transformer derive 問題..

In proving the equation, it is assumed that the primary and secondary coil have no resistance. As such, the voltage applied to the primary is counter-balanced by the back emf.

Apply Faraday's Law to the primary coil,
induced back emf Vp = d(NpAB)/dt = NpA.(dB/dt)
where Np is the number of turns of the primary col
A is the coil cross-sectional area
B is the magnetic flux density

Apply Faraday's Law to the secondary coil again,
induced emf Vs = d(Ns.AB)/dt = Ns.A.(dB/dt)
where Ns is the number of turns in the secondary coil

Hence, Vp/Vs = [NpA.(dB/dt]/[Ns.A.(dB/dt)]

i.e. Vp/Vs = Np/Ns

Refer to your questions,
Q: 係primary戈陣....i同r 都可以assume係0

For an ideal transformer, the primary coil has no resistance. Also, as said above, the applied emf is balanced by the induced back emf, there would not be any current flowing through the primary coil.

Q: 但係secondary戈陣....得i可以assume係0

The secondary coil is in open circuit. As such, there would not be any current flowing. This is not an assumption, but a condition under which the proof of the ideal transformer equation is based.









2011-08-14 12:09:49 補充：
點解secondary個r 5可以係0???
Why not. An ideal transformer should have zero resistance for its coils.

2011-08-14 12:10:54 補充：
Notice that coil resistances do not invlove in the derivation of the "ideal transformer equation".