Magnets Flashcards
What direction do magnetic flux lines go in?
North to South
Define the Tesla
The magnetic flux density which produces a force of 1N in a wire of length 1m with a current of 1A
When the wire is perpendicular to the magnetic field
Flemings right hand rule
Thumb=direction of conventional current
Fingers curl in direction of magnetic field
Flemings right hand rule
Thumb=direction of conventional current
Fingers curl in direction of magnetic field
Flemings left hand rule
Thumb= Force
Index finger= Magnetic field
Middle finger= current
Magnetic force equation (Main)
F=BIL
F=BILsinθ
Equation for magnetic force on a single particle
F=BQV
How to derive radius equation
Centripetal force = magnetic force
Mv^2/r = BQv
r=mv^2/BQv
r=mv/BQ
Derive frequency/time period equation
Centripetal force=magnetic force
mω^2r = BQv
mω^2r = BQωr
mω=BQ
mx2πf=BQ
f=BQ/2πm
T=2πm/BQ
How do you find the time period / frequency of a half rotation?
T=πm/BQ
f=BQ/πm
How does a cyclotron work?
1) Charged particle is inserted in middle
2) Particle experiences a force in electric field -> it’s accelerated to negative plate
3) Particle enters M field and follows circular path
4)Particle re-enters magnetic field. PD must be flipped to accelerate it to higher speed
5) particle enters other Dee. radius is larger because speed is greater
6)Proton re-enters electric field. PD must be flipped again to accelerate particle
7)Process repeats until desired speed is reached
State 2 situations where a charged particle will experience no magnetic force when placed in a magnetic field.
1) It’s moving parallel to field lines
2) It’s stationary
How to induce an EMF and a current
In the wire the positive charges will move up and the negative charges down.
Therefore an EMF has been induced.
If the wire was a complete circuit then a current would be induced too.
The direction of the conventional current would be in the direction the positive charges move.
When does flux cutting/ changing the magnetic flux occur?
When a conductor moves through flux lines
When does flux cutting/ changing the magnetic flux occur?
When a conductor moves through flux lines
What factors affect the size of the EMF induced?
-The speed of movement: more flux lines cut per second.
-The number of coils: like having batteries in series.
-Strength of magnet: greater density of field lines.
-Area cut through: amount of field lines.
Define magnetic flux
The total amount of field lines cut through with the conductor
Magnetic flux equation
Φ=BA
Φ=BAcosθ
Why does including more coils increase the EMF?
Each wire acts like a battery
Looping same wire around multiple times means it gets “cut” multiple times
Like batteries in series
Flux linkage equation
ΦN. (BAN)
BANcosθ
Flux linkage against time graph (when rotating)
Cos graph
Lenz’s law
The direction of the induced EMF/current is in the opposite direction to the change that caused it
Faraday’s law
The induced EMF/current is directly proportional to the rate of change of flux linkage.
Faraday’s law EMF equation
EMF=. - △(ΦN) /△T
Area of EMF against time graph
Change in flux linkage
How to derive EMF equation
EMF= BAN/t (N=1)
EMF= BA/t
A=ld
EMF=Bld/ t
d/t = v
EMF = BLV
Equation for EMF produced by a rotating coil.
Equation for maximum EMF
EMF= BANωsin(ωt)
Max EMF= BANω
How to calculate Vrms (root mean square)
V0/√2
V0=peak voltage
How to calculate Irms (root mean square)
Irms=I0/√2
I0=peak current
What does the Y gain dial do?
Changes the value of each vertical division
What does the time base dial do?
Changes the value of each horizontal division
What do the y shift and x shift dials do?
Move the graph up / left and right
How to get peak voltage from oscilloscope
Find peak to peak voltage
Divide by 2
How to reduce percentage uncertainty when using oscilloscope to measure voltage?
Increase y gain sensitivity to make the curve take up whole screen vertically.
Turn time base off so it’s just a vertical line and easy to measure.
How to reduce percentage uncertainty when using oscilloscope to measure time period?
Decrease the time base sensitivity to make one oscillation take up whole screen vertically
Use freeze function
State the four inefficiencies in a transformer and how they can be reduced.
Flux leakage: place iron core in transformer.
Eddy currents: Laminate iron core.
Resistance of wires: Use a thicker wire or a material with lower resistivity.
Heat: Use cooling vents to reduce heat
Transformer equation
Np/Ns = Vp/Vs
How does flux leakage happen?
As the magnetic field grows and shrinks around primary coil the field lines move left/right
Only needed on right to flux cut secondary coil
So field lines to the left are wasted
How to reduce flux leakage?
Place an iron core in the transformer
Iron is magnetically soft
Helps guide field lines towards secondary coil
How do transformers work?
1) An alternating current is passed through the primary coil
2)Primary coil creates an alternating magnetic field which grows and shrinks
3) Therefore the secondary coil gets flux cut
4)Inducing an alternating current in the secondary coil
Why can’t a direct current be used in a transformer?
The magnetic field would be stationary in this case.
It must be changing so flux cutting occurs
What are Eddy currents?
A current is induced in the iron core.
This can’t go anywhere.
How to calculate efficiency?
Pout/Pin x100
How to calculate efficiency?
Pout/Pin x100
Define root mean square voltage
The square root of the mean of the squares of all the values of voltage in one cycle
What happens when time base it switched off?
It will just be a vertical like
Because dot doesn’t move horizontally
Why does a magnetic material slow down when it goes through a magnetic tube?
-Moving magnet flux cuts walls
-This induced a current in the walls creating a 2nd field
-This opposes the field of the moving magnet slowing it down
When does F=BQv apply?
Magnetic field must be at right angles to the velocity
Why does the balance reading increase when there’s a magnetic field around the wire?
Current through wire creates magnetic field creating a force
An equal and opposite force is present on the magnet pushing the balance down.
Why use an alternating current?
Resistance opposes current reducing power
With a lower current, less power wasted
Alternating current can be reduced using a transformer
Explain why proton follows semicircular path in a Dee
Force due to uniform magnetic field is always at 90 degrees to direction of travel.
This force is the centripetal force for circular motion
Why isn’t is possible for the magnetic field to alter the speed of a proton when it’s in a Dee?
The force is perpendicular to the motion
