EM2 Flashcards
what happens when current passes through a wire?
- a magnetic field is induced
- true for any straight current carrying conductor
- forming concentric rings around the wire
what is magnetic flux density
- measure of the strength of the field
what is one tesla defined as?
- a force of 1N on 1 metre of wire carrying 1A of current perpendicular to a magnetic field
what happens when a current carrying wire is placed in a magnetic field?
- a force is exerted on the wire
- if the current is parallel to the magnetic field, the force is 0N
what equation can be used to find the force exerted on a wire
F = BIL
how can we use FLHR to find the direction of the force exerted
- ThuMb represents the direction of the force
- First finger represents the direction of the field
- SeCond finger represents the direction of conventional current
what is meant by conventional current?
- current that flows in the opposite direction to electron flow
why is there a force exerted on a current - carrying wire?
- a force acts on charged particles moving in a magnetic field
how can we calculate the magnitude of the force exerted on a particle with a charge Q moving at a velocity V perpendicular to a field with flux density B
F = BQv
how can we use FLHR for F=BQV
same principle as F = BIL but use the second finger as direction of travel ( v )
why do the particles follow a circular path inside the magnetic field?
- the force exerted is always perpendicular to the motion of travel
how can we calculate the radius of a particles circular path?
r = mv / BQ
what are cyclotrons used for
- producing ion beams
- radiotherapy
- radioactive tracers
what is a cyclotron made up of
- two semicircular electrodes called dees
- uniform magnetic field acting perpendicular to the plane of the electrodes
- high frequency alternating voltage applied between the electrodes
why does the particles speed not increases inside a cyclotron?
- because of the magnetic field, which is why theres an alternating electric field between the electrodes
how do the particles exit the cyclotron
- particles are accelerated by the electric field
- radius of their circular paths will increase
- alternating electric field changes direction allowing the particles to be accelerated again
- process repeats several times
what is magnetic flux?
- value which describes the magnetic field passing through a given area
- product of magnetic flux density and given area
ø = BA
what is magnetic flux linkage
- magnetic flux multiplied by number of turns N of a coil
Nø = BAN
what happens if the magnetic field is not perpendicular to a coil of wire
- resolve the magnetic field vector into components
BAcosTheta
BANcosTheta
what is meant by electromagnetic induction
- when a conducting rod moves relative to a magnetic field, electrons will experience a force and build up on one side of the rod
- causing an emf to be induced
what is Faraday’s Law
the magnitude of the emf induced is proportional to the rate of change of flux linkage
what is Lenz’s Law
the direction of the induced current is such as to oppos the motion causing it to
how can we demonstrate Lenz’s Law
- measure the speed of a magnet falling through a coil of wire
- measure the speed of the magnet falling through the same height without the coil
why does it take longer for the magnet to reach the ground when it moves through the coil
- as magnet approaches coil there is a change of flux through the coil so an emf is induced
- due to lenz’s law, the direction of the current is such to oppose the motion of the magnet, so the poles match causing repulsion so magnet slows down
- as the magnet passes through the centre there is no change in flux so no emf induced
- as the magnet leavs the coil, there is a change in flux, so a current is induced that opposes the motion of the magnet , therefore an opposite pole, causing attraction so the magnet slows down
give the equation for Faradays law
emf = - N delta flux / delta t
what is the equation for magnitude of emf induced by a straight conductor
emf = B L V
how can we calculate the emf with frequency
emf = BANwsin(wt)
what is peak voltage
- distance from equilibrium to the highest or lowest point
what is peak to peak voltage
- distance from minimum point to maximum point
what is root mean square voltage
- average of all the squares of the possible voltages
what is time period
- distance from peak to peak
why are transformer cores laminated?
- to reduce energy losses due to eddy currents which generate heat
what is the purpose of a cyclotron
a cyclotron accelerates charged particles using a combination of a magnetic field to bend their path and an altenating electric field to increase their speed
how does a velocity selector work
- ensures all ions entering the sprectrometer have the same
speed
link the equation Bqv = Eq
Bqv = Eq
Bv = E
v = B / E
what is flux
a measure of how much magnetism passes through an area
what is the role of the secondary coil ?
- changes the voltage level of the energy supplied
what is the role of the secondary coil ?
- changes the voltage level of the energy supplied
