7 Magnetism and Electromagnetism Flashcards
which direction do magentic field lines go to
North –> South
showing which way a force would act on the North pole if it was put in the field
the closer the lines are…
the stronger the magnetic field
where is the magnetic field strongest
at the poles of amegnet meanign the magnetci forces are also strongest at the poles
which poles whcih attract and which would repel
attract : N-S
repel : N-N and S-S
permananent vs induced magents
- 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
what happens when a current flows through a wire
- 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
how can you increase the strength of a magentic field produced by a wire
- increaing current
- wrapping the wire around a core
- adding multiple coils around the core
what is a solenoid
- wrapping wire into a coil
how does creating a solenoid increase the magnetic field
- 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
what is the formation of magentic field in/outside a solenid
- inside the field lines are strong and unifrom - same strength and difrection
- outside the lines are liek ones around bar
what is an electromagnet
- a solenoid with an iron core
- its magentic field can be turned on and off with an electric current
hwo to find the size of the force acting on a conductor in a magentic field
- force = magentic flux density x density x length
- F = BIL
- can only calculate if current is at 90 degrees to the magnetcu field
how to find the direction of the force
fleming’s left hand rule
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
the motor effect
- 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
how a motor works
- 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
how do loudpseakers work
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
what is the generator effect
- 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
how do altenators/a.c. generators work
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
how do dynamos/dc generators work
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)
how do microphones work
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
what are transforms and what do they do
- 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
how do transformers work
- 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
tranfromer equations
Vp/Vs= Np/Ns
V= voltage
N= number of coils
VsIs=VpVp
V=Voltage
I=Current
p=primary s=secondary
