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X A2 physics AQA > magnetic fields unit 4 > Flashcards

magnetic fields unit 4 Flashcards

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1
Q

what is a magnetic field?

A

An area where a force is exerted on magnetic materials.

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2
Q

In what direction do magnetic field lines go?

A

From north to south.

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3
Q

what happens to a wire when a current goes through it?

A

A magnetic field will get induced around the wire. They are concentric rings forming around the wire. The direction of the field lines follow the right hand rule.

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4
Q

What is the right hand rule?

A

Stick your thumb up and curl your fingers. Have your thumb pointing in the direction of the current. The curl of the things give the direction of the field lines.

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5
Q

What shape of magnetic field will many coils of wire containing a current produce?

A

The shape of that of a bar magnet.

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6
Q

What happens when a wire carrying a current is in a magnetic field?

A

The 2 field will interact. If for example a wire was put between 2 magnets, the field of the 2 magnets would contract to one side and get closer together. This would mean a force was felt by the wire.

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7
Q

What happens when current in a wire is parallel to flux lines of the magnetic field which it is in?

A

No force will be felt.

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8
Q

When a force is felt by a wire carrying a current in a magnetic field, in what direction will the force go?

A

It will be perpendicular to the direction of current and of the direction of the flux lines. Using the left hand rule to find it.

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9
Q

What is fleming’s left hand rule?

A

The First finger is the direction of the Field lines. The seCond finger is in the direction of the second field. The thuMb gives the direction of the force where motion happens.

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10
Q

What determines the size of a force felt by a current carrying wire in a field?

A

It is proportional to the current (I), the length of the wire (l), and the strength of the magnetic field it is in (B)

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11
Q

What is the definition of magnetic field strength?

A

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

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12
Q

What is magnetic field strength also known as?

A

Flux density. Symbol is B and it is measured in teslas, T

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13
Q

what are the units for the Tesla?

A

Wb/m². Think of flux density as the no. of flux lines (webers) per unit area.

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14
Q

what equation gives the force felt by a charged particle moving in a magnetic field?

A

F=BqV.

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15
Q

What type of path does a charged particle have in a magnetic field?

A

A circular one. The left hand rule states that the force will be perpendicular to the direction of movement. It accelerates towards the centre of a circle has shows circular motion.

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16
Q

How is total magnetic flux found?

A

The total magnetic flux passing through an area A, perpendicular to a magnetic field, B is ϕ=BA.
Units are Webers.

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17
Q

How is the size of an EMF in coil moving in a magnetic field found?

A

Product of the magnetic flux passing through the coil and the no. of turns of the coil.
Փ=Nϕ=BAN. The product of this is called the flux linkage.

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18
Q

How is the flux linkage of a coil that is not at right angles to the magnetic flux density?

A

Փ=BAN cosθ. Imagine a large coil of wire with flat ends and nobly sides where the wire is. θ is the acute angle between the nobly side and the field lines.

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19
Q

Why does an EMF form in a conducting rod moving through a B field?

A

The electrons will feel a force and move about the rod as the rod cuts through flux lines. This will induce an Emf across the ends of the rod and if it is part of a circuit, current will flow. This is electromagnetic induction.

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20
Q

When does an EMF get induced?

A

There is relative motion between the conductor and the magnetic field.

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21
Q

What is flux linking?

A

An emf is induced by changing the magnitude or direction of the magnetic flux.(alternating current electromagnet)

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22
Q

What will a change in flux of one weber per second induce?

A

An EMF of 1 volt in a loop of wire.

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23
Q

What is Faraday’s law?

A

The induced emf is directly proportional to the rate of change of flux linkage.

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24
Q

How is farady’s law written as an equation?

A

Induced emf= flux change/time taken= N(ΔΦ/Δt)

25
Q

A graph of Փ against time is drawn. What is represented?

A

The gradient represents the size of the EMF. The area under it gives flux change.

26
Q

What is Lenz’s law?

A

The induced EMF is always in such a direction as to oppose the change that caused it.

27
Q

The induced emf equation on the formula sheet should have a - sign in front. Why?

A

Lenz’s law.

28
Q

Why must Lenz’s law be true?

A

The induced EMF will oppose the change that caused it as this conserves energy. The energy used to pull a conductor through a magnetic field, against the resistance caused by magnetic attraction, is what produced the induced current.

29
Q

How do transformers work?

A

An AC flows through the primary coil and produces a magnetic flux. This is passed through the iron core to the secondary coil where it induced an alternating voltage of the same frequency.

30
Q

What do step up transformers do?

A

They increase the voltage by having more turns on the secondary coil.

31
Q

Why does the national grid use a low current?

A

To reduce the loss of energy to heat. Energy lost =I²R

32
Q

What is the voltage used in the national grid?

A

400,000 volts.

33
Q

How does the output voltage and current change when generated by a generator?

A

A coil is rotated between 2 magnets. The out put changes direction with every half rotation of the coil producing an AC.

34
Q

At a given time when a coil is in a generator, how is the flux linkage found? What happen to flux linkage when the coil is turned?

A

The amount of flux cut the coil
Փ=BAN cosθ
θ will change so the flux linkage varies sinusoidally between +BAN and -BAN.

35
Q

In the Փ=BAN cosθ equation, where is θ measured from?

A

the angle between the field lines and perpendicular of the coil.

36
Q

When a coil is in a generator, what can be changed in Փ=BAN cosθ?

A

The θ can be replaced with ωt

Փ=BAN cosωt

37
Q

If you have the graphs of flux linkage and EMF, what will the relationship between them be?

A

They are 90° out of phase.

Remember, emf is -gradient of flux linakge because of lenz’s law.

38
Q

When a wire is coiled, why mustn’t the coils go back on them selves?

A

They will cancel themselves out.

39
Q

Why is split ring communicator used in a motor?

A

To reverse the direction the current goes in a coil as it turns meaning it will turn in the same direction.

40
Q

How is the force felt by a coil in a generator found?

A

F=BNIAcosθ

area is

41
Q

What is an equation describing the motion of a moving charged particle in a B field?

A

F=BQV and centripetal force=(mv²)/r

equate the 2 and rearrange to get r=MV/BQ

42
Q

In a linear accelerator, why does the distance between the gaps increase?

A

To allow the same amount of time for the changes to swap as the particle will travel faster.

43
Q

When is the right rule used?

A

It is the same as the LDR but is used to find induced current when you provide the force.

44
Q

What is back emf?

A

The emf induced because of the spinning of a coil in a motor. It is back because of lenz’s law.

45
Q

How does back EMF affect the current?

A

The higher the emf, the lower the current.

46
Q

If the speed of the coil spinning in a motor falls, what will happen to the back EMF?

A

It will reduce the back emf, meaning a higher current and larger force felt by the coil.

47
Q

When is the only time back EMF should be considered?

A

When motors spin and when a load causes a motor to reduce in speed.

48
Q

Describe two causes of the energy losses in a transformer and discuss how they can be reduced.

A

Heat it lost due to current going through coils with resistance. Reduce by using lower resistance wire.(thicker wire)
The ac current in the primary coil continually magnetises and demagnetises the iron core which takes up energy as it heats up the core. Use a material that is easily magnetised and demagnetised like iron, instead of steel.
The iron core cuts the flux lines and the change in flux gives EMFs which produce eddies of current which waste heat. Laminate the core with insulation to prevent eddies from forming.
Wind the coils over each other,making them closer to each other meaning more of the flux is transferred between them.

49
Q

What is Colomb’s law?

A

force between two (point) charges is proportional to (product of) charges and inversely proportional to the square of their distance apart.

50
Q

How do you find the sum of electric potential between 2 charges?

A

Just add them together as potential is scalar.

51
Q

How is the eletric field strength between 2 points in a line found?

A

If they are of different charge, add the 2 different products together.
If the point charges are the same, subtract the smallest from the largest.

52
Q

If there is point that isn’t in line between 2 point charges, how is the magnetic field strength found?

A

use Pythagoras.

the resultant of the two is c^2.

53
Q

What are the units for B?

A

teslas.

54
Q

What are webers a measure of?

A

magnetic flux

55
Q

What does NΦ represent on the formula sheet

A

No. of turns X magnetic flux = flux linkage

56
Q

Under what condition does F= BQV apply?

A

The angle between the magnetic field and the direction of velocity is 90°

57
Q

What are the units of magnetic flux linkage?

A

Weber turns

Or tesla per metre cubed

58
Q

What is B

A

Flux density

X A2 physics AQA (8 decks)

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