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A LEVEL PHYSICS > 23 - Magnetic Fields > Flashcards

23 - Magnetic Fields Flashcards

1
Q

ways to draw magnet’s magnetic field

A

dump iron filings or use a plotting compass with a bar magnet

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2
Q

direction of magnetic field lines are always

A

north to south

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3
Q

direction of earth’s magnetic field

A

south pole towards north pole (geographically) therefore bar magnet would have north pole and geographic south pole

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4
Q

define magnetic field

A

the region around a permanent or magnet or a current carrying conductor in which other magnetic objects will experience a force

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5
Q

what is the current carrying conductor responsible for the earth’s magnetic field

A

the earth’s molten core

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6
Q

where is magnetic field strongest around a magnet and why

A

at poles due to field lines being closest

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7
Q

what is responsible for creating a magnetic field around a bar magnet

A

the flowing of free electrons

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8
Q

electromagnetism

A

when a charges particle moves, it creates a magnetic field so when a current passes through a wire, a magnetic field is produced.

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9
Q

how to find direction of magnetic field around current carrying wire

A

using right hand grip rule (thumb is current, rest is field direction)

when representing in exam, make sure each field line gets further away from each other from the wire

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10
Q

magnetic field of solenoid is

A

uniform in the middle

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11
Q

how to find direction of field for solenoid

A

use right hand grip rule but thumb is field direction and rest are current direction

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12
Q

north and south poles for solenoid

A

the solenoid produce north and south poles at their opposite faces

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13
Q

how to determine direction of force on current carrying conductor in external magnetic field

A

Fleming’s left hand rule

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14
Q

force on current carrying wire

A

F=BILsinϑ
force = magfluxdensity x current x length x sinϑ
ϑ is the angle betwee FIELD and CURRENT

if perpendicular F=BIL
if parallel F=0

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15
Q

what is magnetic flux density

A

the strength of the magnetic field measured in Tesla T

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16
Q

what is one tesla equal to

A

the magnetic flux density when a wire of length 1m carrying a current of 1A is perpendicular to the field, experiences a force of 1N

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17
Q

other equations relating to magnetic fields that can be used in qs where factors increase and decrease

A

R=pL/A and V=IR - V is constant

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18
Q

how are northern lights created

A

charged particles from the sun spiral down the Earth’s magnetic field to the poles (strongest), colliding with atoms in the atmosphere to emit light

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19
Q

electron flow and flemings left hand rule

A

electron flow is opposite to conventional current - important in flemings left hand rule

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20
Q

force on electric charge in magnetic field

A

F=Bqvsinϑ
force = magfluxdensity = charge x velocity x sinϑ
F=Bqv if perpendicular

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21
Q

motion of elec charge in magnetic field

A

circular motion

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22
Q

radius of circular motion equation and derived

A

F=mv2/r and F=BQv
cancel out v so
r=mv/BQ

23
Q

electron motion around field lines - radius of circular motion

A

electrons spiral around field lines due to constant component velocity in direction of field lines

24
Q

velocity selector

A

a device that uses both electric and magnetic fields to select charged particles of specific velocity

Forcemagnetic = Forceelectric

BQV=EQ
so V=E/B

25
Q

when to use FLHR

A

motors

26
Q

when to use FRHR

A

generators
when wanting to find direction of current, opposite to LHR due to lenz’s law.

27
Q

how is current induced in a wire

A

when a wire moves relative to a magnetic field a current is induced due to the cutting of magnetic field lines. the amount of cutting is quantised as magnetic flux

28
Q

other way to induce emf

A

moving magnet through coil of wire

29
Q

define magnetic flux

A

the product of the component of the magnetic flux density perpendicular to the cross-sectional area

30
Q

mag flux equation

A

ɸ=BAcosϑ
flux = flux density x cross sectional area
ϑ - angle measured to normal of surface

ɸ=BA if perpendicular

31
Q

unit of magnetic flux

A

Weber (Wb)

32
Q

define one weber

A

the magnetic flux when a field of magnetic flux density 1T passes at right angles through a coil with area of one metre squared

33
Q

things to remember about ɸ=BA

A

ɸ unchanged if loop raised as field is uniform
A is the area only in contact with the field

34
Q

magnetic flux linkage

A

when there are several coils (motors and generators)

flux linkage = Nɸ
number of turns x magnetic flux

UNITS - Wb turns

35
Q

moving bar magnet through coil of wire (N pole to ground)

A

second peak greater and shorter time interval due to acceleration due to gravity
peaks opposite direction due to Lenz’s law
induced emf is 0 when in magnet due to field coil being parallel to flux density

number of coils are direc proportional to induced voltage

35
Q

define magnetic flux linkage

A

the product of the number of turns in the coil and the magnetic flux

36
Q

Faraday’s law

A

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

ε∝ ∆(Nɸ) / ∆t

37
Q

direction of induced emf when magnet moves through coil

A

direction of induced emf and hence the current changed direction when magnet is pulled away and pushed towards the coil due to lenz’s law

38
Q

Lenz’s law

A

the direction of any induced emf or current is always in a direction that opposes the change producing it
conservation of ENERGY

39
Q

combining Faraday’s and Lenz’s laws

A

ε = -∆(Nɸ) / ∆t

40
Q

what is constant for AC generator

A

ε = -∆(Nɸ) / ∆t
ε = -∆(BANcosϑ) / ∆t

BAN is constant

41
Q

how is AC current generated

A

there is magnetic field surrounding the magnet and when the magnet spins, the field lines are cut, causing a change in magnetic flux inducing an emf and current. because the field is rotating, AC is produced, emf induced causes electrons to flow one way then the other

42
Q

emf max occurs when

A

Nɸ is 0

43
Q

emf is 0 when

A

Nɸ is at maximum

44
Q

emf relation to flux linkage and time graph

A

-ve gradient

45
Q

how transformers work

A

an AC current causes the core to be magnetised and demagnetised. this rapid change in flux means equally fast change in flux in secondary coil, inducing AC in secondary coil opposite to direction of flux change causing it

46
Q

the faster the change in flux…

A

higher the induced emf

47
Q

step up and down transformers

A

up - Vs>Vp
down - Vs<Vp

48
Q

transformers coil equation

A

Ns/Np = Vs/Vp = Ip/Is

49
Q

why is core soft iron

A

easy to magnetise and demagnetise quickly, also improves efficiency

50
Q

100% efficiency for a transformer =

A

output power from Vp = Vs

51
Q

why is a transformer not 100% effiecient?

A

heating of P and S coil
eddy currents induced in core
- heating due to current

52
Q

how to improve efficiency of transformers

A

using low resistance to make more efficient and reduce heating effect of current
laminate core to minimise eddy currents

53
Q

voltage for transmission lines - national grid

A

high to minimise heat loss and reduce heating effect of larger current

A LEVEL PHYSICS (16 decks)

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