Electromagnetic Induction

Question 1

Define electromagnetic induction

Answer 1

The generation of an emf when the magnetic flux linkage through a coil changes or a conductor cuts across magnetic field lines

Question 2

For a magnet moved near a wire to induce an emf, how can this emf be increased?

Answer 2

• Moving the magnet faster
• Using a stronger magnet
• Making the wire into a coil and pushing the magnet in and out of the coil

Question 3

For a source of emf, state the equation showing the rate of energy transfer to the component

Answer 3

Rate of Energy Transfer = Induced emf x the current

Question 4

Describe electromagnetic induction for the movement of a metal rod through a magnetic field

Answer 4

When electrons move across a magnetic field, they experience a force at right angles to their direction of motion
Since a metal rod contains free electrons, when it is moved across a magnetic field, the electrons feel a force and are moved to one end, thus creating a charge at either end.

Question 5

State the dynamo rule

Answer 5

The dynamo rule (AKA Fleming’s right-hand rule) shows that the direction of the induced current is opposite to the direction of flow of electrons

Question 6

What does each finger in Fleming’s right-hand rule represent?

Answer 6

ThuMb - Motion of conductor
First finger - Field
SeCond finger - Current

Question 7

In which direction does current flow round the North pole end of a solenoid?

Answer 7

Anticlockwise

Question 8

Define solenoid

Answer 8

A cylindrical coil of wire acting as a magnet when carrying electric current

Question 9

State Lenz’s law

Answer 9

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

Question 10

Explain Lenz’s law in terms of the conservation of energy

Answer 10

Since energy is always conserved, the induced current could never be in a direction to aid the change that caused it; that would mean producing electrical energy from nowhere

Question 11

Define magnetic flux

Answer 11

Magnetic flux is the product of the average magnetic field (magnetic flux density, B) times the perpendicular area that it penetrates (A)
Φ = BA

Question 12

Give the unit for magnetic flux

Answer 12

Weber (Wb) = 1 Tm²

Question 13

State Faraday’s law of electromagnetic induction

Answer 13

The induced emf in a circuit is equal to the rate of change of flux linkage through the circuit
ε = - NΔΦ / Δt

Question 14

Define magnetic flux linkage

Answer 14

The magnetic flux through a coil of N turns
Flux linkage = NΦ = NBA
where B is the magnetic flux density perpendicular to area A

Question 15

Give the equation for the magnetic flux linkage when:

i) The magnetic field is along the normal to the coil face
ii) The coil is turned through 180°
iii) The magnetic field is parallel to the coil area

Answer 15

i) = NΦ = BAN
ii) = -NΦ = -BAN
iii) = 0, since no field lines pass through the coil area

Question 16

Give the equation for the emf induced for a moving conductor of length L through a magnetic field

Answer 16

Induced emf, ε = BLv

Question 17

Give the equation for the emf of a alternating current generator where ε₀ is the peak emf and the coil rotates at a steady frequency

Answer 17

ε = ε₀ sin2πft
where t is the time after θ = 0
therefore: ε = ε₀ sinωt
since the angular velocity of the coi, ω = 2πf

Question 18

For a coil spinning at a steady frequency f, give the equation for the angle from the normal, θ, after time t

Answer 18

θ = 2πft

Question 19

Describe how a spinning coil can be used as a DC generator

Answer 19

By replacing the 2 slip rings of an AC generator with a split-ring. The emf does not reverse its polarity because the connections between the split-ring and the brushes reverse every half cycle

Question 20

For a rotating coil, when is the rate of change of flux the greatest?

Answer 20

When the flux through it is 0

Question 21

Define back emf

Answer 21

Where an emf is induced in the spinning coil of an electric motor because the flux linkage through the coil changes. The emf induced acts against the pd applied to the motor in accordance to Lenz’s law
At any instant:
V - ε = IR
where ε is the back emf, V is the pd applied to the motor, I is the current through the motor coil and R is the circuit resistance

Question 22

Give the relationship between the induced emf (back emf) and the speed of rotation of the motor and thus the current also

Answer 22

The induced emf is proportional to the speed of rotation of the motor, therefore the current changes as the motor speed changes

Question 23

State the equation showing the electrical power for the back emf of a motor

Answer 23

Electrical power supplied by source = electrical power transferred to mechanical power + electrical power wasted due to circuit resistance
IV = Iε + I²R

Question 24

Define transformer

Answer 24

An apparatus for reducing or increasing the peak voltage of an alternating current

Question 25

Describe a transformer and how it works

Answer 25

Any transformer consists of 2 coils: primary and secondary
When the primary coil is connected to a source of alternating pd, an alternating magnetic field is produced in the core. This field passes through the secondary coil so an alternating emf is induced in the secondary coil by the changing magnetic field

Question 26

Describe a step-up transformer

Answer 26

A transformer with more turns on the secondary coil than on the primary coil. So the secondary voltage is stepped up compared with the primary voltage
(Ns > Np so Vs > Vp)

Question 27

Describe a step-down transformer

Answer 27

A transformer with fewer turns on the secondary coil than on the primary coil. So the secondary voltage is stepped down compared with the primary voltage
(Ns

Question 28

Give the transformer rule

Answer 28

Vs / Vp = Ns / Np

Question 29

State and explain 3 reasons why transformers are almost 100% effective

Answer 29

1) Low-resistance windings to reduce power wasted due to the heating effect of the current
2) A laminated core which consists of layers of iron separated by layers of insulator prevents eddy currents (induced currents in the core) so that the magnetic flux is as high as possible
3) A core of ‘soft iron’ which is easily magnetised and demagnetised, which reduces power wasted through repeated magnetisation and demagnetisation of the core

Question 30

Give the equation for the efficiency of a transformer

Answer 30

Efficiency = Power delivered by secondary coil / power supplied to primary coil
Efficiency = IsVs / IpVp x 100%

Question 31

Give the equation for the electrical power supplied to the primary coil for an almost 100% efficient transformer

Answer 31

Power supplied to 1° coil = Power supplied by 2° coil

Question 32

Explain why electricity is carried round the UK in a grid system

Answer 32

Because the transmission of electrical power over long distances is much more efficient at high voltage