chapter 23 - magnetic fields

Question 1

magnet vs magnetic material

Answer 1

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

Question 2

magnetic field lines

Answer 2

• arrow shows direction
• = spaced + parallel shows uniform field
• where stronger field is closer
• N>S

Question 3

motor effect

Answer 3

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)

Question 4

flemmings left hand rule

Answer 4

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

Question 5

right hand grip rule

Answer 5

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

Question 6

magnetic flux density (B)

Answer 6

(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

Question 7

Tesla

Answer 7

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

Question 8

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

Answer 8

if perp
F = BIL
if not
F = BILsinθ

Question 9

direction of field inside solenoid

Answer 9

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

Question 10

magnetic field

Answer 10

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

Question 11

force on moving charges in a magnetic field

Answer 11

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 = mv/Bq

Question 12

helical path of an electron

Answer 12

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

Question 13

accelerators

Answer 13

• 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

Question 14

curved accelerators

Answer 14

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

Question 15

velocity selector

Answer 15

• uses electric and magnetic fields to select charged particles of a specific velocity
• applies a magnetic field perpendicular to the electric field
  Fe = Fm
  Eq = Bqv
  v = E/B

Question 16

time period of the orbit of one electron

Answer 16

r = mv/Bq
v = 2𝞹r/T
T = 2𝞹m/Bq

Question 17

hall probe

Answer 17

• measures magnetic flux density
• Vh = BI/nbq
  where b is the thickness

Question 18

hall effect

Answer 18

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

Question 19

voltage due to hall effect

Answer 19

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

Question 20

electromagnetic induction

Answer 20

• 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

Question 21

magnetic flux

Answer 21

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

Question 22

magnetic flux linkage

Answer 22

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

Question 23

lenz law

Answer 23

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

Question 24

faradays law

Answer 24

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

Question 25

deriving faradays law

Answer 25

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

Question 26

flux linkage graphs and emf graphs

Answer 26

emf graph is the gradient function of the flux linkage graph

Question 27

ac generator

Answer 27

- 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)

Question 28

peak emf induced

Answer 28

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

Question 29

slip/split ring commutator

Answer 29

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

Question 30

3 phase generator

Answer 30

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

Question 31

transformers

Answer 31

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)

Question 32

emf in terms of v

Answer 32

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

Question 33

why transformers not 100% efficient

Answer 33

- 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

Question 34

transformer efficiency

Answer 34

IsVs/IpVp *100

Question 35

power transmission lines

Answer 35

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)

Question 36

finding magnetic flux density in the lab

Answer 36

- place magnets opposite each other on a mass - put a wire in-between them - measure force using the balance = W - B = F/IL - zero balance when no current in the wire - then apply a current