Magnetic Fields Flashcards
How do you calculate the field in a solenoid?
Flux Density (B) =mo x NI/L where N=no. of turns, I=current, L=length of coil & mo=permeability of free space.
What is the direction of magnetic field?
Towards a South Pole.
How do you calculate B for a straight wire in air?
B=moI/2pir.
How do you find flux density?
F=BIL, where F=force, B=flux density, I=current, L=length of wire.
If the current is at an angle to a field: F=BILsin0.
How do you find the velocity of a particle in a magnetic field?
F=Bqv, as I=q/t and L=vt.
For an elector, Bqv is perpendicular to r, as it travels in a circle.
How do you calculate the radius of an electron travelling in a circle?
F=Bev and F=mv^2/r equated gives r=mv/Be.
What is Faraday’s law?
The magnitude of the induced emf is equal to the rate of change of flux linkage.
How do you find flux (φ)?
Flux φ =BAcos0, in Webers (Wb).
Where Bcos0 = the component of magnetic flux density B which is perpendicular to the area A.
What is flux linkage?
Flux linkage = φN, where φ = the flux through a coil, with N turns, so BANcos0.
How do you find the emf?
e.m.f.=Ndφ/dt, so the rate of change of flux linkage.
What is Lenz’s Law?
The direction of any induced current is in a direction that opposes the flux change that causes it, i.e. e.m.f. = -rate of change of flux linkage.
How do you find the emf of an A.C. generator?
e.m.f. = BANωsinωt.
How does a transformer work?
When there is an alternating current in the primary coil, it causes the iron core to be magnetised and then remagnetised in the opposite direction in the opposite direction 50 times a second. This rapid change in magnetic flux means an equally fast magnetic flux through the secondary coil. When more turns are added to the secondary coil, the emf increases by the number of turns, as each turn has the same change of flux through it because they are all in series.
The larger the nuber of turns and the more rapid the flux change the larger the emf.
What is the turns ratio of a transformer?
Ns/Np=r=Es/Ep
Where N=no turns and E=emf.
For energy: r=Ns/Np=Es/Ep=Ip/Is
What is induction?
When the movement of a wire across a magnet induces a current in the wire.
