Magnetism Flashcards
(26 cards)
Magnetic flux density (B)
The strength of the magnetic field
B= magnetic flux/ area
Soft magnetic material
Easy to magnetise and demagnetise
Force
F=BIL
F=BILcos
F=BeV=mv^2/r
Magnetic flux density (phi)
Phi=BA
Phi=BAcos
Electromagnetic induction
- when conductor moves through a magnetic field a pd is induced across the conductors ends
- As moving through the electrons experience a force and move to the end of the conductor inducing a pd as the electrons move through the field
- if conductor’s connected to a full circuit a current will flow
Faradays law
EMF produced proportional to rate of change of flux linkage
EMF=NBA/t
Lenz’s law
Any change must oppose the change the made it
EMF=-NBA/t
Eddy currents
- when a magnet passes through a conductor the conductor cuts the field lines creating small currents called eddy currents
- these produce a magnetic field which opposes the original field causing the conductor to slow down
Magnetic flux linkage
Amount of flux in a coil
How a generator works
- cuts field lines so pd and current generated due to faradays law
- coil sounds between opposite facing poles
How a microphone works
- causes cone to vibrate that moves coil in magnetic field
* cuts field lines so induces emf in opposite direction
Bicycle dynamo
•induced pd doesn’t alternate and charge flows in same direction
Induction hob
- coil under plate runs through occulting magnetic field and cuts field lines
- causes eddy currents in the pan that generate heat
Electric guitar pick up
- strings made of magnetised steel
- when string plucked vibrate above pick up (bar magnet wrapped in 7000 coils) cutting the field lines
- EMF induces
Card readers
Magnetic strip has info on, coil is reader, emf induced due to faradays law
EMF
EMF=BLV
EMF=BANwsin(wt)
Direct current
All charge carriers flow in 1 direction all the time
Alternating current
Current which constantly changes direction
I/V rms
I(rms) = I(0)/root2
V(rms)=V(0)/root2
How transformers work
- apply alternating pd across primary coil
- current induces a magnetic flux which takes the shape of the soft iron core
- secondary could cuts field lines so emf induced as well as current
Transformer equation
Vp/Np=Vs/Ns
Step up transformer
High Vs so decreased current
More turns on 2nd coil
Step down transformer
Lower Vs so increased current
More turns on 1st coil
Efficiency equations
IpVp=IsVs
Ps/Pp=IsVs/IpVp
