Module 6 - 6.3.3 Electromagnetism

Question 1

Magnetic Flux Linkage

Answer 1

Nɸ where
- N = number of coils of wire (turns)
- ɸ = magnetic flux (Wb)

Question 2

Electromagnetic Induction is

Answer 2

the production of an emf across a conductor during its exposure to a changing magnetic flux

Question 3

An induced emf causes an

Answer 3

induced current in the conductor

Question 4

In order to get electrical energy out

Answer 4

work must be done on the system

Question 5

Electromagnetic Induction involves (2)

Answer 5

• moving a coil relative to a magnet in order to get a change in magnetic flux in the region around the circuit
• the system will always try to oppose the change made to it (e.g. moving a magnet towards/away from a solenoid)

Question 6

Fleming’s Right Hand Rule

Answer 6

• thuMb = direction of motion of wire relative to field
• First finger = direction of field
• seCond finger = direction of induced current (conventional)

Question 7

Faraday’s Law

Answer 7

Emf = -Δɸ/Δt

Question 8

Lenz’s Law

Answer 8

the direction of induced EMf is in a direction that opposes the flux change that causes it

Question 9

A generator is a device used to

Answer 9

generate electricity in which the work done to turn the coil within the magnetic field is transferred to electrical energy

Question 10

Alternating Current (a.c.)

Answer 10

electrical current that reverses its direction with a constant frequency
UK mains = 50Hz

Question 11

In a simple a.c. generator (3)

Answer 11

• The rectangular coil is forced to rotate within the uniform magnetic field of the permanent magnet, producing a constantly changing flux linkage through the coil, in turn inducing an alternating emf and so an a.c.
• The two slip rings connected to the two ends of the coil rotate with the coil
• The brushes press against the slip rings and allow electrical contact from the slip rings to an external circuit

Question 12

Generator: when the coil is in its vertical position

Answer 12

there is no induced emf as the rate of change of flux linkage is zero

Question 13

Generator: when the coil is in its horizontal position

Answer 13

there is a maximum induced emf as the rate of change of flux linkage is maximum

Question 14

Transformers consist of

Answer 14

two coils of insulated wire around a core of easily magnetised (soft) iron

Question 15

To induce a current in the secondary coil:

Answer 15

• An alternating p.d. is applied to the primary coil
• This creates an alternating magnetic field
• The flux is linked to the secondary coil
• The alternating flux through the secondary coil induces an emf

Question 16

Current will cause (2)

Answer 16

• a heating effect in the core
• as current ∝ induced voltage

Question 17

Transformers are used in the National Grid to

Answer 17

lower the current for transfer of electricity in order to reduce energy lost through heating

Question 18

Transformers are often laminated in order to

Answer 18

minimise energy lost by heating in the wire’s resistance

Question 19

Transformers are only 100% efficient when

Answer 19

• the resistance of the coils is negligible
• since energy is dissipated by heating in the wire’s resistance