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PT > Electricity and magnetism > Flashcards

Electricity and magnetism Flashcards

1
Q

What is the right-hand grip rule used for?

A

wires and Lenz’s Law

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2
Q

What is the motor effect?

A

When current flows through a wire in a magnetic field that isn’t parallel with the current, a force acts on the wire.

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3
Q

In a motor, how can the direction of the force be reversed? (Two points)

A

Two other:

  1. Reversing the direction of the current
  2. Reversing the direction of the field, by turning magnet around
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4
Q

How does Flemings left hand rule work?

A

thumb-motion (forces)
first- field
second- current

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5
Q

Electric motors are designed to…to..

A

…use the motor effect
…to create a turning movement.

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6
Q

At what position is there no forces acting on the wire?

A

Vertical position. Reliant on momentum to carry itself further round.
The forces have no turning effect. Turning effect changes direction past vertical.

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7
Q

How can the turning effect be increased in motors?

A

FaCT:
-turns
-current
-field

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8
Q

What does a commutator do?

A

A commutator reverses the current in the coil every 180° by reversing the power suply connections,

allowing the coil to keep moving in the same direction.

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9
Q

What is Fleming’s left-hand rule used for?

A

motors and beam of electrons

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10
Q

What should you remember when using the rule for beams of electrons?

A

Electron flow is in the opposite direction of conventional current.

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11
Q

How can the force on the beam of electrons be reversed?

A

By reversing the field.

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12
Q

What is the electron beam effect used in?

A

Particle accelerators, to focus and divert beams of charged particles.

These particles have a lot of kinetic energy, so travel at enormously high speeds. Huge fields are used to divert them.

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13
Q

What do all generators have in common?

A
  • a magnetic field (prod by magnets or electromagnets)
  • a coil of wire (fixed or moving)
  • movement (coil and field move relative to each other)
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14
Q

If the generator is connected to a complete circuit…
If it is not…

A

..induced current flows
..induced e.m.f/voltage will be at its ends ready to make current flow around the circuit.

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15
Q

What is induced e.m.f?

A

The production of emf across an electrical conductor when it cuts through field lines

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16
Q

What is electromagnetic induction?

A

the production of an e.m.f across an electrical conductor when there is relative movement between the conductor and the field.

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17
Q

In terms of electromagnetic induction, a current flows when the wire is moved downwards. Explain what happens when the wire is moved upwards.

A

current flows in an opposite direction

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18
Q

In terms of electromagnetic induction, describe how to make a current flow.

A

move the wire or magnet up and down

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19
Q

A centre-zero meter

A

Needle to the left, the current flows one way, to the right the other way

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20
Q

How do you increase induced e.m.f?

A

-Stronger magnet
-move wire more quickly
-turns

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21
Q

[Induction and field lines] What does the cutting rule tell us?

A

further away –> fewer lines –> smaller e.m.f

quicker–> cut more quickly–> bigger e.m.f

more turns–> bigger e.m.f

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22
Q

What is Fleming’s right-hand rule used for?

A

generators and electromagnetic induction

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23
Q

What does a slip ring do in an a.c generator?

A

Slip rings provide
a continuous connection
as they rotate with the coil.

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24
Q

What are slip-rings?

A

Device used to allow current to and from the a.c generators’ coil. Induced current can flow to where it is needed.

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25
Q

How do you increase the voltage generated in an a.c generator?

A

ratm:

-turn more rapidly
-turns of wire
-coil with bigger area
-stronger magnets

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26
Q

Explain the direction of the turning effect as seen by an observer at O.

A

clockwise
force on left of coil up OR..

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27
Q

Which two structures does a motor have?

A

brush and commutator

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28
Q

Which two structures does a generator have?

A

slip rings and brushes

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29
Q

What is a commutator?

A

device used to allow current to flow to and from coil of d.c motor or generator

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30
Q

Suggest one reason why the DC motor cannot operate without a split-ring commutator

A

The coil cannot continue to rotate in the same direction.

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31
Q

How can you increase electromagnetic induction?

A

Are tm:

  • stronger magnet
  • wire/coil more rapidly
  • more turns
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32
Q

How can you reverse the current when working with electromagnetic induction?

A

reverse magnet

33
Q

How AC generators work? / Explain how rotating the coil continuously causes the galvanometer needle to show an alternating current.

A
  1. Axle is turned so coil rotates. Coil cuts field between the magnets, inducing an e.m.f. in the coil.
  2. This induces a current in the coil which is transferred to the galvanometer via the slip rings and brushes.

Direction of current flow changes with each 180 degree rotation of coil.

34
Q

What is Lenz’s law?

A

the direction of an induced current
always opposes the change in the circuit
or the magnetic field that produces it

N brought close, N induced
(in the direction opp the magnet movement)
in=field opposes, out=field attracts

35
Q

State the position of the rotating coil when the alternating output voltage is at a
maximum value and explain why the maximum output occurs at this position.

A

Horizontal. Maximum number of magnetic field lines are cut at fastest rate.

36
Q

How does a transformer work?

A

A.C in primary generates changing magnetic field. Iron core transfers field to the secondary coil.
Secondary coil cuts magnetic field and an e.m.f. is induced in secondary coil.

37
Q

Why transformers do not work on DC current?

A

An unchanging magnetic field passes through the secondary coil, so no voltage is induced.

38
Q

Explain why the power losses in transmission cables are lower when electrical energy is transmitted at higher voltages.

A

The same power with a smaller current and higher voltage, means less resistance. Reduces the loss of energy due to cables heating up (cables heat up less)

39
Q

How are transformers used in high voltage transmissions?

A

Step-up transformers increase the v of a.c supply to reduce energy loss by heating in cables over long distances. Step-down transformers decrease to a safer level before delivering to homes and businesses.

40
Q

Why are transformers used in high voltage transmission.

A

The same power with a higher voltage, means a smaller current. Reduces the loss of energy due to cables heating up (cables heat up less)

Very efficient= reduced energy loss

41
Q

What is the equation relating voltage in coils and turns in coil?

A

Vp/Vs=Np/Ns

42
Q

Calculating power in transformer.

A

P=IV

43
Q

Calculating power loss.

A

P=I^2 x R

44
Q

Finding the current or voltage in a transformer equation.

A

Ip x Vp = Is x V s

45
Q

A positive rod is brought near object A. A wire is connected from conducting object A to the Earth. State and explain any movement of charge.

A

Electrons flow from Earth to object to balance the positive charge on the object.

46
Q

An electrical device has a metal case. Explain the benefit of earthing the metal case.

A

Earthing provides a low resistance path for the current to flow to the ground. A high current will cause the fuse to melt which stops current to flow to the appliance.

47
Q

Explain the dangers of touching a metal case that has a live wire touching the case.

A

fatal electric shock (electrocution)

48
Q

What is the reason for choosing a particular fuse rating?

A

The current rating should be just above the value of current that flows when an appliance is operating normally.
Too low -> melt during normal operation
Too high -> cables can burn out + fires, dangerous

49
Q

If there is high current why is it necessary to use thick wire?

A

Have smaller resistance. Heated less, so less risk of insulation melting or fire.

50
Q

Explain why a high voltage increases the efficiency of transmission even with thinner wires.

A

Power loss equation. High voltage allows low current. Thin wires have high resistance. Current has greater effect than wire resistance.

51
Q

Name the process by which a changing current in the primary coil P causes a changing
current in the secondary coil S

A

Electromagnetic induction

52
Q

State the component in the transformer to which the a.c. generator is connected.

A

primary coil

53
Q

State the other main component of a transformer and the material from which it is made.

A

iron core

54
Q

State the effect on the motor of reversing the connections to the battery

A

rotation reversed

55
Q

The battery in Fig. 9.1 is replaced with a resistor. The coil is made to rotate by an external
mechanism.
Explain why there is a current in the resistor.

A

Electromagnetic induction. Magnetic field is cut and current is induced.

56
Q

State two ways in which the graph would be different if the coil spins at a faster rate.

A

higher frequency and greater amplitude

57
Q

The magnet is pushed into the solenoid, and then brought to rest with its N-pole just inside the solenoid.
Describe the movement of the pointer of the meter M.

A

pointer deflects then returns to zero

58
Q

The magnet is now pulled to the left out of the solenoid, at a higher speed than in (i).
Compare the movement of the pointer of the meter with that seen in (i).

A

greater deflection in opposite direction for a short time

59
Q

State an assumption you made in your calculation for (b)(i).

A

transformer is 100% efficient

60
Q

The variable resistor is adjusted so that its resistance changes quickly.
State and explain what is seen to happen in the circuit of the second solenoid.

A

Ammeter needle deflects. Electromagnetic induction: changing field is cut and current induced.

61
Q

The variable resistor is adjusted much more slowly.
State and explain the difference in what is seen to happen in the circuit of the solenoid. (3 points)

A

slower rate of cutting field lines, deflection is smaller
reading on ammeter is smaller

62
Q

Explain why the forces on AB and
CD cause the loop to rotate about the axis.

A

forces opposite, causes a moment, rotates

63
Q

State and explain the effect on the output from the secondary coil of when increasing the voltage across the primary coil.

A

Greater field. Turns and voltages equation. Output power increases.

64
Q

When is Earthing useful?

A

when live wire touches metal case (not when excess current flows into the circuit)

65
Q

When is a fuse useful?

A

When excess current flows into the circuit (not when live wire touches metal case)

66
Q

Describe and explain what is observed on the ammeter as the magnet falls from the middle
point of the solenoid to the outside.

A

Meter deflects in opposite direction.
Deflection is greater initially.

magnet moves faster
more field lines cut per second

67
Q

Features: motors Vs generators

A

motor-> motion prod, battery
generator-> crank, current prod

68
Q

A very small negatively-charged oil drop in the air between a pair of oppositely charged horizontal metal plates.

Suggest, in terms of forces, why the oil drop does not move up or down.

A

upward force due to charge of plate=downward force due to weight of drop

no resultant force, forces balanced

69
Q

Without losing any of its charge, the oil drop begins to evaporate.
State and explain what happens to the oil drop.

A

Weight decreases. Upward force is now greater. Moves upwards.

70
Q

The hanging ball is attracted towards the charged rod.
Explain why this happens, given that the ball is uncharged.

A

Negative charges are close. Attraction between opposite charges is greater than repulsion between like charges.

71
Q

An earthed metal wire is touched against the surface of the sphere and the sphere
is discharged. State what happens in the wire as the sphere is discharged.

A

electrons move through wire from the sphere to the Earth

72
Q

A negative point charge Z placed near to sphere S, (full of electrons). The charge experiences a force due to the electric field surrounding S. Draw an arrow to show the direction of this force on the charge.

A

line from sphere, pointing towards Z, through Z

73
Q

Voltage in series and parallel.

A

V=V1+V2+V3 (series)
voltage is the same everywhere (parallel)

74
Q

Current in series and parallel.

A

current is the same everywhere (series)
current is split between branches, adding them gives current in power supply (parallel)

75
Q

Resistance in series and parallel.

A

R=R1+R2+R3 (series)
(fractions for parallel)

76
Q

An LDR and LED are in parallel. How does the reduced resistance of the LDR effect the p.d. across the LED?

A

Resistance of parallel combination of LDR and LED increases, so pd increases.

Greater proportion of p.d. across parallel combination of LDR and LED

77
Q

What do brushes do in generators?

A

Rub against slip rings as they turn, ensuring that brushes have the same emf as the coil sides. They maintain the current.

78
Q

Changing the resistance using area and length.

A

R is directly proportional to L, twice as long means twice R

R is inversely proportional to A, half cross-sectional area means twice R

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