save my exams electromagnetism Flashcards
wha is a magnetic field?
- it is a region of space where a magnetic pole will experience a force
how are magnetic fields created?
- permanent magnets
- moving electric charge ( ie current )
what is magnetic flux density?
- the number of magnetic flux lines passing through a region of space per unit area
what is 1 tesla?
- the flux density that causes a force of 1N on a 1m wire carrying a current of 1A at right angles to the flux
what can we deduce about flux density and field lines?
- higher flux density, stronger magnetic field so lines are closer together
vice versa
explain why a current carrying wire experiences a force?
- current- carrying conductor produces its own magnetic field ( movement of electrons )
- when interacting with an external B field it experiences a force
what is the equation for the force on a current-carrying wire?
F = BILsin@
F = force on current carrying wire ( N )
B = magnetic flux density ( B )
I = current ( A )
L = length of conductor ( m )
@ = angle between conductor and applied B field
how can we use flemings left hand rule to deduce the direction of the force on a current carrying wire?
thumb - direction of force
first finger - direction of magnetic field
second finger - direction of conventional current
why is there a force on a moving charge?
- a moving charge produces its own magnetic field
- when interacting with an external magnetic field it will experience a force
what is the equation for a force on a charged particle in a magnetic field?
F = BQVsin@
F = magnetic force on particle ( N )
B = magnetic flux density ( T )
Q = charge of the particle ( C )
v = speed of the particle (ms^-1)
how can we use FLHR for direction of force on a moving charge?
thumb - direction of force
first finger - direction of magnetic field
second finger - flow of positive charge
why do charged particles undergo circular motion?
- charged particles will experience a force perpendicular to is direction of motion.
- ie a centripetal force
derive the equation for a charged particles radius within a magnetic field
F = mv^2 / r and F = BQv
mv^2/r = BQv
mv/r = BQ
r = mv / BQ
what does the r = mv/BQ equation tell us?
- radius is proportional to the velocity
- radius is proportional to the mass
- radius is inversely proportional to the charge
- radius is inversely proportional to the magnetic field strength
what are cyclotrons?
- particle accelerators that make use of circular trajectory to create a spiral path
what are cyclotrons made up of?
- two semi circular dees
- uniform magnetic field
- alternating electric field
what are uses of cyclotrons in medical industry?
- produce tracers for imaging ( as low half lives )
- high energy beams for radiotherapy
what happens when the particles enter the dees?
- magnetic field applied perpendicular to the path of the particles
- magnetic force acts in a direction perpendicular to the field and path of the particles
- particles undergo circular motion
what happens when a particle crosses a gap between the dees?
- electric field causes particle to accelerate to opposite dee
- causing the particles to enter the dee with a greater v so greater r
what happens when a particle leaves the dee?
- constant speed in the dees
- time spent inside the dees are constant
- direction of e field alternates each time particles reach a gap
- process will repeat until they leave the cyclotron
what is electromagnetic induction?
- the process of inducing an emf in a conductor
- when there is movement between a charge and a magnetic field
when is there electromagnetic induction?
- when a conductor CUTS through magnetic field lines
what is magnetic flux?
- the product of the magnetic flux density and the cross sectional area perpendicular to the direction of the magnetic flux density
what is the equation for magnetic flux?
$ = BA
$ = magnetic flux ( Wb )
B = magnetic flux density ( T )
A = cross sectional area ( m^2 )
what is magnetic flux linkage?
- the product of the magnetic flux and the number of turns on the coil
recap, what is electromagnetic induction?
- the process by which an emf is induced when a conductor moves through a magnetic field
what is faradays law?
- the magnitude of the emf induced is proportional to the rate of change of flux
what is lenzs law?
- the direction of the induced emf is such that to oppose the motion that caused it
what does the gradient of a flux time graph give us?
- the emf
what is the equation for induced emf for a straight conducting rod moving through a magnetic field?
emf = Blv
B = magnetic field strength
L = length of conductor
v = velocity
how is an alternating voltage induced with a rotating coil?
- when a coil rotates in a magnetic field, flux through coil will vary
- as emf is rate of change of flux linkage, emf will change as it rotates
- maximum when coil cuts through most field lines
- minimum when coil cuts through least or zero
explain the phase relationship between flux linkage and emf for a rotating coil
90 degrees out of phase as:
- flux linkage maximum when angle = 0 or 180
- zero when angle = 90 or 270
emf is zero at max flux, as flux isnt changing
what is meant by alternating current?
- current which periodically varies between a positive to a negative value with time
what is peak voltage? or peak current?
- maximum value of the alternating current or voltage
what is peak-to-peak voltage?
- distance between a positive and consecutive negative peak
( i.e double the magnitude of the peak voltage )
what is root mean square current?
- square root of the mean of the squares of all the values of current in one cycle
Irms = Io / root 2
what is root mean square voltage?
- the square root of the mean of the squares of all the values of the current in one cycle
what is a transformer?
- a device which changes high alternating voltage at low current, to low alternating voltage at high current
why is the iron core necessary?
- directs the magnetic field passing from primary coil to the secondary coil
why is an emf induced in the secondary coil?
- in primary coil, alternating current produces alternating voltage, creating an alternating magnetic field inside the core, therefore a changing magnetic flux
- a changing magnetic field passes to the secondary coil through the iron core
- resulting in a change of magnetic flux linkage within the coil so emf induced
give the transformer equation?
Ns / Np = Vs / Vp
what is a step up transformer
Ns > Np, increases voltage of power source
( used between power stations and transmission cables )
what is a step down transformer ?
Np > Ns, decreases voltage of power source
( used between transmission lines and buildings )
in reality transformers are not 100% efficient, give the equation for an ideal transformer efficiency
( IsVs / IpVp )x 100
how do eddy currents arise?
- a changing magnetic field from alternating current
- creating a changing magnetic field in the core that acts against the field that induced them
- emf is therefore induced
- a current flows, as core is made from conducting material
eddy currents dissipate energy by generating heat in the wires, how can we reduce this
- laminate the core
- use high resistivity material
