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A Level Physics > Magnetic Fields > Flashcards

Magnetic Fields Flashcards

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1
Q

Define the term Tesla

A

A force of 1N on 1 metre of wire carrying 1A of current perpendicular to a magnetic field; it is the unit of magnetic flux density

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2
Q

When current passes through a wire what is induced?

A

A magnetic field.

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3
Q

What is Faraday’s Law?

A

The magnitude of the induced EMF is directly proportional to the rate of change of magnetic flux linkage.

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4
Q

What is magnetic flux density?

A

The measurement of the strength of a magnetic field (measured in Tesla)

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5
Q

What design do transformers have to reduce heat energy loss?

A
  • Low resistance wires
  • soft iron core which is easily magnetised and de-magnetised
  • a laminated core made up of insulated sheets of iron which reduces the size of eddy currents
  • the two coils are positioned close together to ensure the maximum amount of flux is passed from the primary coil to the secondary coil
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6
Q

Describe what each finger is used for when using Fleming’s Left Hand Rule

A

ThuMb = The direction of the Motion/ Force
First finger = Direction of the Field
SeCond finger = Direction of the Conventional Current (opposite direction to the electron flow)

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7
Q

If current is parallel to the magnetic field what is the induced current?

A

0 N

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8
Q

What is magnetic flux linkage?

A

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

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9
Q

What is magnetic flux linkage?

A

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

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10
Q

What is Lenz’s law?

A

The direction of the induced emf is always such to oppose the change which has caused the emf to be reduced

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11
Q

Describe a cyclotron and its use

A

It is a type of particle accelerator used to produce ion beams for radiotherapy and radioactive tracers. It contains two semi-circular electrodes, a uniform magnetic field acting perpendicular to the plane of the electrodes, and a high-frequency alternating voltage between the electrodes.

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12
Q

What is magnetic flux?

A

The value describing the magnetic field lines passing through an area, when the field is perpendicular to the area.

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13
Q

What type of current is used in transformers?

A

Alternating Current

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14
Q

Explain how a particle is accelerated in a cyclotron

A

The particles speed increases as they pass between the two semicircular electrodes (the dees), they are accelerated by the alternating electric field. They then travel at a constant speed with a circular path through the uniform magnetic

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15
Q

What is the motor effect?

A

The force exerted on a current-carrying conductor in a magnetic field.

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16
Q

How do charged particles move?

A

In a circle

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17
Q

What does Faraday’s Law state?

A

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

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18
Q

What do you do when the lines of the magnetic field are not perpendicular to the normal of the plane?

A

You have to resolve the magnetic field into vector components.

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19
Q

What is the equation for a charged particle in a magnetic field

A

F = BQv

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20
Q

What varies depending on the angular speed of the coil?

A

Magnetic flux linkage.

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21
Q

Why is an alternating current required in the primary coil of a transformer?

A

Because the primary coil has to generate an alternating magnetic field so that there is a change in magnetic flux experienced by the secondary coil which will cause there to be an induced EMF in the secondary coil

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22
Q

What is required for there to be an induced current rather than just an induced EMF

A

A complete circuit

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23
Q

How are eddy currents formed?

A

The core is also a conductor, an EMF is induced due to the alternating magnetic field causing current to flow in the core which forms eddy currents and releases heat.

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24
Q

What type of transformer increases the input voltage, and which coil has more turns?

A

A step-up transformer, and the secondary coil has more turns than the primary coil.

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25
Q

Describe the path of the particles in a cyclotron

A
  1. The charged particles move from the centre of the electrodes and are deflected in a circular path by the magnetic field.
  2. Once the particles reach the edge of one electrode, they are accelerated by the electric field to move across the gap.
  3. Therefore, the radius of the path will increase.
  4. Once it reaches the gap again, the alternating electric field changes direction to allow the particle to accelerate again.
  5. The process repeats until the particle has exited the cyclotron.
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26
Q

Why are transformers not always efficient?

A

They waste energy as heat, the resistance in the coil produces heat.
The AC current magnetises and de-magnetises the core continuously produced heat
The magnetic flux created by the transformers primary core induces an eddy current within the core. These induce their own magnetic fields and produce heat

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27
Q

What does Lenz’s Law contribute to the equation for induced EMF and why?

A

a ‘-‘ sign because the induced EMF is always in such a direction as to oppose the change that caused it

28
Q

Where do we measure the angle from in a rotating coil within a generator?

A

The angle is measured from the ‘pointer’ an arrow which is perpendicular to the face of the arrow.

29
Q

What is the transformer equation?

A

Ns/ Np = Vs/ Vp
No. coils secondary transformer / no. coils primary transformer = Voltage secondary/ Voltage primary

30
Q

The force exerted is always __________ to the motion of travel.

A

perpendicular

31
Q

How do generators/dynamos work?

A

They induce an electrical current by rotating a coil in a magnetic field

32
Q

How do you reduce eddy currents?

A

Use a laminated core

33
Q

How do you determine the average power supplied by an AC source

A

root mean square current x root mean square potential difference

34
Q

What does the second finger represent in Fleming’s left hand rule?

A

Conventional Current, which flows from positive to negative.

35
Q

What are the Dees in a cyclotron?

A

The Dees are two semi-circular electrodes.

36
Q

Why is it not possible for a magnetic field to alter the speed of a proton while it is in a dee in a cyclotron?

A

The forces due to magnetic field are perpendicular to motion so it can only change its direction and not speed (no work is done)

37
Q

Describe the design features of a transformer that increases efficiency

A
  1. Low resistance, e.g. Copper
  2. Soft iron core: easily magnetised and demagnetised
  3. Laminated core: thin insulated sheets of iron reduces the size of the eddy currents
  4. The two coils are positions close together: ensures the maximum amount of flux is passed from the primary coil to the secondary
38
Q

In a cyclotron at what point are the particles being accelerated?

A

When the particles are passing through the gap between the Dees.

39
Q

Define “eddy current”

A

A localised electric current induced in a conductor by a varying magnetic field

40
Q

What is Faraday’s Law?

A

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

41
Q

What is Lenz’s Law?

A

The direction of induced current is such as to oppose the motion causing it

42
Q

What shape are the field lines of a magnetic field that has formed around a wire?

A

The field lines appear in the shape of concentric rings around the wire.

43
Q

What is the “Anti-Christ” Equation?

A

𝑁Ф = BAN cos 𝜃

44
Q

Why is an alternating current used rather than a direct current when transporting electricity through transmission cables?

A
  • Minimum energy will be lost
  • Lower current means less heat will be lost
  • An alternating current means the current can be reduced using a transformer
45
Q

How do charged particles move in a magnetic field?

A

They follow a circular path.

46
Q

What happens to the direction of force when the direction of current is reversed?

A

Direction of force is also reversed

47
Q

What shaped wire increases the strength of the magnetic field when current passes through it?

A

A solenoid shape

48
Q

Describe the two types of transformer

A

Step-up transformer: increases the input voltage by having more turns on the secondary coil than the primary
Step-down transformer: decreases the input voltage by having less turns on the secondary coil

49
Q

Which formulas do you combine to get the formula to find the radius of a particles circular path in a magnetic field.

A

The formula for centripetal force and the formula for magnetic force on a charged particle.

50
Q

Describe the electromagnetic induction phenomenon

A

When a conducting rod moves relative to a magnetic field, the e- in the rod will experience a force (as charged particles), and build up on one side of the rod = emf to be induced
OR
If you move a bar magnet relative to a coil of wire, if the coil forms a complete circuit, a current is induced

51
Q

What does B represent in magnetic field equations?

A

Magnetic flux density.

52
Q

What does Nϕ represent in magnetic field equations?

A

Magnetic flux linkage.

53
Q

What does N represent in magnetic field equations?

A

The number of turns of a coil.

54
Q

What are the units of flux linkage?

A

Webers (Wb)

55
Q

What does A represent in magnetic field equations?

A

A represents the given area.

56
Q

What shape wave does an alternating current make?

A

A sinusoidal wave

57
Q

Why are step-up transformers used when transmitting electricity over long distances?

A

As the voltage is stepped up, the current will decrease

58
Q

Describe the field lines of an induced magnetic field around a wire

A

Concentric rings

59
Q

What are the names of the two laws for electromagnetic induction?

A

Faraday’s and Lenz’s

60
Q

What is the force when correct is parallel to the magnetic field

A

0N

61
Q

What are the units for magnetic flux?

A

Weber (Wb)

62
Q

How can you demonstrate Lenz’s law?

A

By measuring the speed of a magnet falling through a coil of a wire, and measure the speed of a magnet falling without the coil, and compare them.

63
Q

What is rms Voltage?

A

Root mean square voltage - the average of all squares of the possible voltages, which gives you the average value of voltage output by the supply in either direction

64
Q

How do you calculate the rms Voltage or rms Current on a sine wave?

A

Use the Formulas:
Irms = I0/ root2, Vrms = V0/ root2 ​
Where I​ and V​ are peak values of current and voltage.

65
Q

What does N(Δt/ΔΦ) stand for?

A

The rate of change of flux linkage.

A Level Physics (1 decks)

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