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

The magnetic field produced by a current‑carrying conductor can exert a force on other conductors or magnetic materials. This principle is used in many devices, notably relays and loudspeakers.

Key Formula

The force on a straight conductor of length L carrying current I in a magnetic field of flux density B is

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

where θ is the angle between the direction of the current and the magnetic field.

How the Magnetic Effect is Used

• By arranging a coil so that the magnetic field it creates interacts with a movable iron armature, a small electric current can produce a mechanical movement.
• The movement can be used to open or close contacts (relays) or to move a diaphragm (loudspeakers).

Relays

Principle of Operation

A relay is an electrically operated switch. When a current flows through its coil, a magnetic field is produced that attracts a ferromagnetic armature, causing contacts to change state.

Construction

Operation Steps

1. Supply voltage is applied to the coil.
2. Current I flows, producing a magnetic field B.
3. The armature experiences a force \$F = B I L\$ and moves towards the core.
4. Movement either closes the NO contacts or opens the NC contacts.
5. When the coil voltage is removed, the spring returns the armature, restoring the original contact state.

Applications of Relays

• Automotive ignition systems – to switch high‑current circuits with low‑current control signals.
• Industrial control panels – to control motors, heaters, and lighting from a central control unit.
• Telecommunications – to route signals in telephone exchanges.
• Home appliances – e.g., washing machines and refrigerators for safety cut‑offs.

Loudspeakers

Principle of Operation

A loudspeaker converts an electrical audio signal into sound using the magnetic effect of a current. An alternating current in a coil (voice coil) creates a varying magnetic field that interacts with a permanent magnet, causing a diaphragm to vibrate.

Construction

Operation Steps

1. An audio signal, a varying current \$i(t)\$, is fed into the voice coil.
2. The instantaneous force on the coil is \$F(t) = B\,i(t)\,L\$ (direction depends on the sign of \$i\$).
3. This force moves the coil and attached diaphragm forward and backward.
4. The diaphragm’s motion compresses and rarefies the surrounding air, creating sound waves that correspond to the original audio signal.

Applications of Loudspeakers

• Public address systems – to amplify speech in schools, stadiums, and transport hubs.
• Home audio – televisions, radios, and music systems.
• Automotive audio – car radios and navigation systems.
• Portable devices – smartphones, tablets, and Bluetooth speakers.

Comparison of Relays and Loudspeakers

Summary

The magnetic effect of a current provides a reliable way to convert electrical energy into mechanical motion. In relays, this motion is used to control high‑power circuits with low‑power signals, while in loudspeakers the motion creates sound. Understanding these devices helps students appreciate how fundamental physics principles are applied in everyday technology.