Brainscape's Knowledge GenomeTM

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


See full index

A LEVEL PHYSICS > Magnetic fields + Induction > Flashcards

Magnetic fields + Induction Flashcards

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

Magnetic field in a bar magnet

A

Strongest at its ends - “north and south” seeking poles

Field line/line of force is a line along which a north pole would move in the field

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

Why is the North pole south

A

Magnet needle points to North pole, which means its actually a south seeking pole

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

Motor effect

A

When a current carrying wire is placed at a non zero angle to the lines of force of a magnetic field, it experiences a force due to the field, which is perpendicular to the lines of force and wire.

F=BILsinθ where θ is the angle between the lines of force of the field and the wire

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

Explaining T = BAncostheta

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

Right hand grip rule

A

Curl hand n point thumb in direction of CONVENTIONAL current, curve of fingers shows direction of magnetic field produced by moving current

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

Magnetic field inside a solenoid

A

Assumed to be uniform

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

Faraday’s law

A

Magnitude of induced emf is directly proportional to the rate of change of magnetic flux linkage

ε = -ΔNΦ/Δt (-ΔNBAcosθ/Δt)

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

Importance of negative in Faraday’s law

A

Indicative of Lenz’ law - direction of induced emf acts to oppose the change that caused it.

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

Lenz law w relation to energy conservation (using bar magnet in coil example)

A

If coil didn’t repel magnet, it would attract and accelerate the magnet - energy from nothing.
Moving magnet requires mechanical work - equals electrical energy produced in the coil. If pole wasn’t opposite, no conservation

Also if poles were opposite, wld pull magnet in faster, which in turn wld generate a greater emf, energy from nowhere

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

Why does a current carrying wire experience a force in a magnetic field

A

The electrons are pushed to one side by the force of the field.

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

F=BIL for charged particle leading to F=Bqv

A

L = vt - movement of charge in time t - equivalent to flow of current Q/t in a wire length L

For a charged particle moving across a uniform magnetic field at a speed v perpendicular to the lines of force of the field, force on the particle given by F=Bqv

(Bqvsintheta since u want component of motion perpendicular to lines of force)

Field cld be non uniform

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

Explanation of why hall voltage is proportional to B

A

Hall probe - slice of semiconducting material placed in a circuit, magnetic field with lines of force perpendicular to slice. Electrons are deflected down/up to one side of the slice - causing a potential difference (Hall voltage), once is created, force due to mg field = force due to electric field created by hall voltage – QV/d = BQv (no deflection of oncoming electrons)
where d is distance between top and bottom slice.

Vh = Bvd - for a constant current, v is constant so Vh proportional to B

Edit to clarify condition that hall voltage is reached

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

Conditions for uniform circula acceleration

A

Particle must be moving in a circle - net acceleration towards the centre of gyration

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

Electron gun (Thermionic emission)

A

Electrically heated filament near a positively charged anode - attracts electrons emitted by the filament (thermionic emission).
Electrons pass through small hole in anode to form beam. Greater pd between anode and filament - higher speed of electron when anode reached so faster beam (more power nhf)

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

Cyclotron + why T is constant (+ exception)

A

https://www.youtube.com/watch?v=m2jp0klZHEE

Time taken to complete a semi circle = πr/v = mπ/BQ

(Provided that the speed stays much less than the speed of light c)

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

Electromagnetic induction + induced emf rules

A

Occurs when a current carrying conductor cuts across the lines of force of a magnetic field, if the wire forms a closed circuit, the induced emf forces electrons around the circuit.

No emf induced in wire if wire is parallel to lines of force as it moves as it moves through the field. Wire must cut across lines of force.

When relative motion ceases, induced emf is 0

Rate of energy transfer from source of emf to other components of the circuit = induced current x emf

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

How to increase size of induced emf

A
  • Move wire faster
  • Use stronger magnet
  • Making wire into coil, and
    pushing magnet in and out
    through it
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Right hand dynamo rule

A

First finger field
Second finger current
Thumb - direction of motion of conductor

riGht for Generator

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

Magnetic field inside solenoid assumed to be

A

Uniform

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

Lenz’ law

A

Direction of induced current is always to oppose the change that causes the current

21
Q

Calculating induced size of induced emf for a conductor length L moving a distance s in time t perpendicular to a uniform magnetic field

A

Motion relative to mg field creates induced current, which produces an opposing force BIL, in order to continue at same speed, there must be an opposing force = BIL. Work done by opposing force through a displacement s = BILs, as W=VIT
ε = BIΔs/Δt

LΔs = area swept out so
ε = BA/Δt

22
Q

Magnetic flux (φ) + flux linkage

A

Product of B perpendicular to area swept out by conductor = BAcosθ
(θ is the angle between lines of force and NORMAL to surface swept out)
Unit is the “weber” = Tm^2
Flux linkage through a coil of N turns =NBAcosθ
Note that flux density is flux per unit area passing at right angles through the area

ε = Δφ/t

23
Q

Different cases for flux linkage

A

Max (NBA) when magnetic field is perpendicular to coil face
when turned through 180, is -NBA.
When magnetic field is parallel to coil face - NBA = 0 as no field lines pass through the coil area.

24
Q

Faraday’s law for electromagnetic induction

A

ε = -NΔφ/t

25
Q

All forms of Faraday’s law

A

consider a conductor length l part of a complete circuit cutting ppdly lines of flux density B
Experiences force BIL due to carrying current, opposing motion. Work done = FΔs = BILΔs. Charge transfer in this time is IΔt - using ε = W/Q
ε=BAΔ/t (A = l.s)

For a moving conductor, ε=BLΔs/t === BLv

for a moving magnetic field

ε= ANΔB/t where N is the number of turns on coil

26
Q

Explaining how emf is induced in a fixed coil in a changing magnetic field

A

If current in solenoid changes, emf is induced as magnetic field through the coil changes, so there is a change in flux linkage, inducing an emf.

NΦ = BAN - Suppose flux linkage changes in time Δt to from B to (B+ΔB)

Magnitude of induced emf = NΔΦ /Δt = ANΦΔB/Δt

AS B IS PROPORTIONAL TO CURRENT IN SOLENOID, magnitude of induced emf is proportional to the RATE OF CHANGE OF CURRENT - just remember

27
Q

Rate of change of flux per second for a rectangular coil length l width w entering a uniform magnetic field

A

Time taken for coil to fully enter field (from front face touching) = w/v. During this time, flux linkage increases from 0 to BNlw

therefore rate of change of linkage = BNLv (as coil enters the field)

Once coil is completely in the field, flux linkage doesn’t change, so induced emf is zero. (Induced emf from leading side is cancelled by trailing) [BAcosθ isn’t changing]

28
Q

How to make a dc generator

A

Swap SLIP rings with a SPLIT ring - emf doesn’t change polarity as connections between split rings and contacts switch every half cycle

29
Q

Emf as a function of angle of coil as time

A

ε=ε₀sin(ωt)
ε₀ is peak emf

Flux = BANcosωt ω is with frequency of rotation of coil

induced emf a maximum when coil cuts at right angles to field lines, at this point, ε₀ = BLv, where v is linear velocity, and L is length of conductor within field, so total induced emf is 2BLv

v also equal to ωd/2 where d is width of coil (distance from centre of gyration)

30
Q

Back emf

A

emf induced in a motor as the flux linkage through the coil changes, acts against applied pd according to Lenz’ law. Induced emf proportional to speed of rotation of motor. V-ε=IR where I is current through motor and R is load resistance. Low speed - current high as back emf is low.

Multiplying equation by I gives power relationship

31
Q

Frequency of an alternating current

A

The number of cycles it passes through a second (charge carriers move one way, then reverse, then reverse again)

32
Q

What is the mean power over a cycle

A

Half of the peak power 1/2(I₀)^2R

33
Q

How ac carries energy

A

Even though electrons don’t move along, its the electric field that carries the energy

34
Q

What is the rms current/voltage value of an ac current

A

The value of dc current/voltage that would produce the same power as an alternating current through the same resistance

I/V₀/sqrt2

35
Q

SI unit for flux

A

Weber or Tesla metre^2

36
Q

Drawing a graph of emf when given graph of b/t

A

Take into account sign - a positive B will cause a negative induced emf due to Lenz’ law

37
Q

Ways of increasing emf

A
  • use a stronger magnet
  • use a coil with more turns
  • use a coil of greater area
  • place a soft iron core in the coil
  • use a larger amplitude of oscillation
    of the magnet.
38
Q

Pulses in coil across rotating magnetic axle points

A

Direction of induced emf (or current) opposes
the change that produces it.
* A pulse is produced as a magnet approaches
and leaves a coil.
* Forces on a magnet repel it as it approaches
and attract it as it leaves.
* Current therefore flows one way as the
magnet approaches and the opposite way
as it leaves.

39
Q

Flux linkage units

A

Weber turns

40
Q

Calculating max rate of change of flux from a motor

A

Calculate pd when not rotating,then find back emf by taking difference in voltage when not rotating vs rotating. emf = n(rateorchange…) etc

41
Q

How a transformer can step down voltage

A

The alternating voltage, Vin, supplied to
the input causes an alternating current in
the primary coil
* The ac in the primary coil produces an
alternating magnetic flux in the core
* This alternating flux passes through the
secondary coil
* The induced emf, Vout, in the secondary
coil is proportional to the rate of change
of flux linkage
* There are fewer turns on the secondary
than on the primary, so Vout < Vi

42
Q

Reasons why a transformer might be inefficient

A
  • Thermal energy losses caused by the
    currents in the coils, which have resistance
  • Energy is required to magnetise,
    demagnetise and re-magnetise the core
  • Eddy currents induced in the core cause
    thermal energy losses in the core
  • Imperfect flux linkage: not all the magnetic
    flux produced by the primary coil passes
    through the secondary coil
43
Q

P

A

IrmsxVrms

Irms^2R etc

44
Q

Back emf in transformers

A

Alternating flux from primary coil produces back emf in primary coil as well as emf in secondary coil - Acts against primary voltage making primary current very small when secondary is “off”. When secondary current is on, the magnetic field produced opposes the back emf (what induced it), therefore reducing back emf, so primary current is larger than when secondary current is “off”

45
Q

Why are transformers almost 100% efficient

A

Low resistance windings to reduce power wasted due to heating effect of current

Laminated core which consists of iron layers separated by layers of insulator (current would flow across coils, path of least resistance, rather than along wire). Induced currents, aka eddy currents, are reduced so magnetic flux is high as possible. Also heating effect of induced current reduced

Containing a magnetised SOFT iron core - easily magnetised and demagnetised, reduces power wasted through repeated magne and demagnetisation

46
Q

Efficiency of national grid

A

Current required to deliver power P = P/V. Power dissipated through cables = I²R = P²R/V² - Higher value of voltage - smaller value of wasted power compared to transmitted power

47
Q

How a magnetic pendulum swinging through a magnetic field is heavily damped

A

Bob cuts through magnetic flux of field, Induces an emf in the bob, causing circular (eddy) currents in the bob, which generates a magnetic field that opposes the og one/ Causes thermal energy losses in the bob, which must come from the Ek of the bob, causing it to slow down rapidly

48
Q

R decreasing in r=mv/Bq

A

R decreasing means path is more curved

49
Q
A

A LEVEL PHYSICS (17 decks)

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