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a - physics > chapter 23 - magnetic fields > Flashcards

chapter 23 - magnetic fields Flashcards

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1
Q

magnet vs magnetic material

A

magnet is magnetised
magnetic material has the capacity to be magnetised (and demagnetised)

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2
Q

motor effect

A

force on a current carrying wire due to a magnetic field at a non zero angle (not parallel)
size depends on
- current size
- strength of magnet
- length of wire
- angle between wire and field (min at 0 max at 90)
dircetion of force depeneds on direction of field and current (flemmings left hand)

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3
Q

flemmings left hand rule

A

can use to predict the direction of motor force
thumb = motion
first finger = field
second finger = current

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4
Q

right hand grip rule

A

thumb is current
fingers are magnetic field
or other way round in a solenoid

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5
Q

magnetic field density (B)

A

(field strength)
force per unit length per unit current on a current carrying conductor perp to a magnetic field
- shown by closeness of field lines
measured in Tesla

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6
Q

Tesla

A

unit of magnetic flux density (B)
the strength of flux density that produces a force of 1N in a wire of 1m with 1A flowing perp to the field

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7
Q

force on a wire carrying a current at an angle to a field

A

if perp
F = BIL
if not
F = BILsinθ

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8
Q

direction of field inside solenoid

A

use right hand grip rule in reverse to find direction of field in solenoid
(inside a solenoid field lines are parallel and uniform)

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9
Q

magnetic field

A

a region in which a piece of ferromagnetic material or a magnet or a current carrying conductor or a moving charge experiences a force
- exist around permenant magnets or current carrying conductors
- density of field lines shows strength
- parallel lines = uniform field

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10
Q

force on moving charges in a magnetic field

A

F = Bqv
a charged particle entering a magnetic field perp to the field direction experiences a centripetal force so moves in a circle
mv^2/r = Bqv
r = Bq/mv

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11
Q

helical path of an electron

A

if electron enters field at an angle resolve the velocity so in one direction it moves circularly but in the other v is parallel to the field so moves in a straight line
- moves like a corkscrew

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12
Q

accelerators

A
  • single stage = one electric field
  • multi stage = synchronised fields
    electron goes through magnetic field once it goes through it the field dircetion switches so its repelled from the previous one and attracted to the next
  • the longer the tube the greater the acceleration
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13
Q

curved accelerators

A

same as a linear accelerator but with curved fields
increased radius = increased acceleration
curved path means particles can accelerate through gaps more than once
+ particles can reach higher energy as can keep going round
+ alternating field keeps giving them more energy

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14
Q

velocity selector

A
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15
Q

time period of the orbit of one electron

A

r = mv/Bq
v = 2𝞹r/T
T = 2𝞹m/Bq

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16
Q

hall effect

A

a potential difference is crated across a semi conductor due to a magnetic field across it
- when a wire is put in a field F(B) occurs due to the magnetic field so electrons move down
- as electrons build up on the edge they’re forced to
- an electric field is created which has a force F(E) forcing the electrons upwards
- when in equilibrium F(E) = F(B)
- electrons then travel through the wire in a straight line

17
Q

voltage due to hall effect

A

V = IB/nbq

I = nAqv
F(B) = Bqv
F(e) = EQ
F(B) = F(E) Bqv = Eq
E = Bv = V/d
V = Bvd = BId/nAq = BI/nbq
where b = the thickness

18
Q

electromagnetic induction

A
  • when a wire cuts a magnetic field it produces an induced emf across the wire
  • if the wire is part of a complete circuit current flows
    increase emf by
  • increasing speed
  • increasing field strength
  • increasing length
19
Q

magnetic flux

A

is the produce of magnetic flux density and area
= BA
measured in weber (Wb)

20
Q

magnetic flux linkage

A

= NBA
measured in Weber turns
product of magnetic flux and number of turns

21
Q

lenz law

A

the direction of the induced emf due to a change of flux linkage is such that it will oppose the change of flux producing it
- if it was opposite it would attract so accelerate greatly breaking the law of conservation of energy

22
Q

faradays law

A

the magnitude of the induced emf in a circuit is = to the rate of change of flux linkage in a circuit
ε = dNBA/ dt

23
Q

deriving faradays law

A

WD = Fs
WD = BILs
Q = It WD = VQ
V = WD/It = BILs/It = BLs/t = BA/t

24
Q

flux linkage graphs and emf graphs

A

emf graph is the gradient function of the flux linkage graph

25
Q

ac generator

A
  • consists of a coil spun in uniform field
  • when coil spins at steady rate flux linkage changes continuously
  • when coil is at θ to the field flux linkage = NBAcosθ
    θ = t/T *2𝞹 = 2𝞹ft = wt
    emf = derivative of flux linkage d/dt of NBAcos(wt) = NBAwsin(wt)
26
Q

peak emf induced

A

V =NBAwsin(wt)
max when sin(wt) = 1
V = NBAw

27
Q

slip/split ring commutator

A

slip - generates ac - solid - doesnt change
split - generates dc - split - changes every half turn

28
Q

3 phase generator

A

spinning magnet in middle with coils outside
emf induced in pair of coils
each pairs wave is shifted
using 3 waves gets 3 phases of emf maximising efficiency

29
Q

transformers

A

step up - increase pd
step down - decrease pd
consists of a primary coil and secondary coil connected by a soft iron core
Np/Ns = Vp/Vs = Is/Ip (if 100% efficient)

30
Q

emf in terms of v

A

V = BLs/t = BLv
eg satellite dragging conductive teather through magnetosphere can generate emf

31
Q

why transformers not 100% efficient

A
  • energy loss from current flowing - resistance in wire causes heating effect Plost = I^2R decrease loss by increasing A and decreasing resistivity
  • energy loss from magnetic losses - prevent by putting coils close with soft core - ensures as much if flux as possible is passed onto secondary coil
  • energy loss from magnetising/ demagnetising core - increases temp - use material easily magnetised/ demagnetised
  • eddy currents created in iron core due to magnetic field causing electrons to move - laminated (insulator between layers) prevents this
32
Q

transformer efficiency

A

IsVs/IpVp *100

33
Q

power transmission lines

A

step up to increase voltage which decreases current reducing energy losses due to heating effect
P loss = I^2R (pd lost is not the same as overall pd so cant use P = IV)

a - physics (18 decks)

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