magnetic fields and induction Flashcards
how are magnetic fields produced
by permanent magnets and around conductors carrying a current
definition of magnetic flux density
the force per unit length per unit current carrying conductor at right angles to the magnetic field lines ( B = F/IL )
what does it mean if the field lines are closer together
the field is stronger
what is 1 tesla
the magnetic flux density that produces a force of 1 newton on a wire of length 1 metre with 1 ampere of current flowing through it
what happens when a current carrying conductor is placed in a magnetic field and what can be used to find the direction of this
the conductor experiences a force that can be found using flemmings left hand rule
where is the magnetic field going if there are x’s on the page
in to the page
where is the magnetic field going if there are dots on the page
out of the page
when can we use F=BIL
when the angle between the magnetic field and the current is 90 degrees
what happens when the current is parallel to the field lines
there is no force induced
what is the catapult effect
occurs when a current is passed through two wires connected by a loose wire or bar in a magnetic field
due to flemings left hand rule, the loose wire is then catapulted horizontally away from the magnetic field
this occurs due to the magnetic forces acting on the wires and in the magnetic fields itself
how is a magnetic field formed around a current carrying wire and what do we use to find the direction of the field lines
they form concentric circles around the wire and the direction can be found using the right hand grip rule
what happens when two wires carry current in opposite directions
the net effect of their magnetic fields is added together in the space between the two wires
what happens when two wires carry current in the same direction
the net effect is to cancel out the magnetic field the space between the wires
how to find the direction of current in a solenoid
anticlockwise = north
clockwise = south
what is magnetic flux measured in and what is the equation
magnetic flux = BA
measured in Wb (Weber)
what is the equation for flux linkage if it is at an angle to the normal of the coil
flux linkage = NBAcosx
equation for force on charged particles moving in a magnetic field and when is it valid
F = BQV
valid when the charge is moving perpendicular to the field lines
what happens if the particle is moving parallel to the field lines or if the particle is not moving
there is no magnetic force
equation for force on charged particles moving in a magnetic field that is not at 90 degrees
F = BQvsinx
what direction is current when there is a positive charge
the same direction as the positive charge
what path will a charged particle take in a magnetic field and what is the equation
circular path
r = mv/BQ
what happens if a particle moves through a cloud chamber
it will lose energy because it is giving energy to the molecules in the chamber so it will slow down and spiral inwards
what is a cyclotron
a particle accelerator
how does a cyclotron work
- there is an electric field and therefore a potential difference horizontally between 2 dees
- the protons accelerate when they cross the gap between the dees
- the magnetic field is perpendicular to the motion of the protons
- the protons move in circular motion when inside the dees
- when the speed of the protons increases the radius of the path increases (r = mv/BQ)
- we want the protons to speed up every time they cross the gap
- we do this by switching the direction of the electric field every time the protons undergo one half circle
- we can switch the direction of the electric field using an alternating power supply
- the frequency of the supply should equal the frequency of the circular motion of the protons
- v = rBQ/m
- time for 1 half circle =
pi r / v - f = BQ/ 2 pi m
- the frequency does not depend on the radius of the path so we can use an ac supply of constant frequency
what causes an induced emf
a change in magnetic flux linkage through a coil or when a conductor cuts across magnetic flux lines
what is used to find the direction of the induced emf in a straight conductor moving through a magnetic field
flemings right hand rule
what is lenz’s law
the induced emf and hence current, is in a direction that tends to oppose the change causing it
what is lenz’s law a demonstration of
the conservation of energy
the work done by the movement of the magnet against the opposing force is transferred to the coil providing the energy for the flow of a current in the circuit (energy is dissipated as heat when the current flows.
why is the north pole of the magnetic field on the same side as the north pole of the magnet
the magnetic field produced by the induced current is such that it opposes the movement of the magnet.
what is Faraday’s law
the emf induced in a circuit is directly proportional to the rate of change of flux linkage
what three things does the size of the emf depend on
- the strength of the magnetic field
- the rate at which the coil, magnet or conductor are moved, or the rate at which B changes
- the number of wires loops or turns on the coil
what does the magnitude of the emf depend on
the rate of change of flux linkage
what is the equation for the magnitude of the induced emf when the conductor is perpendicular to the flux lines
e = Blv
if the conductor is connected in a complete circuit what does the current do
flow
if the conductor is not connected in a complete circuit what does the current do
the charge build up at either end of the rod until the potential difference caused by this charge build up is equal and opposite to the induced emf
how do you get the graph for emf againt time from a flux time graph
the negative gradient
as a magnet enters and leaves a coil, what will there be acting on it and what will this cause
there will be an upwards force on the magnet which causes its acceleration to be less than 9.81
what will the magnets acceleration be when its falling through a coil but there is no induced current
9.81
what is the equation for emf induced if the coil is rotated
e = N x change in flux linkage / change in time
what would the flux linkage be if the angle is 0
maximum
what would the flux linkage be if the angle is 90
0
what is the equation for change in flux linkage
NBA
what is an eddy current
when the magnetic flux linkage cutting a large block of conducting material changes, an induced emf is produced. This gives currents that flow within the block
examples of eddy currents
in large vehicles as one of the braking systems - the biggest effect is produced when the vehicle is moving quickly
what happens to the eddy currents if the block is moving
a force opposes it movement (a breaking force) - therefore, the kinetic energy of the wheel is converted into energy dissipated as heat by the eddy currents
three ways to increase the induced emf
- move the wire faster
- use a stronger magnet
- making the wire into a coil
low speed =
(electric motor)
high current and emf is small
high speed =
(electric motor)
low current and emf is high
energy must be transferred as heat when the current flows through the wire, where does this energy come from
collisions between the electrons and the atoms that make up the wire - the electrons transfer some of their kinetic energy
