magnetism

Question 1

magnetic field

Answer 1

the region around a magnet where a force acts on another magnet or on a magnetic material e.g. iron, steel, cobalt or nickel

Question 2

uniform magnetic field

Answer 2

when two poles are held close together
it is a magnetic field that has the same strength and direction at all points

Question 3

how to plot a magnetic field

Answer 3

place a magnet on top of a piece of paper
draw one dot at one end of the magnet near its corner
place a plotting compass next to the dot, so that one end of the needle of the compasspoints away from the dot
use a pencil to draw a new dot at the other side of the compass needle, where the arrow points.
move the compass so that it points away from the new dot, and repeat the process until there is a chain of dots going from one end of the magnet to the other
the remove the compass, link the dots together and draw an arrow on the line thatpoints in the direction that the needle of the compass was pointing to
repeat whole process from different places on the magnet

Question 4

earth’s magnetic field

Answer 4

the geographic north pole is the earth’s magnetic south pole (becuase on a compass, the north pole points to the geographic north pole, but it’s the south pole so it’s actually attracting the compass north pole yk)

Question 5

magnetic field around a wire O

Answer 5

current flowing OUT
right hand thumb rule

Question 6

magnetic field around a wire X

Answer 6

current flowing into the plane
right hand thumb rule

Question 7

how to increase the strength of the magnetic field of the wire

Answer 7

increase current

Question 8

Solenoid definition

Answer 8

a Current Carrying Coil used to generate a uniform magnetic field within its core

Question 9

how to determine the poles of a solenoid

Answer 9

if the current is travelling around in a clockwise direction = south
if anticlockwise = north
if current changes direction, north and south are reversed

Question 10

how to increase the strength of the magnetic field produced around a solenoid

Answer 10

increase size of current
increase number of turns in the coil
reducing length of wire and maintaining the number of turns
adding an iron core through the centre of the coils

Question 11

electromagnets

Answer 11

is a solenoid with an iron core
the magnetic field produced by the electromagent can be switched on and off using current

use iron becuase it is magnetically soft material, so when current is switched off there is no magnetic field so iron easily demagnetises

Question 12

how to increase the strength of electromagnetic

Answer 12

increasing number of turns on the coil
increasing the current

Question 13

examples of electromagnetic devices

Answer 13

scrapyard crane
electric bell
relays
door locks

Question 14

scrap yard cranes

Answer 14

when the electromagnet is switched on, as current flows through the coil, it will attract magnetic materials
when the electromagnet is switched off it will drop the magnetic materials

Question 15

electric bell

Answer 15

electric bell contains an electromagnet consisting of coils of insulated wire (solenoid) wound round iron rods
when an electric current flows through the coil, it induces a magnetic field around the rods and attracts the piece of iron attached to the clapper
the clapper hits the gong and makes it ring, at the same time the electrical contact is broken and the iron core is demagnetised so the armature moves back.
this repeats over and over again so the bell rings

Question 16

door locks

Answer 16

when the switch is closed, current flows through the solenoid and the core becomes magnetised
so it attracts the iron bolt and unlocks the door.
when the electromagnet is switched off, the door will be locked as the iron bolt is back in place

Question 17

relays

Answer 17

relays make use of electromagnetism to allow a LOW current to control the passage of a HIGH current. used in the starter motor of a car

close the switch in the low current circuit
causes electric current to flow through the solenoid
induces magnetism in the soft iron core
the iron switch in the high current circuit is attracted towards the solenoid
causes current to flow in the high current circuit, as the switch is now closed
the spring attached to the switch in the high current sircuit keeps the switch away when there is no current through the circuit

Question 18

motor effect

Answer 18

a wire with current flowing through it is placed in a magnetic field and experiences a force
used to create a simple dc electric motor

Question 19

why an electric motor starts to spin

Answer 19

when the current flows through the coil of wire inside the magnetic field there is a force on each side of the coil
on opposite sides of the coil the current flows in opposite directions, this means that the forces are in opposite directions
the fact that the forces are in opposite directions cause the coil to spin around on its axis

Question 20

commutator why

Answer 20

ensures that the current keeps flowing in the correct direction in the coil so that the forces keep the coil turning in the same direction

Question 21

how to increase the speed of the motor

Answer 21

increase current
increase strength of magnetic field
increase number of turns on the coil

Question 22

how can the direction of the motor be reversed

Answer 22

switch the positive and negative of the cell (reversing the direction of the current supply)
switch the north and south poles of the magnet

Question 23

left hand rule

Answer 23

thumb = movement/thrust
first finger = field
seCond finger = current

Question 24

Loudspeakers

Answer 24

work due to the motor effect
an alternating current passes through the coil of the loudspeaker which creates a changing magnetic field around the coil
as the current is constantly changing direction, the direction of the magnetic field will be constantly changing
the magnetic field produced around the coil interacts with the field from the permanent magnet
the interacting magnetic fields will exert a force on the coil
as the magnetic field is constantly changing direction, the force exerted on the coil will constantly change direction which makes the coil oscillate.
the oscillating coil causes the speaker cone to oscillate
this makes the air oscillate, creating sound waves

Question 25

generator effect

Answer 25

motion being used to create electricity If an electrical conductor moves relative to a magnetic field, or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor

Question 26

generating potential difference

Answer 26

as the coil of the generator turns the wires on the side of the coil cuts magnetic field lines. This generates a pd between the ends of the coil The induced voltage drives a current which flows through slip rings which press against the carbon brushes as the generator spins round, the current in the circuit changes direction. so it generates AC

Question 27

how to increase pd in a generator

Answer 27

increasing the strength of the magnetic field increasing the speed the wire/coil/magnet is moved increasing the number of turns on the coil increasing the area/size of the ocil

Question 28

dynamo

Answer 28

same as AC generator, but instead of slip rings it uses a split ring commutator HAS A SPLIT IN IT OMG so when it rotates round, the side of the coil that is moving down is still connected to the same side of the commutator. so DIRECT CURRENT as the coil rotates it cuts through the field lines this induces a potential difference between the ends of of the coil the split ring commutator changes the connections between the coil and the brushes every half turn in order to keep the current leaving the dynamo in the same direction. this happens each time the coil is perpendicular to the magnetic field lines so the induced pd doesn’t reverse its direction so it generates direct current

Question 29

bicycle dynamo

Answer 29

rotating magnet inside/next to coil rotated by its connection to the bicycle magnetic field lines cut through the sides of the coil induces a potential difference in the ocil since the magnetic field is constantly changing direction it generates alternating current

Question 30

how can you increase induced pd in alternator and dynamo

Answer 30

increase frequency of rotation of the coil increasing the number of turns on the coil increasing the strength of the magnet inserting a soft iron core into the coil

Question 31

Microphones

Answer 31

convert sound waves into electrical current when sound waves reach the microphone, the pressure variations cause the diaphragm to vibrate this causes the coil to move back and forth through the magnetic field produced by the magnet so the coil cuts through the field lines this induces a potential difference in the coil pd is alternation because the coil is continually changing direction due to the vibrations of the diaphragm microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electriclal circits

Question 32

how transformer works

Answer 32

An alternating current is supplied to the primary coil the current is constantly changing direction this induces a changing magnetic field inside the primary coil the changing magnetic field is passed from the primary coil to the secondary coil through the soft iron core becuase the iron core is easily magnetised. !No current flows through the core! the changing magnetic field inside the secondary coil induces an alternating current in the secondary coil

Question 33

why high voltage in national grid

Answer 33

increase pd decrease current so less energy is lost to surroundings, as current in wires heats them so more efficient