7 Magnetism and Electromagnetism

Question 1

which direction do magentic field lines go to

Answer 1

North –> South
showing which way a force would act on the North pole if it was put in the field

Question 2

the closer the lines are…

Answer 2

the stronger the magnetic field

Question 3

where is the magnetic field strongest

Answer 3

at the poles of amegnet meanign the magnetci forces are also strongest at the poles

Question 4

which poles whcih attract and which would repel

Answer 4

attract : N-S
repel : N-N and S-S

Question 5

permananent vs induced magents

Answer 5

• permanent magnets produce their own magentc field
• induced magents are magentci amaterials that turn into a megnet whrn they’re put into a magneti field
• force between magentci and induced magnets is always attractive
• when take away the magentci field , induced magents quicly lose their magentism and stop producing a magentic field

Question 6

what happens when a current flows through a wire

Answer 6

• a maegntic field is created around the wire
• forms a circlur field with the wire in the middle
• changing the direction of the current changes the direction of the magnetic field
• rigth hand thumb rule

Question 7

how can you increase the strength of a magentic field produced by a wire

Answer 7

• increaing current
• wrapping the wire around a core
• adding multiple coils around the core

Question 8

what is a solenoid

Answer 8

• wrapping wire into a coil

Question 9

how does creating a solenoid increase the magnetic field

Answer 9

• the field line around each loop of wire line up with each other
• results in lots of field lines poitning int he same direction that are very close to eachother - the closer the field liens the stronger the field

Question 10

what is the formation of magentic field in/outside a solenid

Answer 10

• inside the field lines are strong and unifrom - same strength and difrection
• outside the lines are liek ones around bar

Question 11

what is an electromagnet

Answer 11

• a solenoid with an iron core
• its magentic field can be turned on and off with an electric current

Question 12

hwo to find the size of the force acting on a conductor in a magentic field

Answer 12

• force = magentic flux density x density x length
• F = BIL
• can only calculate if current is at 90 degrees to the magnetcu field

Question 13

how to find the direction of the force

fleming’s left hand rule

Answer 13

flemings left hand rule
- thumb - thrust = force
- first finger - magntic field
- second finger - current
this shows that if eitehr the current or the magnetci field is reversed then the direction of the force will also be reverse

Question 14

the motor effect

Answer 14

• a current carrying wire, which has its own magentic field, put between magentic poles will winteract with the magnetci field its placed in
• causes the magnet and the conductor to exert a force on eachother

Question 15

how a motor works

Answer 15

• a coil with two arms carrying a current is placed between 2 magnets
• a force acts on the arms (flemings left hand rule)
• the arms move up/down and rotate- depends on which side the poles are, which side current is flowing in from
• ## a split ring commutator is used to keep the ucrrent flowing inf ormt he same directio

Question 16

how do loudpseakers work

Answer 16

uses motor effect
- an a.c current is sent through a coil of wire attached to the base of paper cone
- the coil surrounds one pole of a permanent magnet and is surrounded byt he other pole so the current causes a force on the coil which causes the cone to move
- when the current reverses the force acts int he opposite direction which causes the cone to move in the opposite direction
- so variations in the current make the cone vibrte which makes the air arround the cone vibrate and creates variations in pressure that cause a sound wave
- the freuqncey of the sound wave is the same ad the Hz of the ac, so by controllign the Hz of the ac you alter the cound wave produced

Question 17

what is the generator effect

Answer 17

• a potential difference is induced when a coil moves through a maegnetic field
• if this is a complete circuit it induces a current
• the direction fo the current switches when the direction of movement stops
• happens if the wire is still but magnetic field changes
• has to be 90 degrees to the magentic field

Question 18

how do altenators/a.c. generators work

Answer 18

An alternator is a rotating coil in a magnetic field with commutator rings
- A rectangular coil that forced to spin in a uniform magnetic field
- The coil is connected to a centre-reading meter by metal brushes that press on two metal slip rings (or commutator rings)
- The slip rings and brushes provide a continuous connection between the coil and the meter
When the coil turns in one direction:
The pointer defects first one way, then the opposite way, and then back again
This is because the coil cuts through the magnetic field lines and a potential difference, and therefore current, is induced in the coil
The pointer deflects in both directions because the current in the circuit repeatedly changes direction as the coil spins
This is because the induced potential difference in the coil repeatedly changes its direction
This continues on as long as the coil keeps turning in the same direction
The induced potential difference and the current alternate because they repeatedly change direction

Question 20

how do dynamos/dc generators work

Answer 20

as the coil rotates, it cuts through the field lines
This induces a potential difference between the end 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
Therefore, the induced potential difference does not reverse its direction as it does in the alternator
Instead, it varies from zero to a maximum value twice each cycle of rotation, and never changes polarity (positive to negative)
This means the current is always positive (or always negative)

Question 21

how do microphones work

Answer 21

uses generator effect
- sound waves hit a felxible diaghram that is attached to a coil of wire wrapped around a magnet
- this causes the coil to move in the magentic field which generates a current
- the movement of the coil and so the generated current depends on the properties of the cound wave
- convert the pressure variations of a sound wave into variations in current in an eletric circuit

Question 22

what are transforms and what do they do

Answer 22

• have 2 coils of wire primary and secondary joined with an iron core - one of the coils will have less loops
• change the size of the p.d of an alternating current

Question 23

how do transformers work

Answer 23

• a primary voltage drives an alternating current
  through the primary coil
• the primary coil current produces a magnetic field, which changes as the current changes
• the iron core increases the strength of the magnetic field
• the changing magnetic field induces a changing potential difference in the secondary coil
• the induced potential difference produces an alternating current in the external circuit

Question 24

tranfromer equations

Answer 24

Vp/Vs= Np/Ns
V= voltage
N= number of coils

VsIs=VpVp
V=Voltage
I=Current

p=primary s=secondary