6.3 Electromagnetism (EM Induction, Generators, Transformers)

Question 1

What is magnetic flux defined as?

Answer 1

The product of the magnetic flux density and the area perpendicular to the flux lines.
ϕ = BAcosθ
(θ is the angle between the area and the flux lines)

Question 2

What is magnetic flux linkage?

Answer 2

The product of the magnetic flux and the number of turns in the coil.
Nϕ = BANcosθ

Question 3

What is magnetic flux linkage measured in?

Answer 3

Weber turns (Wb turns)

Question 4

What is electromagnetic induction?

Answer 4

A current is induced due to a change in the magnetic flux linkage.

Question 5

What are the ways to induce a current in a wire via EM induction?

Answer 5

Moving the wire through a non-uniform magnetic field
Passing a magnetic field through a stationary wire.
Varying magnetic field strength over time

Question 6

A wire is moved back and forth with oscillating motion in a magnetic field. What happens?

Answer 6

An alternating current is generated in the wire.

Question 7

State Faraday’s law

Answer 7

The induced e.m.f is proportional to the rate of change of magnetic flux linkage.

ϵ ∝ ΔNϕ / Δt

Question 8

State Lenz’s law

Answer 8

The induced emf is generated in a direction that so it opposes the change that produced it.

Question 9

Give an example of Lenz’s law in action.

Answer 9

If a wire is pushed downwards into a magnetic field, the current will be in a direction that produces a force to oppose its motion into the field.

Question 10

A magnet is moved towards a coil. A current is induced in the coil.

Use ideas about energy conservation to state and explain the polarity of the face of the coil nearest to the magnet.

Answer 10

There is a repulsive force. Work done against this force is transferred into electrical energy in the coil.

Question 11

A bar magnet is pushed into a solenoid. What happens?

Answer 11

The current in the solenoid will be produced in a direction so as to produce a magnetic field that opposes and cancels the field of the bar magnet.

Question 12

What is a search coil?

Answer 12

A flat coil of insulated wire connected to a galvanometer (sensitive ammeter)

Question 13

Experiment: Investigating Magnetic Flux [3]

Answer 13

Place a search coil in a known magnetic field. Quickly move it to a region of space with negligible magnetic field. This calibrates the search coil.

Assuming the coil is removed rapidly, the maximum measured current is proportional to the field strength. Since we used a known field, we can determine the constant of proportionality.

Repeat the process with the magnetic field that needs to be measured and the field strength can be calculated. By measuring the area and number of turns in the coil, magnetic flux can be evaluated from magnetic flux density.

Question 14

What does a generator do?

Answer 14

It converts kinetic energy into electrical energy

Question 15

How does a generator work? [3]

Answer 15

1) A coil of wire is placed in a constant uniform magnetic field.
2) The coil is rotated and so the area perpendicular to the magnetic field is constantly changing.
3) The changing magnetic flux linkage causes an alternating current to be induced in the wire.

Question 16

What do the two graphs of EMF/Flux linkage against time look like?

Answer 16

EMF is a regular sine curve.
Flux Linkage is a cosine curve.

Question 17

Considering a graph of EMF/Flux Linkage against time. At maximum flux linkage, what will the EMF be? Why?

Answer 17

0. The gradient of the flux linkage is 0.

Question 18

Considering a graph of EMF/Flux Linkage against time. At zero flux linkage, what is the EMF? Why?

Answer 18

The induced EMF is at a maximum. This is because the gradient of the flux linkage is at a maximum.

Question 19

What is the opposite of a generator?

Answer 19

An electric motor.

Question 20

How does an electric motor work?

Answer 20

1) A coil is placed in a uniform magnetic field.
2) A current is passed through the coil.
3) A magnetic force is produced on opposite sides of the coil in opposite directions. As the net force on the coil is zero, the forces generate a turning effect (torque of a couple).

Question 21

An electric motor is powered with a DC supply. What component do we need to prevent the coil of wire from turning then stopping once the area is perpendicular to the field lines? How does it work?

Answer 21

A split ring commutator. It reverses the direction of the current once the coil has rotated through 180 degrees. This maintains the constant turning effect.

Question 22

An electric motor is powered with a AC supply. What component do we need to prevent the wires attached to the ring from becoming tangled?

Answer 22

Commutator.

Question 23

What are transformers made up of?

Answer 23

A laminated iron core, with a primary coil (input) and a secondary coil (output)

Question 24

How does a transformer work?

Answer 24

An alternating current flows through the primary coil. This induces a varying magnetic flux in the iron core. This change in magnetic flux induces a current in the secondary coil (in agreement with Faradays law).

Question 25

Why do we place the primary and secondary coils of a transformer on an iron core?

Answer 25

To ensure that the majority of the electrical energy is transferred between the primary and secondary coils.

Question 26

What is the transformer equation?

Answer 26

Np / Ns = Vp / Vs = Is / Ip

Question 27

When a transformer is 100% efficient, what is the input power equal to?

Answer 27

The output power.

Question 28

In reality, transformers are not 100% efficient. What is energy lost as?

Answer 28

Heat in the iron core. Sound

Question 29

What are step up transformers used for?

Answer 29

Reducing power losses in the transmission of electrical energy.

Question 30

How are step up transformers used in reducing power loss?

Answer 30

The step up transformer increases the voltage prior to long distance transmission. This reduces the current in the wire, which greatly reduces power loss.
P = I²R

Question 31

What are step down transformers used for?

Answer 31

Decreasing the voltage from transmission cables, so that it is safer to distribute to consumers.

Question 32

Mobile phones require little voltage to be charged, but mains supply is 230V. What do mobile phone chargers have to lower the voltage supplied to the phone?

Answer 32

A step down transformer.

Question 33

How can we investigate the turn ratio equation of a transformer?

Answer 33

By varying the number of turns in the primary and secondary coils, and measuring the voltages across the input and output using a voltmeter (preferably oscilloscope which can accurately measure changing voltage).

Question 34

How can we investigate the efficiency of a transformer?

Answer 34

Measure the current in each coil with an ammeter and variable resistor (rheostat). The rheostat is used to vary the current at a constant voltage. The efficiency can be estimated by calculating the ratio of output power to input power.

E = (Pout / Pin) x 100 = IpVp / IsVs x 100

Question 35

How can transformers be made more efficient?

Answer 35

Use lower resistance wires in the coils. This reduces power loss to heating from resistance.

Use a laminated iron core. This channels magnetic flux more efficiently by reducing eddy currents which cause power loss to magnetic heating.

Use a soft iron core. It is easier to magnetize and demagnetize therefore they are more susceptible to changing magnetic fields, improving the overall efficiency.