electromagnetism

Question 1

what is a magnetic field?

Answer 1

an area around a magnet where other magnets or magnetic materials experience a force. non contact

Question 2

how do you show a magnetic field in a drawing?

Answer 2

magnetic field lines going from north to south. the closer together lines are the stronger the magnetic field

Question 3

where on a magnet is the magnetic field the strongest?

Answer 3

at the poles.

Question 4

how do compasses work?

Answer 4

a small bar magnet is attracted to earth’s poles.

Question 5

what are the 2 types of magnet?

Answer 5

permanent.
induced.

Question 6

what is a permanent magnet?

Answer 6

a magnet that produces its own magnetic field.

Question 7

what is a induced magnet?

Answer 7

a magnetic material that turns into a magnet when put in a magnetic field. the force between a permanent and induced magnet is always attractive

Question 8

how is a magnetic field created around a wire?

Answer 8

when current flows in a wire a magnetic field is created around the wire made up of concentric circles.

Question 9

what is the right hand thumb rule?

Answer 9

point your right thumb in the direction a current is flowing and curl your fingers showing the direction of the magnetic field.

Question 10

what is a solenoid?

Answer 10

a wire wrapped into a coil that creates a magnetic field when supplied current.

Question 11

how does a solenoid increase the strength of a magnetic field in a wire?

Answer 11

the magnetic field around each loop of wire point in the same direction.

Question 12

what is an electromagnet?

Answer 12

a solenoid with an iron core. the magnetic field can be turned on and off with current

Question 13

what is a use of an electromagnet?

Answer 13

cranes in scrapyards can pick up metals.

Question 14

what is the motor effect?

Answer 14

if a current carrying wire is in a magnetic field the wire will experience a force.

Question 15

how do you get the strongest force out of the motor effect?

Answer 15

the wire is at 90° to the magnetic field.
increase the strength of the magnetic field.
increase current through the wire. or conductor

Question 16

what direction does force act in the motor effect?

Answer 16

the force always acts at right angles to the magnetic field and direction of current in the wire.

Question 17

what is the formula for the force acting on a conductor in a magnetic field?

Answer 17

force newtons = magnetic flux density tesla x current amps x length m
magnetic flux density shows the strength of a magnetic field via how many field flux lines there are in a region. a fancy word for magnetic field strength
the length of the conductor in the magnetic field.
formula is only applicable is the current is 90° to the magnetic field.

Question 18

what is fleming’s left hand rule.

Answer 18

point your left index finger in the direction of the magnetic field your middle finger in the direction of the current and your thumb points in the direction of the force.

Question 19

what is a split ring commutator?

Answer 19

a component of a dc motor that swaps the positive and negative connections every half turn. therefore swapping the direction of the current

Question 20

how can the motor effect be shown?

Answer 20

by using a horseshoe magnet with 2 fixed rails inside it and 1 unfixed rail bridging the fixed rails.
when current is supplied the unfixed rail will roll in the direction of the force.

Question 21

how does a dc motor work?

Answer 21

a loop of wire is placed in a magnetic field.
1 side of the loop experiences upwards force and the other side experiences downwards force causing the wire to rotate.
a split ring commutator swaps the positive and negative power connections every half turn meaning the
forces acting on the wire are always the same direction.

Question 22

how does a loudspeaker work?

Answer 22

ac is sent through a coil of wire attached to the base of a paper cone.
the coil surrounds one pole of a permanent magnet such as the south pole and is surrounded by the other pole.
when current is supplied there is a force on the coil causing the cone to move.
variations in the current make the cone vibrate which makes the air around the cone vibrate too creating variations in pressure creating sound waves.
the frequency of the sound waves is the same as the frequency of the ac which can be controlled.

Question 23

what is the generator effect?

Answer 23

when a wire experiences a change in a magnetic field a potential difference is created across the wire.
if the ends of the wire are connected to make a closed circuit a current will flow around the circuit.

Question 24

how can the strength of the current in the generator effect be increased.

Answer 24

increase the strength of the magnetic field the wire is moving through.
increase the speed the wires moves at.
add more turns to the coil of wire.

Question 25

what does induced current oppose?

Answer 25

the change that made it.

Question 26

what is an alternator?

Answer 26

a generator that creates alternating current.

Question 27

how does an alternator work?

Answer 27

a loop of wire is placed in a magnetic field. 1 side of the loop experiences upwards force and the other side experiences downwards force causing the wire to rotate. instead of a split ring commutator like a motor alternators have slip rings and brushes so the contacts don't swap every half turn.

Question 28

what is a dynamo?

Answer 28

a device that converts energy from motion into a direct current.

Question 29

how can you use an oscilloscope to see generated potential difference?

Answer 29

oscilloscopes show how the potential difference generated in a coil changes over time. for ac a line goes up and down crossing the horizontal axis. *like a sine wave* for dc the line stays above the horizontal axis as the pd is always positive. *the height of the line at any point is the generated pd*

Question 30

how do microphones work?

Answer 30

sound waves hit a flexible diaphragm *a thin flexible structure* attached to a coil of wire wrapped around a pole of a magnet surrounded by the other pole. this causes the coil of wire to move in a magnetic field generating a current. the movement of the coil depends on the properties of the sound wave. *louder sounds give more movement*

Question 31

what is a transformer?

Answer 31

a device that changes the voltage of an alternating current.

Question 32

how does a transformer work?

Answer 32

a transformer has 2 coils *primary and secondary* joined with an iron core. when an alternating voltage is applied across the primary coil the iron core magnetises and demagnetises rapidly inducing an alternating voltage in the secondary coil. *because of the generator effect* if the secondary coil is part of a circuit current is induced. the ratio of turns in the primary and secondary coils match the ratio of voltages.

Question 33

what does a step up transformer do?

Answer 33

increases the voltage. *more turns on secondary coil than primary*

Question 34

what does a step down transformer do?

Answer 34

decreases the voltage. *more turns on primary coil than secondary*

Question 35

what is the transformer equation?

Answer 35

**input voltage ÷ output voltage = primary coil turns ÷ secondary coil turns**

Question 36

why is low current beneficial when transmitting power across the national grid?

Answer 36

low current means less energy is lost through heat. *so power is transmitted at high voltage and low current as P = VI*