A2 - Magnetic Fields Flashcards
What must magnetic fields have in terms of charge?
A movement of charge, or an imbalance of charge
What is the right hand rule?
Curved fingers show direction of field, thumb shows direction of CONVENTIONAL current.
In the equation F=BIL, for what does L stand?
Length of the conductor in the magnetic field.
What kind of motion does a magnetic field induce on a moving charge? Link an equation for the force to the type of motion.
Circular motion. F=BQv = mv^2/r
What is the equation for the force on a charged particle when moving freely in a magnetic field.
F=BQv.
In which direction does the magnetic field move when there is a circle with a cross through it?
Into the paper, like when you put a straw into a lid.
What is the direction of the magnetic field when the circle has a dot in the centre?
Out of the page.
What direction is the force due to a magnetic field acting compared to current and magnetic field lines?
At 90 degrees to current and magnetic field lines. Proved my Flemming’s LHR
For a couple in a magnetic field, where is torque max and min?
When the axis of rotation is parallel to the field lines, the force is max. As the arm moves to perpendicular, the force reduces to zero, as perpendicular distance is zero from the arm to the pivot.
When electrons are propelled onto a card inside a magnetic field, what kind of motion do they follow, and what kind of current do we have to consider when using left hand rule?
They follow circular motion, and will be shown inside card if magnetic field is strong enough. The current is CONVENTIONAL.
What is the hall effect?
Where a current is flowing in a magnetic field, the electrons redistribute to be on one side. If electrons flow from left to right, and the field acts upwards, they move out of the page, due to FLHR
What is FRHR used for?
Generating electricity.
What is the basic premise for a generator? What is is related to?
A conductor moves in a magnetic field, causing the electrons to move. This is the opposite of a motor, in which moving current inside a magnetic field causes the wire to move.
What happens if the speed of the wire increases inside the magnetic field?
The faster the wire, the greater the magnitude of the current.
Define Flux Density with units
magnetic flux per sq m at a given point in the field
Teslas
Define flux with units
amount of magnetic flux in a given area
Wb
Define flux linkage with units
The amount of flux in one unit area * number of unit areas to consider, e.g. 800 coils on a solenoid.
Wb turns
What is Faraday’s law that’s not on the data sheet?
(E) = BA/t (Like with plane wing question) E = (Phi)/t
How is Faraday’s law written in equation form?
induced EMF = flux change/time
What is Lenz’s law?
The EMF induced always opposes the change that caused it, e.g. a magnetic field will oppose as the material enters, and attract as it leaves.
What makes Lenz’s law true?
Energy must be conserved, so the emf will oppose the change. The energy required to pull a material through the magnetic field is against the magnetic field, so is what causes the induced current.
How do transformers work?
An AC current flows through a primary coil of wire, surrounding a soft iron core. This induces a magnetic field in the iron core. This in turn induces a current in the secondary coil. If there are fewer coils, this is a step down transformer, so the voltage is reduced.
What is the purpose of using a low current in the national grid?
The value of power wasted I^2R is lower, increasing efficiency.
What is the relationship between flux linkage and EMF?
They are 90 degrees out of phase. This follows, as the flux linkage is max when perpendicular to field, but the force at this point is zero. And the emf is also zero.
What happens when a wire carrying a current exists in a magnetic field?
There is an interaction between the electric and magnetic fields. The field lines of the magnetic field would move to concentrate based on the direction of the induced magnetic field given by the Right Hand Rule
Define a magnetic field.
An area where a force is exerted on paramagnetic materials.
For a coil rotating in a magnetic field, when is the induced EMF max? What causes induced EMF?
Rate of change of Flux linage determines EMF
this is when the angle between the normal to the coil and MF =90/270.
What is the flux linkage for a rotating coil when the EMF is max?
ZERO
If an electron is orbiting clockwise in circular motion, which way is the magnetic field oriented?
vertically downwards
For a charged particle in circular motion, which way is the current flowing?
Towards the centre :)
How can you apply N2L to circular motion of an electron in a cyclotron? Hence derive an expression for the radius of the circle.
mv^2/r = BQv
r=mv/BQ
State faraday’s law of electromagnetic induction
The induced EMF is equal to the rate of change of flux linkage through the circuit
What is the purpose of a split ring commutator in a DC generator?
It keeps the EMF positive by causing the poles to alternate. makes it a DC signal as always positive
When is the rate of change of flux least?
When the actual flux is maximum
In which direction does the EMF induced act?
Against the PD in the coil
Give an expression for the potential difference in the circuit including circuit voltage and induced emf
V-EMF=IR
Give an equation for the power in the circuit involving circuit voltage and induced EMF
Iv-I(EMF) =I^2R
For IV-I(EMF) =I^2R, give the meaning of each element of the equation
IV - power supplied by the source
IE - electrical converted to mechanical power
I^2R - power lost to heating
State two ways of increasing the peak EMF
increase frequency of rotating coil, increase the number of turns
What does a + in the wire show about the current in the wire?
Flows into the page
What does a dot in a circle show about the current in the wire?
Flowing out of the page
How do you apply the Right Hand Rule (Not Flemming’s)
the thumb shows the direction of the CONVENTIONAL current, the fingers show the direction of the resulting magnetic field
When the hall effect takes place in a conductor, what causes the system to return to equilibrium?
The lorentz force (due to magnetic field) is cancelled out by the electric field due to the build up of charge
Which two factors affect the velocity of particles in cyclotron?
magnetic field strength and radius
What is the application of faraday’s law?
Any situation when a conductor cuts magnetic flux, the induced EMF =BA/t =(BL/\s)/t
What is the shape of the graph of induced EMF vs time for a magnet dropped through a coil?
positive EMF first, then negative EMF of greater magnitude due to acceleration of magnet
What is the rule for determining whether you are looking at the north or south pole of a solenoid?
Can you make N or S out of the flow of current?
Clockwise=south
Anticlock=north
In a rotating coil, what is the equation for flux linkage? What is the meaning of each term?
n(phi)=BANcos(theta)
N(phi) - flux linkage B - flux density/T A - area/m^2 N - number of turns cos(theta) - angle between normal to coil + field
How does Lenz’s law apply to a generator?
The faster the coil spins the greater the back EMF
State faraday’s law of EM induction
EMF = rate of change of flux linkage, N(Phi) in the circuit
