Electromagnetic Induction

Question 1

What is electromagnetic induction?

Answer 1

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

Question 2

What happens when a magnet is moved near a wire?

Answer 2

A small current passes through the wire. This happens because an emf is induced in the wire. If the wire is part of a circuit, the induced emf forces electrons around the circuit. The emf becomes 0 when relative motion between magnet and wire stops. But no emf is induced if the wire is parallel to the magnetic field lines as it moves through the field.

Question 3

What is induced emf increased by?

Answer 3

Moving the wire faster, using a stronger magnet, making the wire into a coil and pushing the magnet in or out of the coil.

Question 4

Why does electromagnetic induction occur?

Answer 4

A wire has lots of free electrons in it. When it is moved in a magnetic field, the electrons experience a force at right angles to their motion. This forces the electrons to one end and away from the other end so one end is negative and the other is positive. In this way, an emf is induced in the wire.

Question 5

Fleming’s right hand rule

Answer 5

Applies to generators. Thumb is direction of motion of conductor. First finger is the direction of the magnetic field. Second finger is direction of the current (the way a positive charge moves).

Question 6

Describe the magnetic field of a long coil (solenoid) connected to a battery with a current passing through it.

Answer 6

A magnetic field is produced in and around the solenoid. The field lines do through the coil and loop around the outside back to where they started from. The current passes anti-clockwise round the north pole end and clockwise round the south pole end.

Question 7

What is Lenz’s law

Answer 7

When a current is induced by electromagnetic induction, the direction of the induced current is always such as to oppose the change that causes the current.

Question 8

What happens when a north pole end of a magnet enters a coil of wire?

Answer 8

A current is induced. The current flows anti-clockwise to create a north pole at the end the north pole of the magnet enters.

Question 9

What is magnetic flux

Answer 9

It is the flux density of a magnetic field multiplied by an area perpendicular to the field. Unit weber (Wb)

Question 10

Define magnetic flux linkage

Answer 10

The magnetic flux linkage through a coil is equal to the magnetic flux density times area perpendicular to field times number of turns in the coil.

Question 11

What is the flux linkage through a coil at different angles to the field?

Answer 11

When the field is perpendicular to the coil face the flux linkage is BAN. When the coil is turned through 180 degrees from this position the flux linkage is -BAN. When field is parallel to the coil area the flux linkage is 0 as no field lines pass through the coil area.

Question 12

What does theta represent in the flux linkage equation?

Answer 12

The angle between the magnetic field and the normal to the coil face

Question 13

What is Faraday’s law of electromagnetic induction?

Answer 13

The induced emf in a circuit is equal to the rate of change of magnetic flux linkage through the circuit.

Question 14

Magnitude of induced emf for a moving conductor in magnetic field.

Answer 14

emf=Blv

Provided the conductor cuts across the field lines perpendicularly.

Question 15

Graph of emf against time for alternating current generator.

Answer 15

Time 0 defined when coil is perpendicular to field lines. Sinusoidal curve starting at origin with amplitude of the peak emf and the time for one cycle is the time period of the cycle.

Question 16

What is peak voltage or current?

Answer 16

The maximum voltage or current in either direction for an alternating current.

Question 17

What is the peak to peak value?

Answer 17

The difference between the peak value one way and the peak value in the opposite direction (peak value x2).

Question 18

What is the root mean square value of the alternating current?

Answer 18

The value of direct current that would give the same heating effect as the alternating current in the same resistor.

Question 19

What is the mean power over a full cycle for a sinusoidal current?

Answer 19

Half the peak power.

Question 20

What do the I0 V0 stand for in the rms equation?

Answer 20

Peak current or peak voltage.

Question 21

What is back emf?

Answer 21

The emf induced in the spinning coil of an electric motor or in any coil in which the current is changing. It acts against the change of applied pd to the motor in accordance with Lenz’s law.

Question 22

Equation for electrical power supplied by source pd to the motor.

Answer 22

Electrical power supplied by source = electrical power transferred to mechanical power + electrical power wasted due to circuit resistance.

IV= Iε + I^2xR

Where ε is induced emf (back emf) that acts against the pd applied to the motor.

Question 23

How oscilloscopes are used.

Answer 23

Connected in parallel to the component. You can adjust the scale of the y and x axes using the voltage gain dial and the time base control dial. If the current is dc, it shows a straight line above rest position. If the current is alternating, it shows a sinusoidal curve.

Question 24

Graph of power against time for alternating current

Answer 24

Sinusoidal curve but troughs just touch x-axis.

Question 25

Describe a transformer and how it works

Answer 25

It changes an alternating pd to a different peak value. There is a primary coil and a secondary coil insulated from each other but have the same iron core. When the primary coil is connected to a source of alternating pd, an alternating magnetic field is produced in the core which passes through the secondary coil so an alternating current is induced in the secondary coil by the changing magnetic field.

Question 26

What is a step-up transformer?

Answer 26

It has more turns on the secondary coil than on the primary coil and therefore the rms pd across the secondary coil is greater than that applied to the primary coil.

Question 27

What is a step-down transformer?

Answer 27

It has more turns on the primary coil than on the secondary coil and therefore the rms pd across the secondary coil is less than that applied to the primary coil.

Question 28

Efficiency of transformer equation

Answer 28

Efficiency is power delivered by secondary coil divided by the power supplied to the primary coil times 100.

Question 29

What are eddy currents?

Answer 29

Induced currents in the metal parts of alternating current machines.

Question 30

How is efficiency of transformers increased?

Answer 30

Use low-resistance windings to reduce power wasted due to the heating effect of current. Core is laminated (consists of layers of iron separated by layers of insulator) to reduce eddy currents so magnetic flux is as high as possible and heating effect of eddy currents in core is reduced. Core is ‘soft iron’ which is easily magnetised and de-magnetised so reduces power wasted through repeated magnetisation and de-magnetisation.

Question 31

How can current fit in the transformer rule?

Answer 31

Replace Vs/Vp with Ip/Is

Question 32

Why is a high voltage used in the transmission of electrical power over long distances?

Answer 32

The current needed to deliver a certain amount of power is reduced if voltage is increased so the power wasted due to the heating effect of the current through the cables is reduced.

Question 33

How to convert peak alternating current or pd to rms value

Answer 33

Divide by root 2

Question 34

Why can’t you use a transformer with direct current?

Answer 34

The current supplied is constant so no alternating magnetic field is induced in the core. Therefore there is no relative motion between a magnetic field and the secondary coil so no emf is induced in the secondary coil.

Question 35

How to see how induced emf varies with angle between coil face and magnetic field

Answer 35

Search coil placed in centre of larger coil and connected to oscilloscope. Large coil connected to ac voltage supply. Vary angle between normal to coil face and plane of search coil. Measure peak to peak value on oscilloscope to use for induced emf value. Graph of induced emf against cos of angle should be straight line through origin.