Magnetic fields Flashcards
what is a force field
an area where an object experiences a non-contact force
what is a magnetic field
an area where a magnet/magnetically susceptible material experiences attraction or repulsion
what way do magnetic field lines point
north to south
what is magnetic flux density
the force on one metre of wire carrying a current of 1A at right angles to the magnetic field
what is 1 Tesla
the flux density that produces 1N in a length 1m with 1A at right angles to the magnetic field
what is flemings right hand rule
when current flows through a wire a circular magnetic field is induced around it
what is flemings left hand rule
when current carrying wire is placed in a field it experiences a force
force on a current equation
F=BIL
force on a moving charge equation
F=BQV
what are the two conditions where a charged particle doesn’t experience a force in a magnetic field
- if it moves parallel to the field line (left hand rule doesn’t apply)
- its stationary (v=0)
what shape path do charged particles follow when in a at right angles in a magnetic field
circular- the force changes the direction the particle is travelling in
what shape path do electric fields follow
parabolic
derive equation for radius with charged particle
r=mV/BQ
derive equation for frequency?time period for charged particle
T=2πm/BQ
f=BQ/2πm
what is a cyclotron
compact particle accelerators often used in hospitals. it consists of two dees where a magnetic field is and an electric field between them
how does a cyclotron work
- a charged particle is inserted into the cyclotron eg proton
- it experiences a force in the electric field and accelerates towards negative plate
- follows circular path in magnetic field (this doesn’t increase speed)
- re-enters electric field which is flipped and accelerates towards other plate.
- enters other “dee” following larger circular path due to r=mv/BQ
- re-enters electric field accelerating again potential difference flipped again (calculated using f=BQ/2πm)
- this repeats until desired speed and exits cyclotron
what happens when a wire is put in a magnetic field
positive charges move up and negatives charges move down creating an EMF. if the circuit is complete a current is induced
what direction does conventional current flows
in the direction of the positive charge
what happens when positive vs negative charge is put in a magnetic field
positive charge moves up
negative charges moves down
what happens when a conductor is put in a magnetic field
it flux cuts the field lines (change in the magnetic flux)
what are the 4 key factors that affect induced EMF size
the speed of movement
number of coils
strength of magnet
area cut through
what is magnetic flux
the amount of lux lines cut with the conductor
what’s the magnetic flux equation
Φ=BAcosΘ
how to achieve flux cutting effectively
wire is formed into square or circle coil, then rotated in magnetic field
why does adding coils increase EMF
each wire acts like a battery, looping the same wire around multiple times means it cuts multiple times. its like putting batteries in series
what is magnetic flux linkage
NΦ=BANcosθ
how do you show flux linkage on a graph
starting at max area- its a cosine curve.
what is Lenz’s law
the induced EMF/current is always in such a direction as to oppose the change that caused it. meaning the EMF/current will be in a direction that creates a force in the opposite direction that first caused it
what’s an example of lens law
dropping a magnet in a copper tube
-falls slower than gravity because magnet field lines are cutting the walls of the tube. inducing an EMF/current in the tube. producing its own magnetic field
what is faraday’s law
the induced EMF/current is directly proportional to the rate of change of flux linkage
what’s the magnitude of induce EMF equation
ε=-NΦ/Δt
what are the gradient and area in faradays law graphs
gradient is -EMF
area under the graph is change in flux linkage
derive faradays law equation
ε=Blv
what does and induced EMF on a rotating coil graph look like
a sine curve (starting at max area)
peak EMF equation
εMAX=BANω
how an EMF graph changes with frequency
if you double magnetic flux density- you double EMF
double frequency squashes graph horizontally and stretches it vertically
what’s the difference between AC and DC
DC is where all the charged particles flow the same way
AC is where the electrons vibrate back and forth many times a second
what is an oscilloscope
effectively a graphical voltmeter, which easy easy to read when it could be changing 50 times a second
how to operate an oscilloscope
-y-gain- increase vertical divisions
-time-base- increase horizontal division (can be turned off)
-y-shift - moves whole graph up/down
-x-shift- moves whole graph left/right
-y-input- plug in power
what does an oscilloscope look like with AC and time base off
a straight line down
what does an oscilloscope look like with AC and time base on
sine curve (moving)
what does an oscilloscope look like with DC and time base off
stationary dot
what does an oscilloscope look like with DC and time base on
stationary horizontal line
how to reduce percentage uncertainty in an oscilloscope
increase y-gain sensitivity
turn off the time base
good practice when measuring time period
shrink curve so it fits the whole screen
freeze the screen if it has that option
what do you need to be able to read of an oscilloscope
peak to peak voltage
peak voltage
time period
frequency
what is rms
you cant calculate an average current or voltage of a AC supply so rms is the equivalent DC voltage delivering the same power
what are the rms equations
Vrms=V0/(root 2)
Irms=I0/(root 2)
why are wires low current in the national grid
because low current means high voltage and if the current is too high then energy is lost through heat
what is a transformer
consists of a soft iron core with two coils of wire. they have no electrical connection, instead making a moving magnetic field
why is AC used in transformers
with an alternating current is used the magnetic field is growing a shrinking constantly which constant cuts the flux lines. with DC the lines cut once and would leave the current to be 0 causing the transformer to not work
how to transformers work
As the field lines produced by the primary coil grow and shrink they
will flux cut the secondary coil * Inducing an alternating current in
the secondary coil
This is why transformers won’t
work with a direct current
.A direct current would create a stationary magnetic field, therefore
no flux cutting
magnetic flux needs to be changing
what causes inefficiencies in transformers
flux leakage- some of the lines aren’t being cut around the primary coil- use a soft iron core to help guide lines
eddy currents-iron core is getting flux cut producing current and wasting energy- laminate the core
resistance-all wires have resistance causing electric energy loss-use thick copper wires (low resistivity)
heat-wires get hot with resistance- use cooling vents
what’s the transformer equation
Ns/Np=Vs/Vp
what’s the efficiency in a normal transformer
80-100%
