Magnetism

Question 1

What is a magnetic field?

Answer 1

A magnetic field is a force field surrounding a magnet, or current carrying wire, which acts on any other magnet or current-carrying wire placed in the field.

Question 2

What is a line of force of a magnetic field / magnetic field line?

Answer 2

A Line of force of a magnetic field is a line along which a ‘free’ north pole would move in the field, acted on by no other forces than that of the field.

Question 3

What is the motor effect?

Answer 3

The motor effect is the force on a current-carrying conductor due to a magnetic field.

Question 4

What is the magnetic flux density?

Answer 4

The magnetic flux density, B, of the magnetic field (the ‘strength’ of the magnetic field) is defined as the force per unit length per unit current on a current-carrying conductor at right angles to the magnetic field lines.

Question 5

unit of magnetic flux density

Answer 5

Unit of magnetic flux density is the Tesla which is 1 N / m / A - i.e. A magnetic flux density of 1 Tesla is that which exerts a 1 Newton force on a 1 metre long conductor carrying a current of 1 Amp.

Question 6

Induced emf

Answer 6

Induced emf: emf generated when magnetic flux linkage through a coil changes, or a conductor cuts across B-field lines.

Question 7

Lenz’s Law

Answer 7

1.Lenz’s Law states that the direction of the induced current is always such as to oppose the change that causes the induced current.

Question 8

Magnetic flux

Answer 8

This product BA of the area swept out (=A m2) and the magnetic flux density component normal to the area (=B / Tesla), is called the magnetic flux ɸ / Wb

Magnetic flux ɸ = BA

Question 9

Unit of Magnetic flux and relationship to B

Answer 9

Unit of magnetic flux is the weber (Wb) = 1 T m2.
Flux density, B, in teslas, is the flux per unit area passing normally through the area.
.: 1 tesla = 1 weber per square m : T = Wb m-2.

Question 10

What is magnetic flux linkage Nɸ?

Answer 10

Magnetic flux linkage through a coil of turns = Nɸ = N(BA) where B is the magnetic flux density perpendicular / normal to area A.

Unit of magnetic flux and therefore magnetic flux linkage is the weber (Wb).

Weber = Volt x Second

Question 11

Faraday’s Law

Answer 11

Faraday’s Law of electromagnetic induction states that the induced emf in a circuit is equal to the rate of change of flux linkage through the circuit.

Question 12

Define back emf.

Answer 12

1 Emf induced in the spinning coil of an electric motor or in any coil in which the current is changing (e.g. the primary coil of a transformer).
2. A back emf acts against the applied pd (Lenz).