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Physics > Magnetic Fielda > Flashcards

Magnetic Fielda Flashcards

1
Q

Magnetic field

A

A region in which a force acts and is exerted on magnets or magnetically susceptible materials

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2
Q

Field lines

A

North to south
Closer the lines
The closer the lines the stronger the field

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3
Q

Magnetic field around a wire
Right hand rule

A

When a current flows in a wire a magnetic field is induced
Stick thumb up in direction of current
Your curled fingers show the direction of the field

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4
Q

Solenoids

A

If you loop a current carrying wire in one plane the surrounding magnetic field is doughnut shaped
While a coil with length (solenoid) forms a field like a bar magnet

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5
Q

left hand rule

A

first finger - magnetic field
second finger - current
thumb - force

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6
Q

how to make a wire vibrate

A

by passing an alternating current through a wire in a magnetic field, the wire can be made to vibrate
as the direction of the force is always perepndicular, when the direction of the crrent alternates, so does the force, from up to down causing the wire to vibrate

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7
Q

magnetic flux density

A

the force on one meter of wire carrying a current of one amp at right angles to the magnetic filed
measured in telsas
vector

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8
Q

one telsa equal to …

A

1N per amp per m
1 telsa = 1 N/Am

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9
Q

equation for maximum force a wire could experience

A

F = BIL

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10
Q

charged particles in a magnetic field

A

a force acts on charged particles in a magnetic filed
this is why current carrying wire experiences a force in a magnetic filed due to the elctrons

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11
Q

equation for force with charge and velocity

A

F = BQv

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12
Q

FLHR direction of current for a positive charge

A

current point direction of motion

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13
Q

FLHR of current for a negative charge

A

opposite to motion

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14
Q

what is the force indepent of in a magentic filed

A

force experienced by a particle in a magnetic field is independent to the particles mass although the centripetal acceleration it experiences does depend on the mass

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15
Q

equation for radius

A

r = mv / BQ

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16
Q

radius curvature increases if…

A

the mass or velocity of the particle increases

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17
Q

radius curvature decreases if…

A

strength of magnetic field or if charge on the particle increases

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18
Q

equation for frequency

A

f = v / 2pi r
r = mv / BQ

f = BQ / 2pi m
so frequency is independent from velocity

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19
Q

what does time taken depend on

A

frequency is independent from velocity
the time taken for a particle to complete a full circle depend only on magnetic flux density and its mass and charge

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20
Q

increasing the velocity in a magnetic field

A

make it follow a larger radius but itll take the same amount of time to complete it

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21
Q

cyclotron

A

made up of 2 hollow semi-circular electrodes with a uniform magnetic field applied perpendicular to the plane of the electrodes
and alternating p.d. is applied between the electrodes

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22
Q

particle accelerators

A

charged particles are produced and fired into one of the electrodes, where the magnetic field makes them follow a semi-circular path and then leave the electrode
an applied p.d. between the electrodes then accelerates the particles across the gap until they enter the next electrode

as the speed is slightly higher it will follow a larger radius before leaving the electrode
p.d. will be reversed and it’ll accelerate the particle again before entering the other electrode
this process repeats until the particle exits the cyclotron

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23
Q

total magnetic flux

A

Φ = BA
only possible if B is normal to A

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24
Q

electromagnetic induction

A

if there is relative motion between a conducting rod and a magnetic field, the electrons will accumulate at one end of the rod
This induces an emf across the rod

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25
Q

inducing emf in a flat coil or solenoid

A

happens in the same way
the emf is caused by the magnetic field that passes through the coil changing
if the coil is part of a complete circuit, an induced current will through through it.

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26
Q

magnet moves away from coil

A

positive
anticlockwise

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27
Q

magnet moves towards the coil

A

negative
clockwise

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28
Q

flux linkage

A

when a wire coil is moved in a magnetic field the size of the emf induced depends on the magnetic flux passing through the coil and the number of turns on the coil cutting the flux

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29
Q

flux linkage equation

A

N Φ = BAN

30
Q

what does the rate of change in flux linakge tell you

A

how strong the emf is

31
Q

a change in flux of one wbber per second will induce

A

an emf of 1 volt in a loop of wire

32
Q

faradays law

A

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

33
Q

magnitude of emf

A

rate of change of flux linkage

34
Q

emf equation

A

-N △Φ / △t

35
Q

lenzs law

A

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

36
Q

lenzs law and the conservation of energy

A

the energy used to pull a conductor to pull a conductor through a magnetic field against the resistance caused by the magnetic field against the resistance caused by the magnetic attraction is what produces the induced current

the idea that induced emf opposes the change that caused it agrees with the conservation of energy

37
Q

lenzs law and motion of a conductor

A

lenzs law says that induced emf will produce a force that opposes the motion of the conductor (resistance)

38
Q

induced emf in a rotating coil

A

when a coil rotate uniformly in a magnetic field, the coil cuts the flux and an alternating emf is induced

39
Q

the amount of flux cut by the coil (flux linkage) is given by

A

NΦ = BAN cosx
as the coil roates, x changes so it varies sinusoidally between +BAN and -BAN

how fast x changes depends on the angular speed, w, of the coil

x = wt
NΦ = BAN coswt

40
Q

equation for emf for graph

A

NΦ = BAN w sinwt

41
Q

graph for NΦ

A

cos
thingy perpendicular at +BAN and -BAN
paralell at 0

42
Q

graph for emf

A

sin
perpendicular at 0
parallel at +-emf

43
Q

peak emf equation

A

2NBlV
BANw
BAN 2pi f

44
Q

the graph of the induced emf can be altered by

A

-increasing speed of rotation increasing frequency and increasing max emf
-increasing magnetic field density will increase max emf butll have no effect on frequency

45
Q

generators

A

or dynamos, convert kinetic energy into electrical energy
they induce an electric current by rotating a coil in a magnetic field

46
Q

simple alternator

A

a generator of alternating current
it has sliprings and brushes to connect to the coil to an external circuit
the output voltage and current and current change direction with every half rotation of the coil producing an alternating coil

47
Q

alternating current

A

one that changes with direction with time means the voltage across a resistance goes up and down in a regular pattern

48
Q

using an oscilloscope to display the alternating current ( and d.c.)

A

the trace you see is made by an electron beam moving across the screen
you can set this using a dial on the front of the oscilloscope
oscilloscope are basically just voltmeters
the vertical height of the trace at any point shows the input volatge at that point
produces at sinusoidal waveform
d.c = horizontal line

49
Q

oscilloscopes if you turn off time base

A

ac voltage = vertical line
dc voltage = dot

50
Q

three basic pieces of info to get from an oscilloscope

A

-time period
-peak voltage
-peak-to-peak voltage

51
Q

root mean square volatge

A

an ac supply of 2v will be below 2v most of the time, means wont have as high power output as a 2v dc supply
have to average them out, need to sqaure as + and - bit cancel out
Vrms = V0 / root2

52
Q

root mean current

A

Irms = I0 / root2

53
Q

avearge power equation for ac supply

A

p = VI

Irms x Vrms = V0 / root2 x I0 / root2

54
Q

uk main electricty supply voltage

A

230

55
Q

tranformer

A

devices that make use of electromagnetic induction to change the size of the voltage for an alternating current

56
Q

how a transformer work

A

an ac flowing in the primary coil causes the core to magnetise, demagnetise and remagnetise continuously in opposite directions
this produces a rapidly changing magnetic flux across the core
becuase of this, a magnetically soft material is needed (usually iron)

57
Q

how a secondary coil works

A

a rapidly changing magnetic flux in the iron core passes through the secondary coil where it induces an alternating voltage of the same frequency

58
Q

primary/secondary coil equation

A

(Vp/s) = (Np/s) x △Φ/△t

Ns / Np = Vs / Vp

59
Q

step-up transformers

A

increase the voltage by having more turns on the secondary than the primary

60
Q

step-down transformer

A

reduce voltage by having fewer turns on the secondary coil

61
Q

ineffiency in a transformer

A

have small loses in power mostly from heat
the metallic core being cut by the continuously changing flux which induced an emf in the core
in a continuous core this causes currents called eddy currents which causes it to heat up and energy to be lost

62
Q

eddy currents

A

a looping current induced by the changing magnetic flux in the core of the transformer

63
Q

how to reduce the effects of eddy currents

A

laminating the core
involves having layers of the core separated out by thin layers of insulator so a current cant flow

64
Q

reducing the effect of energy needed for magnetism

A

using a material easily susceptible to magnetism reducing the energy needed

65
Q

how to reduce magnetic loss

A

ideally the magnetic flux created by the primary coil would cut through the secondary coil, but this isnt the case in practice (especially if the coils are far apart)
to reduce this magnetic loss, a core design in which the cores are as close as possible can be used - including winding the coils on top of eachother around the same part of the core rather than round different parts of the core

66
Q

for an ideal transformer where power in = power out

A

Ip x Vp = Is x Vs
Vp / Vs = Is / Ip

67
Q

calculating the effieciency of a transformer

A

efficiency = Is x Vs / Ip x Vp

68
Q

transformers on the national grid

A

electricty from power stations are sent round the country at the lowest possible current because a high current causes greater loss in energy and power loss in cables (P = iiR)
using cables with the lowest possible resistance also reduces energy loss
P = VI so low current = high voltage

69
Q

volatages for stations for national grid

A

power station –> step up = 25kV
step up –> plylons / cables = 400kV
cables –> step down –> homes = 230V

70
Q

investigating relationship between voltage and number of coils

A

-wrap wire around 2 C-cores
begin with 5 turns in primary and 10 in secondary = 1:2 ratio
-turn on ac supply - use low volatge as transformers increase voltage
-record voltage in each coil
-repeat with different ratios

71
Q

transformers experiment equation

A

Ns / Np == Vs / Vp

72
Q

find relationship betwwen current and voltage of the transformer for a given number of turns in the coil

A

same set up but add a variable resistor to primary coil and ammeter to both circuits
Ns / Np = Vs / Vp = Is / Ip

Physics (15 decks)

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