Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Physics > Electromagnetic Induction COPY > Flashcards

Electromagnetic Induction COPY Flashcards

You may prefer our related Brainscape-certified flashcards:
Physics 101 flashcards AP® Physics flashcards
1
Q

What is the circuit symbol for a DC voltage supply?

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is the circuit symbol for an AC voltage supply?

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is the equation for magnetic flux?

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the formula for flux linkage?

A

Flux linkage = Flux x Number of Turns

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What are the units of flux linkage?

A

Weber-Turns

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is the flux linkage if the perpendicular to a coil of wire with N turns and cross-sectional area A is at an angle ø to the B field with strength B?

A

Flux linkage through the coil = BANcosø

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

If there are half the number of turns on a coil how is the voltage affected?

A

The voltage is halved

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Define Faraday’s Law

A

When the flux through a loop changes, an emf is induced which is proportional to the rate of change of flux

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is the equation for Faraday’s Law?

A

emf = N∆flux / ∆t

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Describe and explain how the emf is affected if the wire is ‘cut’ down through the field.

A

There is an emf induced

The flux through the loop of wire changes

by Faradays law this causes emf.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Describe and explain how the emf is affected if the wire is held stationary in the field.

A

There is no emf induced

there is no change in flux

Faraday’s Law states that there needs to be a change in flux to induce emf

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Describe and explain how the emf is affected if the wire is ‘cut’ up through the field.

A

The flux throught the loop reduces

An emf is induced

in the opposite direction to the emf induced when the wire is cut down the field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Describe and explain how the emf is affected if the wire is ‘cut’ through the field at a greater rate.

A

By Faradays Law this will induce a greater emf

The rate of change of flux is greater

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Describe and explain how the emf is affected if more turns of the wire are cut through.

A

Using Faraday’s Law

I the number of turns increases the emf will increase

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is the equation for the emf induced in a straight wire moving at right angles to the B field?

A

Emf = BLv

(B = B field strength, L = length of wire, v = velocity)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Derive the equation Emf = BLv

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Define Lenz’s Law

A

The direction of the induced emf opposes the change producing it

It comes about as a result of conservation of energy

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What is Fleming’s Right Hand Rule ONLY used for?

A

The direction of induced emf when a wire cuts through a magnetic field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What does each finger in Flemings Right Hand rule stand for?

A

First Finger Field

thuMb = direction of Motion of wire

second finger = direction of emf

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

Explain why dropping a magnet into a coil will cause it to become an electromagnet

A
  • As the magnet falls, its field lined cut through the coil and induce an emf in it by Faraday’s Law
  • The emf causes a current to flow in the coil and so it becomes an electromagnet
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q
A
  • As the magbet falls its field lines cut through the coil and induces an emf by Faraday’s Law
  • The emf causes a current to flow which makes it become an electromagnet
  • When the magnet is in position A, the top of the coil is a north pole to oppose the motion od the magnet, according to Lenz’s Law
  • When the magnet is in position B, the bottom of the coil becomes a north pole to attract the south pole and stop it moving away
  • therefore the current changes direction
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Why must a data logger be used ot measure induced emf?

A

It can take many measurements each second and so the rapid changes in emf can be measured effectively

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Draw the graph of the emf induced against time for a magnet dropped thorugh a coil.

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

Draw the graph of the emf induced against time for a magnet dropped thorugh a coil. Also draw the curve if the magnet was dropped from a greater height.

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

When a piece of metal and a magnet are dropped through copper tubing the magnet takes much longer to fall. Why?

A
  • As the magnet falls, the B field lines around it are cut, so there is a change of flux in the tubing.
  • This induces an emf in the copper, by Faraday’s Law, which induces a current.
  • The current produces a B field which puts a force on the magnet
  • By Lenz’s Law, this force opposes the downwards motionof the magnet and so the magnet takes much longer to fall through the tube. The metal doesn’t have this resistive force.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

Why does an aluminium plate stop in a short time after it falls between the poles of a magnet?

A
  • When the plate falls between the poles of the magnet, it cuts the field lines between them and so there is a change of flux between the plates
  • This induces an emf in the plate (by Faradays Law), which causes an electric current to flow in the plate
  • By Lenz’z Law the force will be in the opposite direction to the motion and so will decelerate the aluminium plate
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

Why does an aluminium plate stop in a shorter time after it falls between the poles of a magnet compared to an aluminium pate with grooves in it?

A
  • When a plate with grooves is used, an emf is still induced
  • However current cannot flow through the grooves so the current is lower than the plate with no grooves
  • This means the force on the current from the magnetic field is lower and so there is less deceleration
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

How do you calculate the emf of an AC generator?

A

emf = BANw sin wt

w (omega) = angular velocity

B = magnetic field strength

N = number of turns

A = Area

t = time of rotation from the vertical starting position

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

How can you increase he amplitude of the emf produced by an AC generator?

A

Increase B, A, N or omega

30
Q
A

A & C

31
Q
A

B & D

32
Q
A

A & C

33
Q
A

B & D

34
Q
A

B & D

35
Q

What is a direct current?

A

Where the current flows one way round a circuit

36
Q

What is an alternating current?

A

Where the current keeps changing directions

37
Q

Draw a graph of ac current.

A
38
Q

Define the root means square of an ac voltage or current

A

The dc voltage or current which would produce the same power in a resistor

39
Q

What is the equation for Vrms?

A

Vrms = V0 / √2

40
Q

What is the equation for Irms?

A

Irms = I0 / √2

41
Q

What are the power equations for ac voltage?

A

P = Vrms x Irms

P = Irms2R

P = Vrms2 / R

42
Q

What are the equations for resistance when there is ac voltage?

A

R = Vrms / Irms

R = V0 / I0

43
Q

How do you work out voltage on a cathode ray oscilloscope?

A

Voltage = no. of divisions x volts per devision

44
Q

How would you get a taller trace on a CRO?

A

Decrease the volts per division

45
Q

Draw the magnetic field lines in wire wrapped around an iron core

A
46
Q
A
  • When the switch is pressed a current flows in coil A and producesd a magnetic field. the flux lines produced go through coil B.
  • The change in flux through B induces an emf in coil B and the voltmeter shows a reading for a short time.
47
Q
A

when the current in coil A is steady, the B field in the core will be steady.

As there is no change in flux in B, there is no emf induced and the voltmeter reads 0.

48
Q
A

The current in A drops dramatically and the magnetic field inside the iron core collapses

This causes a large negative emf to be induced in B

49
Q
A

It becomes an electromagnet.

50
Q
A

The atoms in the iron line up with the magnetic field

51
Q

Describe what happens during the floating ring experiment

A
  • The current constantly changes direction in the ac coil
  • This creates a constantly changing B field in the coil and rod
  • The changing B field in the iron core goes through the aluminium ring causing a change in flux in the ring
  • This changing flux induces an emf in the aluminium which causes a current to flow
  • The current creates a B field around the aluminium
  • The B fields from the iron core and from the current flowing through the aluminium interact to cause an upwards force in the aluminium which balances its weight
52
Q

Draw a transformer

A
53
Q

How does a transformer work?

A
  • An ac current in the primary produces a changing magnetuic field in the primary
  • The changing B field lines in the primary go through the secondary (they are linked with it)
  • There is a constant change in flux in the secondary
  • By Faraday’s Law, this will induce an ac emf in the secondary
54
Q

Why is an ac voltage needed in a transformer and not dc?

A

With a dc voltage there is no change in flux and so no p.d. is induced. There is only a tiny voltage to start as the flux is changing initially.

55
Q

What is the formula for Vs / Vp?

A

Vs / Vp = Ns / Np

Vs = secondary voltage

Vp = primary voltage

Ns = No. of turns on secondary

Np = No. of turns on primary

56
Q

Explain a step-up transformer

A

A step up transformer has more turns on the secondary coil than the primary coil to increase the voltage.

The emf flowing is very great which induces a high current.

57
Q

Explain a step-down transformer

A

A step-down transformer has less turns on the secondary coil than the primary

so the voltage is less.

58
Q

What is the power fromula for a transformer and when does it apply?

A

FOR A 100% EFFICIENT TRANSFORMER

vsIs = VpIP

so power in = power out

59
Q

What is the efficiency formula for a transformer?

A

(power out / power in) x 100

60
Q

How do you lower power losses in wires? (3)

A
  • Decrease resistance
  • Increase resistivity
  • Have a large cross-sectional area
61
Q

What is meant by the term ‘linked’?

A

A B field goes through a coil

62
Q

What are the three major causes for energy losses in transformers?

A
  • Copper losses in wires
  • Eddy currents
  • Magnetic Hysteresis
63
Q

Why are there copper losses in wires?

A
  • There are energy losses in the primary ans secondary coils due to the electrical energy being converted to heat energy because of the resistance in the wires
64
Q

How can copper losses be reduced?

A

Use wires with a low resistivity with a large cross-sectional area that have a low resistance.

Having a low resistance reduces power loss

65
Q

Why are eddy currents a cause for energy loss in a transformer?

A
  • The changing flux in the iron core induces an emf in the core which creates currents called eddy currents
  • Energy is lost as heat in the iron core
66
Q

How can we reduce the power loss in a transformer through eddy currents?

A

Use laminated strips to make the core. Current can’t flow between the strips so the current is lower meaning less heat loss.

Laminated strips stop large currents flowing.

67
Q

Why is magnetic hysteresis a cause for energy loss in a transformer?

A

Energy is required to continually change the direction of the magnetisation of the core.

68
Q

How can we reduce the power loss in a transformer through magnetic hysteresis?

A

Make the core out of an easily magnetised material.

This means there is less energy required for the change in direction of magnetism.

69
Q

Describe the melting nail experiment

A
  • The number of turns on the secondary is much less than on the primary, so the secondary voltage will be much lower than the primary voltage
  • If the transformer is 100% efficient IsVs = IpVp
  • If Vs is small then Is must be large to compensate. Since the heating effectof he current will be equal to the power and the power = I2R, then a high current means a large heating effect.
70
Q

Explain how power is transmitted and why it is efficient

A
  • The power stations produce electricity at a relatively low volatge. This is fed into a step-up transformer which increases the voltage
  • Assuming the transformer is 100% efficient, the power in = the power out
  • This means that increasing the voltage in the power lines will decrease the current
  • Since power loss = I2R, this decrease in current will mean a much lower powe rloss and less energy will be lost as heat to the atmosphere

Physics (15 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions