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AQA ALevel Physics > Magnetic fields > Flashcards

Magnetic fields Flashcards

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1
Q

What is a magnetic field?

A

A region surrounding a magnet or current-carrying wire that will
exert a force on any other magnet or current-carrying wire placed within it.

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2
Q

What objects experience a force in a magnetic field? What is its direction relative too its velocity?

A

A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the magnetic field

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3
Q

What direction is conventional current?

A

Towards the positive charge (opposite direction too the flow of e-)

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4
Q

What is magnetic flux?

A

Measure of the number of field lines passing through an area enclosed by a loop of conductor.

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5
Q

What unit is magnetic flux measure in?

A

Weber (Wb)

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6
Q

What equation links Magnetic flux, magnetic flux density and area?

A

Φ = B. A

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7
Q

What value of the magnetic flux do you take as the loop is rotated around an axis?

A

The component of the mag flux that is perpendicular too the field.

Φ = B. A. cos x

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8
Q

What is the magnetic Flux Density (B)?

A

The force per unit current per unit length on a current-carrying
wire placed at 90º to the field lines. Sometimes also referred to as the magnetic field strength.

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9
Q

What is the unit of measurement for magnetic flux density?

A

Tesla

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10
Q

What is the left hand rule?

A

FBI The direction of:
F(force), thumb
B(field), first finger
I(current), second finger

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11
Q

What equation links Force, Charge, Field strength and velocity? What is its use?

A

F=Bqv

Used too model the circular motion of a single particle in a magnetic field.

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12
Q

Why do single particle in a magnetic field experience circular motion?

A

The force experienced by a charged particle in a magnetic field is ALWAYS perpendicular too its velocity. Therefor, it is constantly accelerating (changing direction) towards the centre of its circular path.

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13
Q

What is a solenoid?

A

A long coil of wire

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14
Q

How do you find the direction of field/current in a loop of wire?

A

Reverse the right hand rule, thumb = field and fingers = current

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15
Q

How do you find the direction of the field induced by a current carrying wire?

A

Right hand rule, thumb = current and fingers = field

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16
Q

What is the equation for the radius of the path taken by a charged particle in a magnetic field? Why can this be derived?

A

r = mv / Bq

Let mv²/r = Bqv

By ignoring gravity, the resultant / only force acting on the particle is the magnetic force.

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17
Q

Using circular motion equations, derive a formula too show that T, time period, of the motion of a particle is independent from r.

A

T = 2πm / Bq

r = mv/ Bq
v = rBq / m
ω = v/r
2π / T = (rBq / m) / r
T = 2πm / Bq

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18
Q

What equation links the force experienced by a current carrying wire, magnetic field strength, Current and length of the wire?

A

F = BIL

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19
Q

What is the equation for the kinetic energy of a charge in a magnetic field, and its derivation?

A

rBq / 2m

EK = 1/2 mv²
v=rBq / m
v² = (rBq)² / m²
1/2 x m x (rBq)² / m² = rBq / 2m

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20
Q

What is one tesla ?

A

The magnetic flux density which produces 1N of force on a conductor of length 1m carrying 1A.

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21
Q

How do you model a pair of wires interacting with each other?

A

Each wire experiences a force exerted on it by the field of the other wire. Model the wires with two seperat4e drawings of the direction of the current and the field of the wire and therefore the direction of force it applies on the other.

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22
Q

How does a cyclotron work?

A

T is constant for a particle of a fixed charge in a constant magnetic field.

r=mv / Bq
T = 2πm / Bq

A charged particle is placed in the centre of a vacuum chamber with two electrodes ‘dees’ separated by a small gap.

The particle is accelerated across the gap by a change in potential difference (gain in kinetic energy) between the D’s.

Its velocity increases every time the particle crosses the gap between the two D’s and so its radius increases.

The polarity of the D’s alternates at every crossing as to ensure the particles only experience an increase in energy ( a positive potential difference).

Increases until a maximum kinetic energy/ radius is met and the particle leaves the chamber.

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23
Q

Name 3 uses of charged particles in magnetic fields

A

Cloud Chamber
Mass spectrometer
Cyclotron

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24
Q

What is the use of a Cloud chamber and how does it work?

A

Determining types of particles by observing their collisions within a field.

After a collision, the velocities of the particles involve decreases and so does the radius of their path. Images of these events can be analysed.

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25
Q

What is the use of a Mass Spectrometer and how does it work?

A

Determining the masses of charged particles.

Charged particles are accelerated by a potential difference and pass through a region of magnetic and electric fields, allowing particles of specific velocity too pass through.

When the two forces induced by the fields are equal (they act in opposite directions), particles can pass through. Eq = Bqv so
v = E/B

r= mv/ Bq

All of the particles velocities are equal and the field is uniform so the radius of the path of the particle leaving the spectrometer must be determined by mass or charge.

26
Q

What is the purpose of a split ring commutator?

A

To swap the direction of the current so the couple doesn’t oppose the initial direction of motion and return to where it came (almost half turn oscillations) in a simple motor.

27
Q

How do simple motors work?

A

Loop of current carrying wire placed between a north and south pole of two permanent magnets.

Forces are exerted on the loop of wire with current flowing through it. The forces on the opposite sides of the loop act as a couple resulting in a circular motion.

As it rotates, the horizontal distance between the forces and the couple decreases until vertically above one another and the couple is zero.

The coils angular velocity carries it through this point until it again experiences a couple.

28
Q

What is magnetic flux linkage?

A

The magnetic flux multiplied by the number of turns in a coil.

NΦ = Φ x n

29
Q

What unit is magnetic flux linkage measured in?

A

Weber (Wb) or Weber turns

30
Q

What is faradays law?

A

A change in magnetic flux linkage will result in an induced emf.

31
Q

What is the equation expressed by Faraday’s law?

A

ε = -d(NΦ)/ dt
ε= Δ B.A.n (turns) / Δ t

32
Q

What is Lenz’s Law?

A

The direction of the induced ε always opposes the change that made it

33
Q

State three changes that can be made to change the NΦ (and induce an emf)

A

Change the area enclosed
(through rotation, change of shape or moving through the field)

Change the flux density

Change the number of turns

34
Q

How would you find the average ε?

A

NΦ final - NΦ initial / Δ t

35
Q

How do you find the emf at a point on a NΦ against t graph?

A

Take a gradient of the line at time t

36
Q

What id the relationship between NΦ/t and ε/t graphs ?

A

A ε/t graph is the negative gradient function of a NΦ/t graph

37
Q

What are eddy currents?

A

Closed loops of current induced within conductors by faraday induction that flow in planes perpendicular to the magnetic field.

38
Q

What is the ε when the NΦ is at a maximum, and why?

A

ε = 0V

The ε = -d(NΦ)/ dt
At a local maximum of any graph, the gradient is 0 (the rate of change for any value)

39
Q

Describe the changes that take place when a magnet falls through a coil

A

As the magnet approaches the coil, the coil experiences a ΔNΦ inducing an ε. The d(NΦ)/ dt is increasing as the magnet accelerates under gravity and the field strength increases as the magnet grows closer.

When the magnet is in the middle of the coil, the coil experiences a NΦ max where ε = 0.

The coil experiences a decrease in NΦ, inducing an ε, as the magnet moves further away from it. B decreases until it reaches 0.

ε falling away > ε falling towards as it is accelerating

t falling away < falling towards

40
Q

For a magnet falling through a much longer coil, how does the ε differ? (compared too a ‘normal’ graph)

A

The ε remains at 0 for a longer amount of time while it is completely in the coil

41
Q

Why does a magnet take a long time too fall through a metal tube?

A

The ΔNΦ induces an ε in the tube whose free electrons create ‘eddy currents’.
By Lenz’s law, the ε is in the upwards direction, inducing a field in the tube that exerts a resistive force against magnet.

42
Q

What determines the effect of eddy currents?

A

Resistivity and geometry of a material (prevent formation of large eddy currents)

43
Q

On a graph of B/ x (displacement), how do you find the ε ?

A

ΔB/Δx * v = ΔB/ Δt
ε = n.A . ΔB/ Δt

44
Q

What is the root mean square value?

A

The DC Voltage which provides the equivalent output as the AC signal

45
Q

What is the equation for the root mean square voltage?

A

Vrms = Vpeak / √2
(DC) = (AC peak) / √2

46
Q

What is the equation for AC average power?

A

Ipeak . Vpeak = 2(average power)
V²peak / 2R total = average power
I²peak . Rtotal = 2(average power)

47
Q

What is the relationship between AC peak power and average power?

A

Peak power = average power x 2

48
Q

What does the y shift/ x shift do on an oscilloscope ?

A

Moves the trace up/down OR left/right respectively

49
Q

What does the y gain determine on an oscilloscope ?

A

The height of the signal in V / Division or V / cm on the screen

50
Q

What does the time base determine on an oscilloscope ?

A

The duration of the trace in secs/ Division secs/ cm

51
Q

What scale of values is convention on an oscilloscope ?

A

1,2 or 5 x10 ^(x)

52
Q

What is the purpose of a transformer?

A

A device that changes the voltage of AC current

53
Q

What are the primary components of a transformer?

A

A primary coil, a secondary coil and an iron core

54
Q

What is equivalent too the ratio of Ns / Np? (turns in secondary : turns in primary)

A

Vs / Vp

55
Q

Which coil has the larger number of turns in a step up transformer?

A

Secondary coil

56
Q

Which coil has the larger number of turns in a step down transformer?

A

Primary coil

57
Q

What does the AC in the second coil depend on?

A

Efficiency and turns ratio

Is CANNOT be determined without the efficiency

58
Q

Why is the current induces in the second coil alternating?

A

The field generated by the AC in the first coil is constantly changing direction so the ΔNΦ and ε induced are constantly changing direction.

59
Q

How do you calculate the efficiency of a transformer?

A

Ps / Pp = Is.Vs / Ip.Vp

60
Q

How can efficiency of a transformer be improved?

A

Low resistivity windings (less power lost too I^2R heating)

Soft iron core (easily magnetised so less e lost upon magnetisation)

Laminate the core (reduces the effect of eddy currents as the cannot form between the layers of insulator in the iron0

61
Q

Why are step up and step down transformers used?

A

Power lines require High voltages and the lowest currents possible to avoid I^2.R heating.

High Voltages are dangerous in residential areas so if not stepped down must be kept incredibly high so not too ionise the air around us.

AQA ALevel Physics (4 decks)

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