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PHYSICS > SP13: electromagnetic induction > Flashcards

SP13: electromagnetic induction Flashcards

1
Q

what is needed to make electric current flow in a circuit?

A

voltage or potential difference

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2
Q

how can a potential difference be induced in a conductor?

A

-when there is movement between the conductor and a magnetic field

  1. a coil of wire is moved in a magnetic field
  2. a magnet is moved into a coil of wire
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3
Q

electromagnetic induction/the generator effect

A

when motion between a conductor and a magnetic field creates electricity

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4
Q

what does an induced voltage produce if the conductor is connected in a complete circuit?

A

it produces an induced current, this creates a magnetic field around itself & the magnetic field opposes the original change

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5
Q

what does the direction of the induced potential difference or induced current depend on?

A

the direction of movement

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6
Q

the current is reversed when:

A

-the magnet is moved out of the coil
-the other pole of the magnet is moved into the coil

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7
Q

an induced potential difference or induced current will increase if:

A

-the speed of movement is increased
-the magnetic field strength is increased
-the number of turns on the coil is increased

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8
Q

what is a generator?

A

device that converts kinetic energy into electrical energy

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9
Q

what is an alternator? (AC generator)

A

-a coil of wire rotating in a magnetic field, which induces current as it rotates

-the coil is connected to two metal slip rings (commutators), which allow the current to pass out of the coil
-each side of the wire always connects to the same commutator

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10
Q

alternator output on a voltage-time graph

A

-alternating sine curve

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11
Q

coil at 0° degrees (alternator)

A

coil is moving parallel to the direction of the magnetic field, no potential difference is induced

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12
Q

coil at 90° (alternator)

A

the coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum

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13
Q

coil at 180° (alternator)

A

coil is moving parallel to the direction of the magnetic field, no potential difference is induced

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14
Q

coil at 270° (alternator)

A

-the coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum
-here, the induced potential difference travels in the opposite direction to what it did at 90°

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15
Q

coil at 360° (alternator)

A

-back at its starting point, having done a full rotation
-coil is moving parallel to the direction of the magnetic field, no potential difference is induced

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16
Q

the maximum potential difference or current produced by an alternator or dynamo can be increased by:

A

-increasing the rate of rotation
-increasing the strength of the magnetic field

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17
Q

what is a dynamo? (DC generator)

A

-a coil of wire rotating in a magnetic field, which induces current and voltage as it rotates

-the coil is connected to a split ring commutator
-the side of the coil that is moving down is connected to one part of the commutator, the side that’s moving down is connected to another part
-the split ring commutator changes the coil connections every half turn
-as the induced potential difference is about to change direction, the connections are reversed
-this means that the current to the external circuit always flows in the same direction

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18
Q

dynamo output on a voltage time graph

A

sine curve that stays in the same direction all the time

19
Q

coil at 0° (dynamo)

A

coil is moving parallel to the direction of the magnetic field, so no potential difference is induced

20
Q

coil at 90° (dynamo)

A

coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum

21
Q

coil at 180° (dynamo)

A

coil is moving parallel to the direction of the magnetic field, no potential difference is induced

22
Q

coil at 270° (dynamo)

A

-moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum
-the induced potential difference travels in the same direction as at 90°

23
Q

uses of alternators

A

to charge batteries of ships, cars & motorbikes

24
Q

uses of dynamos

A

-toys
-power tools
-bicycle lights

25
Q

steps of a moving-coil microphone

A
  1. pressure variations in sound waves cause the flexible diaphragm to vibrate
  2. the vibrations of the diaphragm cause vibrations in the coil
  3. the coil moves relative to a permanent magnet, so a potential difference is induced in the coil
  4. the coil is part of a complete circuit, so the induced potential difference causes a current to flow around the circuit
  5. the changing size and direction of the induced current matches the vibrations of the coil
  6. the electrical signals generated match the pressure variations in the sound waves
26
Q

headphones

A
  1. variations in an electric current cause variations in the magnetic field produced by an electromagnet
  2. this causes a cone to move, which creates pressure variations in the air and forms sound waves
27
Q

how to produce sound waves from the alternating current provided to a loudspeaker:

A
  1. a current in the coil creates a magnetic field
  2. the magnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards
  3. the current is made to flow in the opposite direction
  4. the direction of the magnetic field reverses
  5. the force on the cone now pulls it back in
  6. repeatedly alternating the current direction makes the cone vibrate in and out
  7. the cone vibrations cause pressure variations in the air - which are sound waves
28
Q

how do you make a loudspeaker cone vibrate correctly?

A

elastic current must vary in the same way as the desired sound

29
Q

what is a transformer?

A

a device that can change the potential difference or voltage of an alternating current

30
Q

step up transformer

A

increases the voltage

31
Q

step-down transformer

A

reduces the voltage

32
Q

structure of a transformer

A

-a basic transformer is made from two coils of wire, a primary coil from the alternating current (ac) input and a secondary coil leading to the ac output
-coils are wound around an iron core
-the core is easily magnetised and can carry magnetic fields from the primary coil to the secondary coil

33
Q

can transformers use direct current?

A

no, they can only work with alternating current

34
Q

when a transformer is working:

A
  1. a primary voltage drives an alternating current through the primary coil

2, the primary coil current produces a magnetic field, which changes as the current changes

  1. the iron core increases the strength of the magnetic field
  2. the magnetic field passes through the secondary coil
  3. the changing magnetic field induces a changing potential difference in the secondary coil
  4. the induced potential difference produces an alternating current in the external circuit
35
Q

the ratio of potential differences on the transformer coils matches the ratio of…

A

the numbers of turns on the coils

36
Q

equation connecting primary and secondary transformers

A

V(p)/V(s) = N(p)/N(s)

37
Q

V(p)

A

the potential difference in the primary (input) coil in volts

38
Q

V(s)

A

the potential difference in the secondary (output) coil in volts

39
Q

N(p)

A

the number of turns on the primary coil

40
Q

N(s)

A

the number of turns on the secondary coil

41
Q

equation for power output of a transformer

A

V(s) x I(s) = V(p) x I(p)

(potential difference across primary coil × current in primary coil = potential difference across secondary coil × current in secondary coil)

42
Q

what is the assumption when calculating the power output of a transformer

A

a transformer is 100% effective

43
Q

how does the national grid transport electricity?

A

-the higher the current in a cable, the greater the energy transferred to the surroundings by heating
-high currents waste more energy than low currents

-to reduce energy transfers to the environment, the national grid uses step-up transformers to increase the voltage from power stations to thousands of volts, which lowers the current in the transmission cables
-step-down transformers are then used to decrease the voltage from the transmission cables, so it is safer to distribute to homes and factories

PHYSICS (15 decks)

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