Electromagnets, Solenoids, Relays and Logic

An electromagnet is a type of non-permanent magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current passing through the wire creates a magnetic field which is concentrated in the hole through the centre of the coil. The magnetic field disappears when the current is turned off. The wire turns are often wound around a magnetic core made from a ferromagnetic or ferrimagnetic material such as iron; the magnetic core concentrates the magnetic flux and makes a more powerful electro-magnet.

A solenoid using a moving magnetic core, or sometimes a coil and a stationary magnet, to produce linear or radial motion of a part(in the case of a hard disk drive). The motion of the part being used to do some work or function. If the core is a permanent magnet then the device is usually termed a linear motor.

In the case of a relay, or contactors, they contain a coil that mechanically actuates electrical contacts via a moving magnetic material (arm or carrier).

In the case of a voice-coil of a speaker, alternating current from program material flows through the voice-coil and causes a diaphram to move that translates the content to sound pressure so it can be heard by the diaphram in one's ear.

Relays with small control current may be used to actuate electric circuits with higher current, and/or higher voltage, and are used as isolated switches that may be used to switch mains or high voltage equipment from small voltage outputs, e.g. micro controllers, computers or battery operated equipment. (With the advent of MOSFETs and other silicon switching devices relays have been replaced with more reliable and faster solid-state devices.)

The following link is a very education presentation of electromagnetic devices:

• Solenoids and Relays https://www.youtube.com/watch?v=fdj_g_1tj4c