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A-Level Physics > 12: Magnetic Fields > Flashcards

12: Magnetic Fields Flashcards

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1
Q

Which direction to field lines go on magnets?

A

North to south

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2
Q

What is a transformer?

A

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

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3
Q

What is meant by a magnetically soft material

A

A material whose magnetisation disappears after the current is removed

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4
Q

how do transformers work

A
  • an alternating current 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
  • the rapidly changing magnetic flux in the iron core passes through the secondary coil, where it induces an alternating voltage of the same frequency
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5
Q

Why is magnetically soft material needed for the core of a transformer?

A

due to the rapidly changing magnetic flux across the core
- could be iron or a special alloy

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6
Q

How to work out the voltage across primary coil

A

Vp = Np x ΔΦ/Δt

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7
Q

How to work out the voltage across the secondary coil

A

Vs = Ns x ΔΦ/Δt

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8
Q

What is the equation for an ideal transformer

A

Ns/Np = Vs/Vp

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9
Q

What do step-up transformers do

A

Increase the voltage by having more turns on the secondary coil than the primary

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10
Q

What do step-down transformers do

A

Reduce the voltage by having fewer turns on the secondary coil than the primary

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11
Q

What are eddy currents

A
  • looping current induced by the changing magnetic flux in the core
  • they create a magnetic field that acts against the field that induced them, reducing the field strength
  • they also dissipate energy by generating heat
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12
Q

How can you reduce the effects of eddy currents?

A

laminating the core
- this involves having layers of the core separated out by thin layers of insulator, so a current can’t flow

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13
Q

other ways transformers can be inefficient (apart from eddy currents) and how to fix it

A
  • Heat is generated by resistance in the coils
  • this can be reduced by using wires with low resistance (thick copper wires)
  • energy is needed to magnetise and demagnetise the core, and this energy is wasted as it heats the core
  • this can be reduced by using a magnetically soft core
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14
Q

how do you work out the efficiency of a transformer

A

Is x Vs/ Ip x Vp

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15
Q

What is an alternating current

A

A current that changes direction with time

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16
Q

how can you measure alternating current using an oscilloscope

A
  • the trace is made by an electron beam moving across a screen
  • the vertical height of the trace shows the input voltage at that point
  • you can control the volts per division of the grid using the Y-gain control dial
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17
Q

What is the pattern produced by an alternating current source

A

A regularly repeating sinusoidal waveform

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18
Q

What is the pattern produced by a direct current source

A

The source is always at the same voltage, so you get a horizontal line

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19
Q

What happens if you turn off the time base of an oscilloscope

A
  • ac displayed as a vertical line
  • dc displayed as a dot
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20
Q

What are the three pieces of information you can get from an ac oscilloscope trace

A
  • time period T
  • peak voltage V0
  • peak-to-peak voltage
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21
Q

what is the purpose of a root mean square voltage

A

creates an average ac voltage so it can be compared with dc

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22
Q

How do you work out the Vrms of a sine wave

A

Vrms = V0/sqrt(2)

where V0 is the peak voltage

23
Q

How do you work out the Irms of a sine wave

A

Irms = I0/sqrt(2)

where I0 is the peak current

24
Q

How do you work out the average power of an ac supply

A

average power = Irms x Vrms

25
Q

What is the Vrms of UK mains voltage

A

230

26
Q

What is Faraday’s law

A

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

27
Q

What does the gradient of a graph of flux linkage against time represent

A

magnitude of emf

28
Q

What does the area under a graph of magnitude of emf against time represent

A

flux linkage change

29
Q

What is Lenz’s law

A

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

30
Q

How can you alter the shape of the graph of induced emf

A
  • Increasing the speed of rotation will increase the frequency and increase the maximum emf
  • increasing the magnetic flux density B will increase the maximum emf, but will have no effect on frequency
31
Q

How do you induce emf in a rotating coil

A
  • when a coil rotates uniformly in a magnetic field, the coil cuts the flux and an alternating emf is induced
  • the amount of flux cut by the coil is given by NΦ = BANcosθ
  • as the coil rotates, θ changes so the flux linkage varies sinusoidally between +BAN and -BAN
32
Q

How can you investigate flux linkage

A
  • with a search coil
  • the stretched metal spring acts as a solenoid when connected to an alternating power supply
  • the alternating supply means the magnitude field field of the solenoid is constantly changing, so the flux is changing therefore an emf is induced
  • search coil is connected to an oscilloscope to record the induced emf
  • position the search coil so that it is about halfway along the solenoid and within the inside of the solenoid
  • ## rotate the search coil so its angle to the solenoid and magnetic flux lines changes by 10 degrees and the record the induced emf
33
Q

what is magnetic flux

A
  • the magnetic flux density multiplied by the area
  • the number of magnetic field lines passing through an area
34
Q

What is the equation for magnetic flux

A

Φ = BA

35
Q

What is electromagnetic induction

A
  • when there is a relative motion between a conducting rod and a magnetic field, electrons in the rod experiences a force which causes them to accumulate at one end of the rod
  • this induces an electromotive force across the ends of the rod
36
Q

what is flux linkage

A

the magnetic flux in a coil multiplied by the number of turns on the coil

37
Q

what is the equation for flux linkage

A

NΦ = BAN (cosθ) if not normal

38
Q

what is the equation for a force acting on a single particle moving through a magnetic field

A

F = BQv

39
Q

what is the equation for the radius of the path followed by charged particles in a magnetic field

A

r = mv/BQ

40
Q

what does the radius equation tell you

A
  • the radius of curvature increases if the mass or velocity of the particle increase
  • the radius of curvature if the strength of the magnetic field or the charge on the particle increase
41
Q

the frequency of rotation of a charge particle in a magnetic field…

A

is independent of its velocity

42
Q

what are some uses of cyclotrons

A
  • producing radioactive tracers
  • high-energy beams of radiation for use in radiotherpay
43
Q

what is a cyclotron made up of

A
  • two hollow semicircular electrodes with uniform magnetic field applied perpendicular to the plane of electrodes, and an alternating potential difference applied between the electrodes
44
Q

how does a cyclotron work

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 potential difference between the electrodes then accelerates the particle across the gap until they enter the next electrode
  • as the particle’s speed is slightly higher, it will follow a circular path with a larger radius before leaving the electrode again
  • at this point the direction of the potential difference is reversed so the particle is accelerated again before entering the next electrode
  • this process repeats as the particle spirals outwards, increasing in speed, before eventually exiting the cyclotron
45
Q

how can you investigate the force on a current-carrying wire

A
  • use a top pan balance
  • the dc current should be connected to a variable resistor so that you can alter the current
  • use the variable resistor to change the current and record the new mass
  • repeat until you have tested a large range of currents
  • convert mass into force by xg
  • do a graph of F against I
  • gradient = Bl
46
Q

the closer together magnetic field lines are…

A

the stronger the field is

47
Q

what does your thumb represent in right hand rule

A

current

48
Q

what do other fingers represent in right hand rule

A

magnetic field

49
Q

what does your first finger represent in the left hand rule

A

magnetic field

50
Q

what does your second finger represent in the left hand rule

A

current

51
Q

what does your thumb represent in the left hand rule

A

motion/force

52
Q

what is magnetic flux density

A

the force on one metre of wire carrying a current of one amp at right angles to the magnetic field

53
Q

how do you work out the force of a current-carrying wire perpendicular to a magnetic field

A

F = BIl

A-Level Physics (17 decks)

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