SP13: electromagnetic induction

Question 1

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

Answer 1

voltage or potential difference

Question 2

how can a potential difference be induced in a conductor?

Answer 2

-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

Question 3

electromagnetic induction/the generator effect

Answer 3

when motion between a conductor and a magnetic field creates electricity

Question 4

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

Answer 4

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

Question 5

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

Answer 5

the direction of movement

Question 6

the current is reversed when:

Answer 6

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

Question 7

an induced potential difference or induced current will increase if:

Answer 7

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

Question 8

what is a generator?

Answer 8

device that converts kinetic energy into electrical energy

Question 9

what is an alternator? (AC generator)

Answer 9

-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

Question 10

alternator output on a voltage-time graph

Answer 10

-alternating sine curve

Question 11

coil at 0° degrees (alternator)

Answer 11

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

Question 12

coil at 90° (alternator)

Answer 12

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

Question 13

coil at 180° (alternator)

Answer 13

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

Question 14

coil at 270° (alternator)

Answer 14

-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°

Question 15

coil at 360° (alternator)

Answer 15

-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

Question 16

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

Answer 16

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

Question 17

what is a dynamo? (DC generator)

Answer 17

-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

Question 18

dynamo output on a voltage time graph

Answer 18

sine curve that stays in the same direction all the time

Question 19

coil at 0° (dynamo)

Answer 19

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

Question 20

coil at 90° (dynamo)

Answer 20

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

Question 21

coil at 180° (dynamo)

Answer 21

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

Question 22

coil at 270° (dynamo)

Answer 22

-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°

Question 23

uses of alternators

Answer 23

to charge batteries of ships, cars & motorbikes

Question 24

uses of dynamos

Answer 24

-toys
-power tools
-bicycle lights

Question 25

steps of a moving-coil microphone

Answer 25

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

Question 26

headphones

Answer 26

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

Question 27

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

Answer 27

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

Question 28

how do you make a loudspeaker cone vibrate correctly?

Answer 28

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

Question 29

what is a transformer?

Answer 29

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

Question 30

step up transformer

Answer 30

increases the voltage

Question 31

step-down transformer

Answer 31

reduces the voltage

Question 32

structure of a transformer

Answer 32

-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

Question 33

can transformers use direct current?

Answer 33

no, they can only work with alternating current

Question 34

when a transformer is working:

Answer 34

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

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

Question 35

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

Answer 35

the numbers of turns on the coils

Question 36

equation connecting primary and secondary transformers

Answer 36

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

Question 37

V(p)

Answer 37

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

Question 38

V(s)

Answer 38

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

Question 39

N(p)

Answer 39

the number of turns on the primary coil

Question 40

N(s)

Answer 40

the number of turns on the secondary coil

Question 41

equation for power output of a transformer

Answer 41

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)

Question 42

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

Answer 42

a transformer is 100% effective

Question 43

how does the national grid transport electricity?

Answer 43

-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