Magnetic fields Flashcards by Laura McMahon

what is a force field

an area where an object experiences a non-contact force

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what is a magnetic field

an area where a magnet/magnetically susceptible material experiences attraction or repulsion

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what way do magnetic field lines point

north to south

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what is magnetic flux density

the force on one metre of wire carrying a current of 1A at right angles to the magnetic field

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what is 1 Tesla

the flux density that produces 1N in a length 1m with 1A at right angles to the magnetic field

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what is flemings right hand rule

when current flows through a wire a circular magnetic field is induced around it

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what is flemings left hand rule

when current carrying wire is placed in a field it experiences a force

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force on a current equation

F=BIL

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force on a moving charge equation

F=BQV

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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)

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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

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what shape path do electric fields follow

parabolic

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derive equation for radius with charged particle

r=mV/BQ

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derive equation for frequency?time period for charged particle

T=2πm/BQ
f=BQ/2πm

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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

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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

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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

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what direction does conventional current flows

in the direction of the positive charge

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what happens when positive vs negative charge is put in a magnetic field

positive charge moves up
negative charges moves down

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what happens when a conductor is put in a magnetic field

it flux cuts the field lines (change in the magnetic flux)

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what are the 4 key factors that affect induced EMF size

the speed of movement
number of coils
strength of magnet
area cut through

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what is magnetic flux

the amount of lux lines cut with the conductor

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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%