Magnetic Fields (General) Flashcards
Definition of magnetic flux density and Tesla
Exam definition
Measure of strength of the field
- Force on one metre of wire carrying a current of one amp at right angles to a magnetic field
- Unit = Tesla
- Tesla = One newton per amp per metre
Equation for finding the force exerted on a wire when inducing a magnetic field
F = BIL
Explain the movement of electric current in respect to magnetic fields
When charges move they create both magnetic and electric fields
hence magnetic is due to moving charge and electric is due to static charge
These charged particles experience a force due to the fields == which allows them to move along wire
Equation for finding force exerted on charged particles in a magnetic field
F = Bev
Describe how the direction of force can contribute to the motion of a charged particle
The force exerted on a moving charged particle is ALWAYS perpendicular to its motion FOR MAGNETIC FIELDS
Hence the force acts a centripetal force thus the particles experience circular motion
Explain how you can find an equation to give the radius of the motion of a charged particle
Force for charged particle can be equated to centripetal force equation
Giving:
r = mv/BQ
What is a cyclotron
A type of particle accelerator that is used to accelerate particles at high speeds/energies for use in medical applications
Describe the structure of a cyclotron (including fields)
Cyclotron consists of two semi circular electrodes (dee’s) (with a magnetic field) separated by a gap
Between this gap a high frequency alternating voltage is applied (creating an electric field between the dee’s)
Describe how a charged particle can be accelerated in a cyclotron
A charge particle moves from the centre of the gap to the electrode of which it is attracted due to its charge (hence due to electromagnetic force)
It is accelerated to the electrode since it is experiencing a force in the same direction as its motion when in the electric field (of which is applied due to the voltage)
Once it enters the electrode, it now experiences a magnetic field where the force is perpendicular to its motion (hence it moves in a circular path) and does not increase in speed
The particle then leaves the electrode, at which time the alternating voltage has switched directions, meaning the particle is now attracted to the opposite electrode
Hence the process is repeated
Explain how and why the radius of the particles motion in a cyclotron changes
The radius of the circular path increases since the velocity increases every time the particle experiences the electric field
This is due to the force being in the same direction as the motion of said particle
Using the equation r = mv/BQ if all other components are constant then radius is proportional to velocity hence it increases as such
What is meant by magnetic flux
A value that describes the magnetic field (or magnetic field lines) passing a through a given area
Represented by phi == Φ
Equation for magnetic flux
BA
Magnetic flux density x area
What is meant by magnetic flux linkage and how can it be calculated
Denoted as == NΦ
It is a measure of the magnetic flux passing through a coil
Which can be calculated from BAN if perpendicular
Magnetic flux x area x number of turns
Equation for finding magnetic flux linkage when magnetic field is not perpendicular to coil
NΦ = BAN cos θ
If in doubt you resolve to find component perp
Explain how electromagnetic induction occurs
When a conducting rod moves relative to a magnetic field, the electrons in the rod experience a force (as they are charged particles)
This force causes charge to build up on one side of the rod
Creating an induced emf
(only occurs if you move a bar magnetic relative to a coil wire)
What is meant by Faraday’s law
States that the magnitude of induced emf is equal to the rate of change of flux linkage
What is meant by Lenz’s law
States that the direction of induced current is such as to oppose the motion causing it
Essentially the induced current will be in the direction that causes its magnetic poles to repel to magnetic poles of the magnetic object
N to N for example
What is meant by motor effect and dynamo effect and when does flemings rules apply?
Motor effect == Current creates a force
(Use fleming left hand rule)
Dynamo effect == Force creates a current (emf induction)
(Use fleming left hand rule BUT WITH RIGHT HAND)
Explain how a change in flux linkage induces an emf
A change in magnetic flux induces an electromotive force (EMF) according to Faraday’s law
ε=−dΦ / dt
When there is a change in either area or field strength, the magnetic flux through the conductor changes (hence moving positions will change field strength)
When a change occurs, an electric field is induced which drives free electrons hence inducing an emf
For a magnet falling down a hollow conductor explain why its acceleration is zero
(may be slightly more advance than spec but helps understanding)
When a magnet interacts with a wire (hollow conductor) and emf is induced (due to a change in magnetic flux linkage following Faradays law)
At points where the magnetic field of the magnet and the motion of the magnet are perpendicular == There will be a circular electric field with a current (hence eddy currents) (this is the emf induced btw)
These circular currents act like solenoids and produce their own magnetic field
Due to Lenz’s law the magnetic field (poles) they produce must oppose the motion of the magnet (North pole near a north pole ect)
Due to the repulsion felt by this, the magnet reaches terminal velocity hence has zero acceleration
How can you work out the direction of the current (electric field) produced when a magnet falls through hollow conductor
Use the North South rule
If the magnetic field of the current induced is North at top then anticlockwise
If the magnetic field at the current induced is South on top then clockwise
(To know if its S or N just do the pole which will cause repulsion to the falling magnet)
Explain what happens when a magnet falling through a hollow conductor exits
Technique due to the poles the magnet should repel at the bottom and shoot out
However == it always wants what it cant have like a shower
However we need attraction due to Lenz’s law (the poles must be opposite essentially)
Therefore the current and poles flip half way
Describe the graph of Flux linkage against time
Straight line through origin (Proportional)
Emf is the gradient
Describe graph of emf against time
Constant horizontal line
Area underneath = Change in flux linkage
