Magnetic Flashcards
What is the motor effect?
when you put a current carrying wire in a magnetic field at a non-zero angle, a force acts on the wire
Magnetic flux density
the force per unit length per unit current on a current-carrying conductor at right angles to the magnetic field lines
What is a simple electric motor
- it consits of a coil of insulated wire which spins between the poles of a U-shaped magnet
When a current passes through round the coil, the wires at opposite edges of the coil are acted on by forces in opposite directions
the force on each coil makes it spin about its axis
How does the split ring commutator work?
The direction of the current round the coil is reversed by the split ring commutator each time the coil rotates through a half turn. This ensures the current along an edge changes direction when it moves from one pole face to the other.
Why do electrons travel in curved paths when entering a magentic field?
The direction of force on each electron is perpendicular to the direction of motion
r= mv/BQ
At angle to the lines of a field
There is a component of B
therefore F = BQvsinθ
Cyclotron how does it work
- Two hollow metal Dee’s with a magnetic field acting perpendicular
- Proton source situated in the middle in an air gap between the Dee’s
- Alternating voltage across the Dee’s
- Electric field between the two Dee’s
- A particle is emitted and is accelerated to one plate
- As it enters the Dee it obeys circular motion as the direction of motion is perpendicular to the magnetic force
- As it leaves the Dee the potential difference reverses and the particle accelerates to the other Dee
- the particle gains speed while accelerating to the other Dee and the radius increases
- The radius of the particle increases until the particle fires out of the cyclotron
What is the Time period of a cyclotron?
T = 2mπ/BQ therefore alternating frequency for voltage f = BQ/2mπ
What is electromagnetic induction?
the generation of EMF when the magnetic flux linkage through a coil changes or a conductor cuts across magnetic field lines
Peak to peak voltage
the difference between peak negative and peak positive voltage
Dc voltage graph is a horizontal line at the value of Vrms
it is a horizontal line at the value of Vrms
Two examples when a charged particle will not experience a force in a magnetic field
- when a charged particle is at rest it will not experience a force as BQr =mv
- when a charged particle moves parallel to magnetic field lines. Sin(0) is equal to 0 therefore no magnetic force
Peak emf
BANω
When you place an object in a hollow tube which is a permanent magnet
- the object falls slowly through the tube
- the object experiences a rate of change of flux linkage as it cuts through magnetic flux
- this causes an induced current in the object (as it is a metal)
- the current then produces its own field
- the two fields interfere
- this causes an upwards force on the object which causes it to slow down
- you could also say Ek is lost to the current causing a heating effect on the tube
How would the hollow tube experiment change with a magnet of a greater resistivity with the same dimensions?
- resistivity is proportional to resistance
- this means the resistance is greater
- a smaller magnetic field is produced
- therefore smaller induced emf
- the object will fall more quickly
- the opposing magnetic field will be smaller
What causes Eddy currents?
Induced current in the iron core itself which produces a heating effect. This reduces the efficiency of the transformer.
Soft iron core in transformers
- iron core focuses and directs the magnetic field from the primary to the secondary coil
- soft iron is used as it s easily magnetised and demagnetised
How to keep high efficiency of transformers?
- low-resistance windings to reduce power wasted due to the heating effect of the current
- a laminated core which consists of layers of iron separated by insulators help prevent eddy currents from forming
- a core of soft iron which is easily magnetised and demagnetised reduces power wasted
- eddy currents can be reduced by having a core made from a metal of high resistivity
Flux density and time graph
EMF is proprtional to delta B divided by delta t
Therefore a straight line on this graph indicates constant induced EMF
EMF time graph
- horizontal line with 0 gradient indicates constant EMF
- a straight line with gradient indicates there is a change in EMF
What is the phase relationship between EMF and magmetic flux linkage
they are π/2 rad out of phase
This is because when you have max magentic flux, the gradient is 0
This means there is no rate of change of flux linkage therefore there is no EMF
What happens when you drop a coil through a magnetic field?
- As it enters the magnetic field, the rate of change of flux linkage increases due to the increase of speed
- When it is halfway through the magnetic field, the EMF and current flips at this point
- This is due to Lenz’s law
- The rate of change of flux linkage at this point is now greater as the coil is moving even faster
- Therefore induced EMF is greater
What to do when calculating maximum magnetic flux linkage?
Dont use an angle it should be 90 degrees! therefore x 1
Units for Flux linkage
Wb turns
Define the tesla
The tesla is the strength of the magnetic field that produces a force of 1 newton in a wire of length 1m with 1 ampere.
The time taken by a proton to travel round a semi circle in a cyclotron is independent of the radius of the path
this is because t = πm/BQ
Using circular motion = magnetic force
removing v and r with time
