Magnetic Fields Flashcards
(24 cards)
What is a magnetic field and how do you represent it
A region where a force is exerted on magnetic materials
Represented by field lines (flux lines)
-go from north to South Pole of magnet
-closer lines = stronger field
- at a neutral point, magnetic fields cancel out
-shows the direction of force in a MF along which a stationary North Pole would move in the field
How is a magnetic field produced in a wire
When current flows in a wire or any long straight conductor, a MF is produced around the wire
-the field lines are concentric circles centred on the wire
-use right hand rule to find the direction of MF around a current carrying wire
What happens when you put a current-carrying wire into an external magnetic field (e.g. two magnets)
The field around the wire and the field around the two magnets will add together
-causes a resultant field- lines closer together shows a stronger MF
-these close lines cause a pushing force on the wire
Describe Flemings left hand rule
The direction of the force is always perpendicular to both current and MF
If current is parallel to field lines the size of the force is 0N
-there is no component of MF perpendicular to the current
FBI
Force, B=Magnetic field (N to S), I=current (+ to -)
Define magnetic flux density
The force per unit length per unit current on a current-carrying conductor at right angles to the magnetic field lines
Measure of the strength of a magnetic field
What is force proportional to in an external MF and when current is 90 degrees to the MF
The force on a current-carrying wire at right angles to an external magnetic field is proportional to the magnetic flux density
When current is 90 degrees to the MF, the size of the force is proportional to the current
What type of quantity is flux density and what is it measured in
Vector quantity and is measured in Tesla
1 Tesla= Wb/m^2
Define a Tesla
The magnetic flux density if a wire of length 1m carrying a current of 1A has a force of 1N exerted on it in a direction perpendicular to both the flux and the current
Where are magnetic fields found
Around permanent magnets, induced magnets or current-carrying wires
What is the notation for current going into the page and out of the page
Into the page = cross inside a circle
Out of the page= a dot inside of a circle
What will happen if 2 parallel wires are placed near each other and their currents are in the same direction? What if the currents were anti-parallel
Parallel currents attract (weak field between wires, strong outside)
Anti parallel currents repel (strong field between wires, weak outside)
What is the motor effect and what is the equation for the force on a current carrying wire of length L
A forced produced when a current carrying wire is placed in a magnetic field and the two magnetic fields interact
F=BIL
In the motor effect what will happen if you:
-reverse the direction of current
-flip the magnetic poles
-place the wire parallel to the MF
-causes the force to act in the opposite direction
-causes the force to act in the opposite direction
-there is no force when wire is parallel to MF
Explain the structure of a motor and the processes involved in making it spin
In a motor, a rectangular current carrying coil is inside a MF
The sides of the coil perpendicular to the MF experience a force
These forces act in opposite directions and form a coupe as the current is in opposite directions
The couple formed causes the coil to rotate (the sides parallel to the MF do not experience a force)
When the coil reaches its vertical position the magnitude of the couple is zero as the forces are acting through the axis of rotation
The SPLIT RING COMMUTATOR ensures the coil continues to rotate as it reverses the direction of the current in the coil
What is the equation for the force on a charged particle moving in a MF, when the field is perpendicular to velocity
F=BQv
Why: a force acts on charged particles MOVING in a MF
Electric current is the FLOW of negatively charged electrons
Explain why the path of a particle with a positive charge moving at a velocity v at right angles to a MF at magnetic flux density B is circular?
Since the force acts perpendicular to velocity NO WORK is done on the particle (i.e. kinetic energy is constant, therefore speed is constant
The particle is accelerating so direction mush change
Therefore the particle moves in a circle
BQv=mv^2/r
r=mv/BQ
-r increases (particle deflected less) if the mass or velocity of particle increases
-r decreases (particle deflected more) if the strength of the MF or the charge on the particle increases
-if particles with a larger specific charge (Q/m) are used, r decreases
What is a cyclotron
A type of particle accelerator used to accelerate charged particles to large speeds approaching the speed of light
The protons are kept moving in a circular path by a uniform magnetic field at right angles to the plane of the path
An alternating EF is used to accelerate the particles onto spiral paths of increasing radius
What does a cyclotron look like and how does it work
-consists of 2 hollow Dees (d shaped electrodes) in a vacuum chamber
-a MF is applied perpendicular to plane of the dees, and the dees are connected to an AC supply which produces an alternating EF between the dees
-charged particles are fired into one electrode (dee). The MF makes them follow a circular path and then leave the electrode
-an applied p.d. between the electrodes accelerates the particles across the gap to the next electrode
-since the speed is slightly higher, it will follow a circular path with a larger radius before leaving the electrode again
-the p.d. is reversed so the particle accelerates again before entering the next electrode
-this process repeats as the particle spirals outwards, increasing in speed before eventually exiting the cyclotron
Explain why frequency is constant in a cyclotron and give the equation that shows this
The time to complete each orbit is independent of their speed because although they travel faster in each successive orbit, they also have further to travel
v=2πr/T 1 T=time for one full cycle of the alternating voltage
r=mv/BQ 2
2—> 1
T=2mπ/BQ
f=BQ/2πm
What is magnetic flux passing through an area perpendicular to a magnetic field equal to
(Not the same as magnetic flux density)
Φ=BA where B is normal to A
How can you induce an emf in a rod
If there is relative motion between a conducting rod and a magnetic field, the electrons in the rod will experience a force which causes them to accumulate at one end of the rod
This induces an emf across the ends of the rod
How do you induce an emf in a flat coil or solenoid
-moving the coil towards or away from the poles of a magnet
-moving a magnet towards or away from the coil
-the emf is caused by the MF changing
-if the coil is apart of a complete circuit, an induced current will flow through it
What is flux linkage and the rate of change of flux linkage
The product of the emf induced when moving a coil in a MF, the magnetic flux passing through the coil and the number of turns in the coil that cuts the flux
NΦ=BAN
The rate of change of flux linkage tells you how strong the emf will be in volts
State Faradays Law
The induced emf is directly proportional to the rate of change of flux linkage
EMF=flux linkage change/time taken
