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Physics - A Level > Magnetic fields > Flashcards

Magnetic fields Flashcards

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1
Q

What is the function of the core of a transformer?

A

Provides a path for the magnetic flux to link the primary and secondary coils efficiently. It is made of soft iron, which is easily magnetised and demagnetised. When an alternating current flows in the primary coil, it produces a changing magnetic field - the core guides this changing flux through to the secondary coil

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2
Q

What is the function of the secondary coil in a transformer?

A

Has a varying magnetic flux passing through it, which induces an emf that is determined by the number of turns.

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3
Q

Explain how the efficiency of a transformer is increased by constructing the core out of thin iron sheets.

A

The sheets of laminations are made from insulator/ high resistivity material. This reduces eddy currents/ charge flowing in the core, which means that smaller emfs are induced in the core.

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4
Q

Why shouldn’t the voltage in a long-distance transmission be too low?

A

If the voltage is too low then power is transmitted at a high current = energy loss due to heating.

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5
Q

Why shouldn’t the voltage in a long-distance transmission be too high?

A

If the voltage is too high, it creates major insulation difficulties - electrical breakdown of insulation

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6
Q

What is Lenz’s law?

A

The direction of the induced current is such that it will oppose the change of flux that is producing it.

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7
Q

What shape are the magnetic field lines induced by a current in a wire?

A

Cocentric rings around the wire

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8
Q

What is the definition of a Tesla?

A

A force of 1N on 1 metre of wire carrying 1A of current perpendicular to a magnetic field.

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9
Q

What do the fingers represent in Fleming’s left hand rule?

A

Force (or motion)
Field
Current (conventional current = opposite to direction flow)

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10
Q

What is the condition for using Fleming’s left hand rule for a charged particle moving in a magnetic field?

A

The charge must be positive. If the charge is negative, the force is reversed.

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11
Q

What direction is the force on a moving charge in a magnetic field compared to its velocity?

A

Perpendicular

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12
Q

How do you find the formula for radius of a charged particle’s circular path and what is it?

A

F = BQv and F = (mv^2)/r

r = (mv)/(BQ)

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13
Q

What are some of the uses of a cyclotron?

A

Producing ion beams for radiotherapy, radioactive tracers

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14
Q

In what direction does the magnetic field in a cyclotron act?

A

Perpendicular to the plane of the electrodes

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15
Q

What type of voltage is used in a cyclotron?

A

AC

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16
Q

Why does changing the strength of a magnetic field not impact the speed of a particle?

A

Because the force exerted by the magnetic field is always perpendicular to the direction of travel.

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17
Q

What happens when particles reach the edge of the ‘dee’?

A

The move across the gap, where they are accelerated by the electric field, meaning that the radius of their circular path will increase as they move through the second electrode. When the particle reaches the gap again, the alternating electric field changes direction, allowing the particles to be accelerated again.

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18
Q

What are the two laws of electromagnetic induction?

A

Faraday’s law and Lenz’s law

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19
Q

What is Faraday’s law?

A

The magnitude of induced emf is equal to the rate of change of flux linkage

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20
Q

What is Lenz’s law?

A

The direction of induced emf is such as to oppose the change of flux causing it.

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21
Q

What is the formula for flux that contains velocity?

A

B x l x v x delta t

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22
Q

What is the formula for emf that contains v?

A

emf = Blv

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23
Q

What’s the word for a graph shaped like a sine curve?

A

Sinusoidal

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24
Q

What is the peak voltage on an oscilliscope?

A

V 0 (subscript)
The distance from the equilibrium to the highest / lowest point

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25
What is the peak to peak voltage?
Distance from the minimum point to the maximum point
26
What is the root mean square voltage?
The average of all of the squares of the possible voltages
27
What value is the 230V supplied in homes actually?
The rms value of voltage
28
What type of current do you use with a transformer?
Alternating current
29
What does a step-up transformer do?
Increases the input voltage by having more turns on the secondary coil than the primary.
30
What does a step-down transformer do?
Decreases the input voltage by having fewer turns on the secondary coil
31
What is the main cause of energy loss in a transformer?
The production of eddy current induced by the alternating magnetic field in the primary coil. Due to Lenz's law, they oppose the field that produced them, reducing the field's flux density and generating heat, which causes energy to be lost.
32
In a wire with moving charge, what is the right hand thumb rule for?
Thumb - direction of current Fingers - direction of field
33
How does the motor effect work?
Wire is put in a magnetic field. Superposition of fields results in a resultant force on the wire - opposing field directions cancel and weaken the field, same direction fields reinforce and strengthen the field.
34
What is a split ring commutator and what does it do?
Reverses the direction of the current every half rotation - keeps the motor rotating as the direction of the force is reversed (motor effect).
35
What is the tricky thing about Fleming's left hand rule to remember about charged particles moving in a magnetic field?
The conventional current is opposite to the direction of electron flow - and the left hand rule uses conventional current for middle finger.
36
Do time period and frequency depend on the radius of the circle or the velocity of the particle, for a particle moving in a magnetic field?
No (derive the time period equation to prove it!)
37
When does the speed increase in a cyclotron?
When particles are only moving through the electric field between the dees.
38
What is the time period of the alternating current in a cyclotron?
The same as the electron's rotation
39
What needs to be true for a particle to be 'selected' in a velocity selector?
Fe and Fm need to be equal, so its direction doesn't change (one is up, one is down).
40
How do you find the selected velocity in a velocity selector?
Fe = Eq Fm = Bqv Eq=Bqv v=E/B
41
How can you change the velocity selected in a velocity selector?
Change the ratio of the field strengths.
42
How does induction work (briefly lmao)?
Moving a wire in a magnetic field causes an EMF across the wire, which induces a current in the direction that will create an opposing force due to the motor effect.
43
What is the right hand thumb rule when applied with Lenz's Law for a current through a coil?
Thumb - points towards north pole Fingers - direction of current
44
Explain the graph shape for current/time when a magnet is dropped through a coil of wire.
Smaller positive curve followed by larger negative curve.
45
Why is a current induced when a bar magnet is dropped through a coil of wire?
As the magnet moves towards the coil, flux density (B) increases, so flux linkage increases (so an emf is induced). As it falls away, flux linkage decreases.
46
Why is the peak negative current induced in a coil when a magnet is dropped through larger than the peak positive?
The magnet accelerates, and induced emf = rate of change of flux linkage. Smaller time for same change - higher emf/current. (V=IR)
47
Why do magnets take longer to fall when dropped through a copper pipe?
Circular currents are induced that, for the first half of the fall, repel the magnet upwards, and for the second half, attract it upwards - so there is a resistive upwards force.
48
What happens when a metal pendulum swings through a magnetic field (first half)?
Eddy currents are induced such that a field is created that will repel the existing magnetic fields - poles match (can find current direction using right hand rule)
49
What happens when a metal pendulum swings through a magnetic field (second half)?
The direction of the eddy current is reversed and the field created now attracts the existing field, pulling the plate towards the equilibrium point - force is always opposite to velocity, so SHM is damped.
50
How can the impact of eddy currents in a metal pendulum swinging through a magnetic field be reduced?
Using a plate with slits, so that the eddy currents can't form, or using a non conductive plate and no current can be induced.
51
Why are magnets and eddy currents used as car brakes?
They aren't worn down.
52
What is a generator?
Rotating coils of wire in a magnetic field produces a sinusoidal alternating current.
53
What do rms values tell us?
The DC V or I that would deliver the same power.
54
Is the 230V of the mains supply V0 or Vrms?
Vrms
55
What is the time-base of an oscilloscope?
The scale in the x direction
56
What is the y-gain of an oscilloscope?
The scale in the y direction
57
Do transformers work for DC currents?
No, they only work for alternating currents.
58
Why are transformers not 100% efficient?
The changing flux causes eddy currents in the core. These create a magnetic field in the opposite direction to the magnetic field we are trying to induce in the second core - reduces the efficiency of the transformer.
59
How can you reduce the eddy currents in a transformer?
Laminate the core with non-conducting material parallel to the flux change - reduces the eddy currents as they cannot flow through the laminate.
60
How does laminating the core of a transformer reduce power loss?
The laminations increase the resistance of the core, P = V^2/R
61
What 3 things should be done to prevent heat loss in a transformer?
1 - choose a core that can easily be magnetised 2 - choose a core that is laminated to reduce eddy currents 3 - choose low resistance coils to reduce the work done against resistance
62
Give the formulas for power transferred and power lost
Power transferred: P=IV (constant, so high voltage means low current) Power lost: P=I^2R (with R as a constant) So, for a given power output, a high voltage results in low current, so less power is lost.

Physics - A Level (19 decks)

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