Magnetic Fields Flashcards
What happens a magnetic field is perpendicular to a current-carrying wire
A force is exerted perpendicular to both the direction of current and the magnetic field lines
What does each finger represent in Flemings Left hand rule
What is magnetic flux density?
The number magnetic field lines passing through a unit area of material
Measured in Teslas or Webers/meters^2
What is the equation for the force felt by moving charge in a magnetic field
What assumption has to be made?
F = BQV
. Only applies when the motion to the charged particle is perpendicular to the magnetic field direction
Derive the F=BQV equation
Which fields do cyclotrons use?
An electric and magnetic field
How does a cyclotron work? (7)
A cyclotron is made up of 2 semi circular electrodes called “dees”
with a magnetic field applied perpendicular to the dees
and an alternating pd applied between the Dees
Each Dee is a metal electrode with alternating and opposite charges to oneanother,
this creates a an electric field between the two dees,
Which accelerates each particle every half revolution
The magnetic field causes the particles to move in circular motion, apply a constant centripetal force towards the centre
As the particles speed up their radius of motion increases,
until it breaks free tangential to one of the dees
What is magnetic flux?
ф=BA
Flux = Flux density x Area
Where B is normal to A (perpendicular)
What is flux linkage?
Nф= Number of turns cutting the magnetic flux at one time
What is the flux linkage of a rectangular coil rotating through a magnetic field
Nф= BAN cosx
What is Faraday’s Law?
The emf induced is directly proportional to the rate of change of flux linkage
What is Lenz’ law
The direction of the induced emf is such as to oppopse the change that induces it
to conserve energy
What happens when you move a straight conductor through a magentic field
The electrons experience a force pushing them to one end of the conductor creating an EMF across the conductor
How do you derive the radius of a charged particle when it is perpendicular to the magnetic field equation
Equate Centripetal force equation MV^2 / r to BQV
What is a couple
A pair of equal and opposite forces acting on a body but not at the same point
What is the equation for calculating force on a wire
F=BIL sinx
where sinx= 1 when current is perpendicular to magnetic field
What is the purpose of a carbon brush in a electrical motor
To make electrical contacts between the circuit and the roatating motor
What is the purpose of a split ring commutator in a motor
Changes the direction of the current every half rotation to make sure the coil rotates in the same direction
The speed of a motor can be increased by
. Increasing the size of the current
. Number of turns on the coil
. Strength of the magnetic field
How does an electron move in a circuit
Current shows directions a positive test charge would take
So electrons flow in the opposite direction to the current.
This is used to make the field line rules universal
How is time period of a moving particles affected when accelerated in a cyclotron.
v = 2pi * radius / Time period (circular motion)
Put into MV/BQ = r
T = 2pi*M / BQ
No factors of radius or velocity so it stays constant
What is the frequency of changing electric field in a cyclotron
T = 2piM / BQ
f = Bq/ 2pim
have to times frequency by two as has to change polarity every half turn to accelerate the particle.
f = B/pi * specific charge
