Electromagnetic Induction

Question 1

Magnetic flux density and magnetic flux

Answer 1

It’s useful to think of magnetic flux density as the number of field lines per unit area

And magnetic flux as the total number of field lines in a given area

So that makes magnetic flux the product of the area swept out by a coil in a magnetic field and the magnetic flux density

Magnetic flux = BA the product of magnetic flux density and the area normal to the field

Question 2

Electromagnetic induction in a rod

Answer 2

If there is motion of a conducting rod relative to a magnetic field, then the electrons will experience a force

This causes them to accumulate at one end

Hence inducing an EMF due to the p.d.

Question 3

EMF in a solenoid or a flat coil

Answer 3

Move the coil towards or away from the poles of the magnet

Move the magnet towards or away from the coil

EMF is induced by the magnetic field changing as it passes through the magnet

Question 4

Change in flux linkage and EMF

Answer 4

A change in flux linkage of 1 Weber per second will induce an electromotive force of 1 Volt in a loop of wire

Question 5

Faraday’s law

Answer 5

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

Question 6

Lenz’s law

Answer 6

The induced e.m.f is always in such a direction as to oppose the change that caused it

The direction of induced current is always such as to oppose the change that causes the current

The induced current generates its own magnetic field that opposes the magnetic field that caused it

Use the right hand grip rule to find what direction the current will go in if the direction of the magnetic field is known

Question 7

Alternator

Answer 7

A generator of alternating current

Slip rings and brushes to connect the coil to an external circuit

The output voltage and current change direction with every half rotation

Question 8

Transformer

Answer 8

A device that uses electromagnetic induction to change the size of the voltage for an alternating current

Alternating current in the primary/input coil coil produces magnetic flux

Changing magnetic field passes through the iron core to the output coil where it induces an alternating voltage of the same frequency as the input

Question 9

Increasing induced EMF in a wire

Answer 9

Move the wire faster

Use a stronger magnet

Make the wire into a coil

Question 10

Root mean square value of alternating current

Answer 10

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

Question 11

Step up transformer

Answer 11

More turns on the secondary coil than on the primary coil

Thus it increases the voltage

Question 12

Step down transformer

Answer 12

Fewer turns on the secondary coil than on the primary coil

Thus it decreases the voltage

Question 13

Flux linkage

Answer 13

The product of magnetic flux passing through the coil and the number of turns in the coil

It can be visualised as the volume of the magnetic field passing through the coil multiplied by the strength of the field

It’s a measure of the amount of a magnetic field passing through the coil

= BAN

Question 14

Back EMF

Answer 14

EMF induced in the spinning coil of an electric motor or in any coil in which the current is changing

It acts against the applied pd

Question 15

Flemings right hand rule/Dynamo rule

Answer 15

Used to determine the direction of an induced current

Thumb - direction of motion of conductor

First finger - direction of field

Second finger - the direction of the induced current

Question 16

Eddy currents

Answer 16

Induced currents in transformer cores and other metal parts of AC machines

Question 17

Increasing transformer efficiency

Answer 17

Low resistance windings (e.g. thick copper wires) to reduce power wasted due to the heating effect of the current

Laminated core which consists of layers of iron and layers of insulation - reduces eddy currents so the magnetic flux is as large as possible, also reduces the heating effect of induced currents in the core as currents cannot flow in a discontinuous conductor or when the resistance is very high

A core of soft iron - easily magnetised and demagnetised, reducing power wasted through repeated de/magnetisation

Design transformers so could are as close as possible - not all magnetic flux is transferred from primary to secondary coil

Question 18

Why don’t we want eddy currents in the transformer core?

Answer 18

The dissipate power as heat

They create opposing magnetic fields reducing rate of change of flux linkage

Question 19

Inefficiencies in transformers

Answer 19

Resistance in coils

Heat loss through eddy current

Energy wasted demagnetising and magnetising the core

Not all the magnetic flux is transferred from primary to secondary

Question 20

Operational controls for oscilloscopes

Answer 20

Focus - making the wave clearer

Brilliance - brightness

Variable - time base e.g. ms per division, change time base to fit more waves on

Volts/division - increases the number of volts per box can be used to make the wave look larger, decreases error

Question 21

Relationship between induced emf and magnetic flux/flux linkage for a rotating coil in a magnetic field

Answer 21

when induced emf = max, magnetic flux/flux linkage = 0

when induced emf = 0, magnetic flux/flux linkage = max

Question 22

Conservation of energy in dropping a bar magnet through a copper tube

Answer 22

Induces a magnetic field to oppose the motion of the magnet, the work done against this repulsive force is transferred to electrical energy in the coil

Question 23

Why does the magnitude of magnetic flux vary as the coil rotates?

Answer 23

The component of the magnetic flux density, B, perpendicular to the area of the coil changes

Flux linkage is greatest when the area of the coil is perpendicular to the magnetic field

It varies depending on cos theta
Magnetic flux is zero when the coil is parallel to B and maximum when coil is perpendicular to B

Question 24

Key units

Answer 24

Weber (Wb) 1 Wb = 1 Tm^2

Tesla (T) = N/Am NA^-1 m^-1

Question 25

Draw the following graphs:

(a) a conductor moving at a constant speed perpendicular to a uniform magnetic field (flux linkage against time)
(b) a wire coil rotating at a constant speed in a uniform magnetic field (flux linkage against time and emf against time)

Answer 25

See CGP textbook

Question 26

What creates the trace on an oscilloscope?

Answer 26

An electron beam moves across the screen to represent a sinusoidal wave produced by an alternating current

Question 27

Why is root mean square needed?

Answer 27

The voltage of an AC power supply is below the peak value most of the time - so the peak value can’t be used in comparison to a DC power supply

To compare with DC, an average is needed but a normal average won’t work as the positive and negative values will cancel out

So we use the root mean square values of current and voltage

Question 28

Why does the reading on a top pan balance increase when there is a uniform magnetic field and a wire perpendicular to this field (clamped in place) on the balance?

Answer 28

There is an interaction between the magnetic field and the current-carrying wire

The wire experiences an upwards force due to the magnetic field

Hence the wire exerts an equal and opposite reaction force on the magnets - increasing the balance reading

Question 29

Equation for a moving conductor in a magnetic field and emf

Answer 29

emf = BLv

Where L is length of the conductor passing through the field and v is velocity at which the conductor moves through the field