questions about electrical induction

Discussion in 'General Electronics Chat' started by logicman112, Dec 27, 2008.

  1. logicman112

    Thread Starter Active Member

    Dec 27, 2008
    First experiment i want to talk about is a wire which is shaped like a circle and a permanent magnet is moved through it back and forth.

    We know that the induced current by an external magnetic field , makes a new variable magnetic field which opposes the first one. But the both changing magnetic fields induce emf into the circuit so that if one of them is positive , the second one is negative and the total electrical voltage will be a sum of the two and it can be even zero! so how we say that always a net voltage is induced into a loop of wire by a moving magnet?

    Now i describe another experiment where we have a transformer with two coils, primary and secondary. while the primary is connected to a resistor and AC voltage source serially and the secondary is short circuited.

    AC voltage source magnetizes the core and a variable magnetic flux will pass through the secondary. On the other hand the induced current in the secondary will make an opposing flux. We name the currents of the primary and the secondary, i1 and i2.

    If N1*i1=N2*i2 then according to the above idea the total flux passing through the secondary coil will be zero, means i2=0! because f = L * i (f is the magnetic flux).

    so how we can have non-ideal transformers with identical inductors and i1=i2?
    Is i1=0 really? Does that mean that we have short circuited the heads of the AC voltage source of the primary circuit?
  2. duffy

    Active Member

    Dec 29, 2008
    In both cases you are dealing with shorted coils, so attempting to define "voltage" is pedantic. Where would you even measure voltage in a circle of wire?

    The effect of opposing voltage in an inductor is used to describe what happens when you initially apply a voltage source to a coil. The electons can't move at first, so they oppose it.

    It's because of this that the flux is initially zero. No moving electrons, no flux. This is also why you don't get a short circuit because the induced emf on the primary exactly opposes the applied voltage. This only happens for an infinitesimally brief moment, then the current builds and so does the flux.

    THAT'S when you get the short circuit. Now the current is at its maximum, and the shorted secondary is shading the core so the primary sees a much smaller inductance than it was designed for and blows the fuse!