magnetic field B (right‑hand rule).
3. The magnetic force on the armature is F = BIL (θ ≈ 90°), pulling it toward the core.
4. The moving contacts either close the NO set or open the NC set, changing the state of the external circuit.
5. When the coil voltage is removed, the spring returns the armature, restoring the original contact arrangement.

Everyday examples

• Automotive ignition relay – low‑current control of the high‑current spark‑plug circuit.
• Domestic washing‑machine control – safety cut‑off for the motor.
• Industrial PLC‑controlled relay bank – switching motors, heaters, and lighting from a central controller.
• Telecommunication exchange relay – routing of telephone lines (historical but illustrative).

Loudspeakers (Dynamic Speakers)

Principle of operation

A loudspeaker converts an alternating audio current into sound. The audio current in the voice coil creates a time‑varying magnetic field that interacts with the steady field of a permanent magnet, producing a force that makes the diaphragm vibrate.

Construction (key components)

Operation steps

1. The audio source supplies a varying current i(t) to the voice coil.
2. At any instant the magnetic force on the coil is F(t)=B\,i(t)\,L (direction given by Fleming’s left‑hand rule).
3. The coil and attached diaphragm move forward when the force is in one direction and backward when it reverses, reproducing the waveform of the audio signal.
4. The vibrating diaphragm creates alternating high‑pressure and low‑pressure regions in the surrounding air – these are the sound waves we hear.

Everyday examples

• Public‑address (PA) systems in schools, stadiums, and transport hubs.
• Home audio – televisions, radios, hi‑fi stereo sets.
• Car audio – speakers in the dashboard, doors, and rear deck.
• Portable devices – smartphones, tablets, Bluetooth speakers.

Comparison of Relays and Loudspeakers

Link to other syllabus topics (IGCSE 0625)

• 4.5.4 Force on a current‑carrying conductor – the same formula F = BIL sin θ is used for both relays (linear pull) and loudspeakers (oscillatory pull).
• 4.5.5 DC motor – a rotating armature experiences a continuous torque from the magnetic force; conceptually similar to the linear force in a relay.
• 4.5.6 Transformer – relies on magnetic flux linking two coils; demonstrates that the magnetic field produced by a current can affect other conductors, just as the relay coil’s field moves an armature.
• 4.5.7 Electromagnetic induction – the motion‑induced emf ε = BLv is the counterpart of the force equation and underpins the operation of d.c. motors and generators.

Summary

The magnetic effect of a current provides a direct route from electrical energy to mechanical motion:

• Relay – a modest current in a coil creates a magnetic field that pulls an armature, allowing a low‑power control signal to switch a high‑power load.
• Loudspeaker – an alternating audio current in a voice coil produces a rapidly changing magnetic force that makes a diaphragm vibrate, reproducing the sound waveform.

Both devices illustrate the core IGCSE concepts of magnetic fields, Fleming’s left‑hand rule, and the force equation F = BIL sin θ, and they show how electromagnetism is exploited in everyday technology.