Applied Electricity MCQ 2

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.