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Magnets Flashcards

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1
Q

What direction do magnetic flux lines go in?

A

North to South

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2
Q

Define the Tesla

A

The magnetic flux density which produces a force of 1N in a wire of length 1m with a current of 1A

When the wire is perpendicular to the magnetic field

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3
Q

Flemings right hand rule

A

Thumb=direction of conventional current
Fingers curl in direction of magnetic field

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4
Q

Flemings right hand rule

A

Thumb=direction of conventional current
Fingers curl in direction of magnetic field

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5
Q

Flemings left hand rule

A

Thumb= Force
Index finger= Magnetic field
Middle finger= current

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6
Q

Magnetic force equation (Main)

A

F=BIL
F=BILsinθ

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7
Q

Equation for magnetic force on a single particle

A

F=BQV

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8
Q

How to derive radius equation

A

Centripetal force = magnetic force
Mv^2/r = BQv
r=mv^2/BQv
r=mv/BQ

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9
Q

Derive frequency/time period equation

A

Centripetal force=magnetic force
mω^2r = BQv
mω^2r = BQωr
mω=BQ
mx2πf=BQ
f=BQ/2πm

T=2πm/BQ

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10
Q

How do you find the time period / frequency of a half rotation?

A

T=πm/BQ

f=BQ/πm

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11
Q

How does a cyclotron work?

A

1) Charged particle is inserted in middle
2) Particle experiences a force in electric field -> it’s accelerated to negative plate
3) Particle enters M field and follows circular path
4)Particle re-enters magnetic field. PD must be flipped to accelerate it to higher speed
5) particle enters other Dee. radius is larger because speed is greater
6)Proton re-enters electric field. PD must be flipped again to accelerate particle
7)Process repeats until desired speed is reached

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12
Q

State 2 situations where a charged particle will experience no magnetic force when placed in a magnetic field.

A

1) It’s moving parallel to field lines
2) It’s stationary

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13
Q

How to induce an EMF and a current

A

In the wire the positive charges will move up and the negative charges down.

Therefore an EMF has been induced.

If the wire was a complete circuit then a current would be induced too.

The direction of the conventional current would be in the direction the positive charges move.

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14
Q

When does flux cutting/ changing the magnetic flux occur?

A

When a conductor moves through flux lines

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15
Q

When does flux cutting/ changing the magnetic flux occur?

A

When a conductor moves through flux lines

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16
Q

What factors affect the size of the EMF induced?

A

-The speed of movement: more flux lines cut per second.
-The number of coils: like having batteries in series.
-Strength of magnet: greater density of field lines.
-Area cut through: amount of field lines.

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17
Q

Define magnetic flux

A

The total amount of field lines cut through with the conductor

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18
Q

Magnetic flux equation

A

Φ=BA
Φ=BAcosθ

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19
Q

Why does including more coils increase the EMF?

A

Each wire acts like a battery

Looping same wire around multiple times means it gets “cut” multiple times

Like batteries in series

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20
Q

Flux linkage equation

A

ΦN. (BAN)

BANcosθ

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21
Q

Flux linkage against time graph (when rotating)

A

Cos graph

22
Q

Lenz’s law

A

The direction of the induced EMF/current is in the opposite direction to the change that caused it

23
Q

Faraday’s law

A

The induced EMF/current is directly proportional to the rate of change of flux linkage.

24
Q

Faraday’s law EMF equation

A

EMF=. - △(ΦN) /△T

25
Q

Area of EMF against time graph

A

Change in flux linkage

26
Q

How to derive EMF equation

A

EMF= BAN/t (N=1)
EMF= BA/t
A=ld

EMF=Bld/ t

d/t = v

EMF = BLV

27
Q

Equation for EMF produced by a rotating coil.

Equation for maximum EMF

A

EMF= BANωsin(ωt)

Max EMF= BANω

28
Q

How to calculate Vrms (root mean square)

A

V0/√2

V0=peak voltage

29
Q

How to calculate Irms (root mean square)

A

Irms=I0/√2

I0=peak current

30
Q

What does the Y gain dial do?

A

Changes the value of each vertical division

31
Q

What does the time base dial do?

A

Changes the value of each horizontal division

32
Q

What do the y shift and x shift dials do?

A

Move the graph up / left and right

33
Q

How to get peak voltage from oscilloscope

A

Find peak to peak voltage
Divide by 2

34
Q

How to reduce percentage uncertainty when using oscilloscope to measure voltage?

A

Increase y gain sensitivity to make the curve take up whole screen vertically.

Turn time base off so it’s just a vertical line and easy to measure.

35
Q

How to reduce percentage uncertainty when using oscilloscope to measure time period?

A

Decrease the time base sensitivity to make one oscillation take up whole screen vertically

Use freeze function

36
Q

State the four inefficiencies in a transformer and how they can be reduced.

A

Flux leakage: place iron core in transformer.
Eddy currents: Laminate iron core.
Resistance of wires: Use a thicker wire or a material with lower resistivity.
Heat: Use cooling vents to reduce heat

37
Q

Transformer equation

A

Np/Ns = Vp/Vs

38
Q

How does flux leakage happen?

A

As the magnetic field grows and shrinks around primary coil the field lines move left/right

Only needed on right to flux cut secondary coil

So field lines to the left are wasted

39
Q

How to reduce flux leakage?

A

Place an iron core in the transformer
Iron is magnetically soft
Helps guide field lines towards secondary coil

40
Q

How do transformers work?

A

1) An alternating current is passed through the primary coil
2)Primary coil creates an alternating magnetic field which grows and shrinks
3) Therefore the secondary coil gets flux cut
4)Inducing an alternating current in the secondary coil

41
Q

Why can’t a direct current be used in a transformer?

A

The magnetic field would be stationary in this case.

It must be changing so flux cutting occurs

42
Q

What are Eddy currents?

A

A current is induced in the iron core.
This can’t go anywhere.

43
Q

How to calculate efficiency?

A

Pout/Pin x100

44
Q

How to calculate efficiency?

A

Pout/Pin x100

45
Q

Define root mean square voltage

A

The square root of the mean of the squares of all the values of voltage in one cycle

46
Q

What happens when time base it switched off?

A

It will just be a vertical like
Because dot doesn’t move horizontally

47
Q

Why does a magnetic material slow down when it goes through a magnetic tube?

A

-Moving magnet flux cuts walls
-This induced a current in the walls creating a 2nd field
-This opposes the field of the moving magnet slowing it down

48
Q

When does F=BQv apply?

A

Magnetic field must be at right angles to the velocity

49
Q

Why does the balance reading increase when there’s a magnetic field around the wire?

A

Current through wire creates magnetic field creating a force
An equal and opposite force is present on the magnet pushing the balance down.

50
Q

Why use an alternating current?

A

Resistance opposes current reducing power
With a lower current, less power wasted
Alternating current can be reduced using a transformer

51
Q

Explain why proton follows semicircular path in a Dee

A

Force due to uniform magnetic field is always at 90 degrees to direction of travel.
This force is the centripetal force for circular motion

52
Q

Why isn’t is possible for the magnetic field to alter the speed of a proton when it’s in a Dee?

A

The force is perpendicular to the motion

Physics (10 decks)

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