Applied Electricity MCQ 2

Study Notes

Electromagnetic Relays

An electromagnetic relay is an electromechanical switch made up of an electromagnet and a set of contacts.
It operates by closing a switch in the primary circuit, turning on the electromagnet, attracting the armature, and forcing the contacts together in the secondary circuit.
Uses of electromagnetic relays include controlling high-current circuits with low-current signals, controlling high-voltage circuits with low-voltage signals, and as protection relays by opening and closing circuit breakers.

Electromagnetic Forces

A current-carrying conductor experiences a force in a magnetic field.
This force can be demonstrated by completing an electrical circuit using thin tin foil and placing it in a magnetic field.
A simple d.c.motor is based on this principle, where a coil of wire rotates between the poles of a magnet, creating torque.

Moving-Coil Devices

A moving-coil loudspeaker is based on the principle that a current-carrying conductor in a magnetic field experiences a force.
It consists of a coil of wire carrying current, wrapped around a cardboard tube and positioned in a magnetic field, creating a linear force.
A moving-coil galvanometer is also based on this principle and is used to measure current.

Electromagnetic Induction

Electromagnetic induction is where an emf is induced by a change in the magnetic flux linking a circuit.
An induction coil is an electronic device used to get a very high voltage from a low voltage d.c. source.
It is made up of a d.c. supply of low voltage, a primary coil, a secondary coil, and a soft iron core.
Induction coils are used in car spark plugs and electric fences.

Alternating Current

An alternating current (a.c.) is an electric current whose direction reverses periodically.
It is produced by an alternating voltage.
A simple a.c. generator is based on the principle that an emf is induced in a conductor as it moves through a magnetic field.
Induction motors are used in fans, fridges, vacuum cleaners, washing machines, etc.

Transformers

A transformer is an electrical device used to change the voltage of a.c. power supplies.
Transformers allow for voltage to be stepped up and stepped down, which is useful in applications such as high voltage power lines.

Semiconductor Devices

A diode is an electronic device that allows current to flow in one direction only, made from semiconductor material.
It is used in a.c. rectification.
An LED is a semiconductor device that emits light when current flows through it.
A photodiode is a semiconductor device used to detect the presence of light or to measure its intensity, or as part of a light-sensitive relay.

Description

Learn about the components and operation of an electromagnetic relay in an electrical circuit. Understand how current flow in one circuit can control the current in another circuit using this essential electromechanical switch.