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physics > magnetic feilds (everything - detailed) > Flashcards

magnetic feilds (everything - detailed) Flashcards

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1
Q

what is a magnetic feild

A

a region where force is exerted on magnetic meterial

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2
Q

what is another word for magnetic field lines

A

flux lines

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3
Q

field lines go form _____ to _____ pole

A

north to south

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4
Q

a _____ ____ is generated by a _____ _____ ___

what rule do you use to find the direction of these flux lines

A

magnetic field, current carrying wire

fleming right hand rule

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5
Q

explain why there is a resultnent force when a current carrying wire is incedent perpendicular between to 2 fixed magnets

A
  • because the current carring wire generats a magnetic field perpendicular to the direction of the conventinal current
  • this magnetic field is then parrelel to the magnetic field generated by the fixed magents
  • the magnetic field then adds where the flux lines are going in the same direction and minus were they are going in opposing directions
  • this then means there is a greater magnetic flux density above or below the wire depending on the direction of conventinal current
  • this means there is a resultent force ‘pushing’ the wire down or up depending on the direction of conventinal current

(note the reletive high magnetic flux density region also pushes the opposit way as well)

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6
Q

if conventinal current is parallel to field lines of a fixed magnet what is the size of the force generated

why…

A

0N
because there is no component of the magnetic field perpendicular to the current

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7
Q

force in a current carrying wire is preportinal to…
F∝

A

magnetic flux density
F∝B

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8
Q

magnetic flux density (B) is defined as …
therefore one of B si units are…
the other is… wich is…

A

the force on 1m of wire carrying a current 1A perpendicular to the magnetic field
Nm^-1A^-1
Tesla(T) = WB/m^2 (as B=Φ/A)

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9
Q

when current and flux lines are perpendicuar what is the equation for F

if they are not perpendicular then the equation for F is…

A

F=BIL

F=BILcos(θ)
where θ is the angle bwetween the normal to the wire and the magnetic flux lines

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10
Q

what is L in BIL equation

A

the lenth of wire that is cutting the flux lines (lenth if wire in the magnetic field)

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11
Q

why does a current carring wire experince a force when in a magnetic field (moleculer explenation)

A
  • there are charged particals (e-) moving within a current carring wire
  • since the current carrying wire is in a magnetic field the charged particals are also moving within a magnetic field
  • this induces a force on the charged particals (e-)
  • this force is then by extension experienced by the wire as a whole
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12
Q

what is quation for F experinced by a partical in a magnetic feild…

derive this equation

A

F=BQV

F=BIL
I=Q/t, s=d/t d=sxt l=vxt
F=BQ/tvt
F=BQv

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13
Q

charged paricals in a magnetic feild are deflected in a ______ path

A

circular

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14
Q

force on a moving charged partical in a magnetic field is always _______ to its direction of motion

A

perpendicular

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15
Q

a charged partical moving in magnetic field travel with ______ motion

this is because…

A

circular

  • the magnetic force acting on the partical is perpendicular to its direction of travel
  • this magnetic force acts as cetripetal acceleration
  • thereofre causing circular motion
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16
Q

what is the equation for the radius of the circle a moving charged partical creats in a magnetic field

derive this equation

A

r=mv/BQ

circular motion equation = F=mv^2/r
F equation for a moving charged partical in a magnetic feild = F=BQv
F=F
mv^2/r=BQv
1/r=BQv/mv^2
1/r=BQ/mv
r=mv/BQ

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17
Q

when is F=BQv true

(not sure about this one)

A

when the partical is moving perpendicular to the flux lines (magnetic feild)

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18
Q

explain how a cyclitron works

A

see pg 143 for diagram
- charged particals are fired into one of the electrodes from the center of the cyclitron
- the magnetic field within the electrode induces a magnetic force that acts a centripetal acceeration thereofre making the particals follow a semi circular path
- as the charged particals leave the electrode an applied p.d between the electrodes accelerates the charged partical invreseasing its velocity
- because the particals speed is slightly high it follows a circular path with a slightly higher raduis within the other electrode
- as the chrages partical leaves the elecrode again the p.d is reversed accelerating the charged partical again before entering the next electrode
- this process repeats as the charged partical spirals outwards, increaseing in speed, before eventually exiting the cyclitron

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19
Q

in a cyclitron T is…

A

constent

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20
Q

what is magnetic flux

magnetic flux equarion

A

the total number of magnetic flux lines (magnetic field lines) passing through defined region of space (area) (magnetic flux density x area)

magnetic flux = magnetic flux density x area
Φ=BA

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21
Q

when is emf induced (assuming there is a changeing magnetic field)

A

when conductors cut magnetic flux lines

22
Q

in electromagnetic induction:

if there is a _____ ______ between a ________ ____ and a magnetic field, the electrons in the rod will experince a ____. wich cause them to…

what does this do…

A

reletive motion, conducting rod, force

accumilate at one end of the rod

this induces an emf across the ends of the rod as there is a p.d across the rod

23
Q

emf is induced in a coil if… (think soliniod)
if this coil is part of a complete circiut…

A

if magnetic flux (magnetc field) that passes through the coil changes

induced current will flow (AC)

24
Q

the more turns in a coild means…

A

bigger emf will be induced

25
Q

magnetic flux linkage equation

magnetic flux linkage equation if the loop of wire is not perpendicular to the flux lines

A

NΦ=BAN

NΦ=BANcos(θ)

26
Q

a change in flux linkage of 1 Wbs^-1 will induce an emf of ____ in a loop wire

A

1v

27
Q

give faradays law

give the equaition

therefore
draw NΦ/t graph
what is the gradient of this graph

draw ε/t graph
what is the area

A

the induced emf is directly preportinal to the rate of change of flux linkage

ε=ΔNΦ/Δt

see pg 147
gradient = ε

see pg 147
area = ΔNΦ

28
Q

ε∝…

A

ε∝rate of charnge of flux linkage
ε∝ΔNΦ

29
Q

what is lenzes law

this is in agreement with the…

A

the induced emf is always in such a direction as to oppose the change that causes it

principle of conservation of enegy

30
Q

when trying to find the direction of current in a moving rod within a magnetic feild what is the force (the thimb) when using fleming left hand rule

A

the force (thumb) is the resistence force porduced by the iduced emf
thereofre the thumb force is in the opposit direction to the motion of the rod within the magnetic field

31
Q

what is the equation that combines lenses law and faradyas law

A

ε=-ΔNΦ/Δt

32
Q

what is a alternator

A

a generator of alternating current (AC)

33
Q

what is another word for generators

A

dynamos

34
Q

in an alternator the V and I change direction every…

A

half rotation

35
Q

when dealing with alternators:

give the flux linkage equation

give the induced ε equation

give a extra equation for ε (this one is not on the silibis but can help)

A

NΦ=BANcos(θ)
but we are dealing with circular motion of the alternator so
ω=θ/t
θ=ωt
NΦ=BANcos(ωt)

ε=BANωsin(ωt)
(ε = rate of change of BANcos(ωt)) so i assume the extra ω came from diffrentiating BANcos(ωt))

ε=BLv

as
ΔΦ=ΔBA
A=lenth x distentce
distence =vt
ΔΦ=ΔBLvt
ε=NΔBLvt/Δt
ε=NBLv

36
Q

when dealing with alternators:
draw the graphs for NΦ (flux linkage) over t

drwa the graph for induced ε

what degrees is ε out of phase with NΦ

A

see pg 147

90 degrees

37
Q

oscilloscope graphs:
draw a graph of AC with time peried

draw a graph of AC withought time base

draw a graph of dc with time base

A

see pg 148

38
Q

will an ac power supply of 2V be 2V all of the time

what has a higher power supply 2V ac or 2V dc

A

no 2V would be the max voltage
the voltage will follow a sin graph with a max and min V of 2V

2V dc as it is contently at 2V

39
Q

rms equations

thereofre avarge power of an ac supply will be

A

Vrms=V(o)/root2
V(0) is max voltage
Irms=I(0)/root2
I(o) is max current

avarge power = Irms x Vrms

40
Q

what is the UK mains Vrms electricity supply

what is the max V(o)

A

Vrms=230V

V(o) = 330V

41
Q

what is the uk main frequenncy

A

50Hz

42
Q

transformers make use of ________ ________ to change the size of the ________ for an ________ ______.

A

electromagnetic induction, voltage, alternating current

43
Q

how does a tranformer work (simplistic version)

A
  • an alternating current flowing in the primary coil produces a changing magnetic flux (acting as a solinoid)
  • this changing magnetic flux (field) is then passed through the iron core of the transformer to the secondary coil
  • the secondary coil then experinces the changing magnetic flux linkage inducing an alternating voltage of the same frequency as the input voltage

(the magnetic flux is changing at the primary coil as the current is changing direction (AC))

44
Q

when dealing with transformers:
give the induced ε equation for the primary and seconday coils

therefore combine these 2 equations and the 100% power efficiency ideal equation to give the equation for ideal transformers

A

V(p)=N(p)ΔΦ/Δt
V(s)=N(s)ΔΦ/Δt

N(s)/N(p) = V(s)/V(P) = I(s)/I(p)

45
Q

step-up transformer ______ the voltage by having _____ _____ on the secodary coil then the primary coil.

step-down transformer ______ the voltage by having _______ _____ on the secondary coil.

A

increases, more turns

decreases, less turns

(more turns = high Voltage)

46
Q

what are the main ways a transformer losses power

A
  • eddy currents in the transformer core (produces heat in the iron core)
  • resistence in the coils (produces heat)
47
Q

what are the ways to prevent power loss in trensformers for:
eddy currents

resistence in the wires

A

reduce the effect of eddy currents by laminating the iron core with layers of insolation

use thick wiers with low resistnce

48
Q

what are eddy currents

A
  • looping currents induced by the changing magnetic flux in the core. they create a magnetic field that acts against the field that produces them, reducing the overall field strenth.
  • they also dissipate enegy by generating heat.
49
Q

give power loss equation

A

P (power loss) = I^2 x R

50
Q

what is roughly the transmission voltage used in UK

A

400KV

51
Q

emam question:
define Tesla (1mark)

A

1 Telsa is the magnetic flux density (B) that produces 1N of foce on 1m if wire carrying 1A of current

physics (17 decks)

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