# Electromagnetic Induction Flashcards Preview

## Physics > Electromagnetic Induction > Flashcards

Flashcards in Electromagnetic Induction Deck (29)
1
Q

Magnetic flux density and magnetic flux

A

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

2
Q

Electromagnetic induction in a rod

A

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.

3
Q

EMF in a solenoid or a flat coil

A

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

4
Q

Change in flux linkage and EMF

A

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

5
Q

A

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

6
Q

Lenz’s law

A

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

7
Q

Alternator

A

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

8
Q

Transformer

A

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

9
Q

Increasing induced EMF in a wire

A

Move the wire faster

Use a stronger magnet

Make the wire into a coil

10
Q

Root mean square value of alternating current

A

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

11
Q

Step up transformer

A

More turns on the secondary coil than on the primary coil

Thus it increases the voltage

12
Q

Step down transformer

A

Fewer turns on the secondary coil than on the primary coil

Thus it decreases the voltage

13
Q

A

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

14
Q

Back EMF

A

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

15
Q

Flemings right hand rule/Dynamo rule

A

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

16
Q

Eddy currents

A

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

17
Q

Increasing transformer efficiency

A

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

18
Q

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

A

The dissipate power as heat

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

19
Q

Inefficiencies in transformers

A

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

20
Q

Operational controls for oscilloscopes

A

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

21
Q

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

A

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

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

22
Q

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

A

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

23
Q

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

A

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

24
Q

Key units

A

Weber (Wb) 1 Wb = 1 Tm^2

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

25
Q

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)

A

See CGP textbook

26
Q

What creates the trace on an oscilloscope?

A

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

27
Q

Why is root mean square needed?

A

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

28
Q

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?

A

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

29
Q

Equation for a moving conductor in a magnetic field and emf

A

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