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Some questions on electromagnetism

  1. Apr 4, 2003 #1
    dear friends
    there are some theoretical questions in my mind about electromagnetism
    1) in RL circuits the induced emf starts to decrease from its maximum value to zero
    and this is caused by the changes of flux
    but why does the magnetic flux changes initially very rapidly and then starts to change more slowly

    2)what is the physical explanation of faradays law of induction

    3)what is the physical explanation of lenzs law
  2. jcsd
  3. Apr 5, 2003 #2


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    Science Advisor

    Greetings Murad !

    Hmm... I'm not sure about what you're
    asking in number 1. Could be because
    the energy of the electromagnetic fluctuations
    is gradualy lost to the resistor - if that's
    related to your question.

    2. What Faraday's law means to say is that
    if the flux increases at a steady rate
    then the induced potential difference remains
    the same.

    The "physical explanation" part is that if you
    increase the area of the closed circuit loop
    or alternativly the other parameter of the
    flux - the value of the magnetic field, then -
    the energy of the flux within the closed circuit
    loop will change and the charges will "adept"
    to the new energy change of the magnetic flux.

    In much the same way an electric charge
    in an electrostatic field will have greater/lesser
    energy as it moves "up"/"down" the potential
    difference that is the field.

    For a magnetic field the story is slightly different.
    Moving in the direction of the field does
    not cause energy differences. What does is
    movement perpendicular to the field because
    that's the way the magnetic field effects
    charges - a perpendicular force.

    Magnetic flux is the measure for the
    energy content of a closed circuit loop. The energy can
    increase if the magnetic field increases or
    if the area increases - increase in magnetic flux.
    Each case requires a greater current which in turn
    means that the circuit has greater energy.

    Now, a change in the electrostatic field creates
    an electric energy "wave" that pushes or pulles
    the electric charge in the field. In much the
    same way a change in the magnetic flux is actually
    an energy change for a closed electric circuit loop
    which manifestes itself as a potential difference
    experienced by the loop's charges in a direction
    perpendicular to the magnetic flux change.

    3. This is a simple implication of the way
    the magnetic field effects moving charges and
    the way the moving charges generate the field.

    If the inducing magnetic flux increased and
    thus increased the current then the electric
    charges move faster than before and the direction
    of the magnetic flux produced by the increase in induced
    current in the circuit is reversed to the magnetic
    flux relative to the previous rest frame of the current.

    And, if the magnetic flux decreased and the
    current decreased then the current is now negative
    relative to the rest frame of the previous current
    and the magnetic flux induced by this changing current
    actually increases the decreasing inducing flux.

    It also has a "deeper" reason in light of my
    explanation of Faraday's law - suppose that
    our closed circuit loop is at energy level
    zero when a ball on a string is at rest.
    (This doesn't necessarily mean there is
    no potential difference or magnetic flux
    change or induced current - that is just
    a private case. It could be that the rate
    of change of magnetic flux is constant
    and this rate suddenly changes.)

    If the ball were to move to the left then
    the energy of the circuit will be negative
    (Relative to the "zero" rest frame. In case
    that there is no change of magnetic flux
    enitialy - any change will mean positive
    energy and the distinction will be the
    direction of the induced current/electric field.)
    and if it moved to the right then the energy
    will be positive.

    The ball always "wants" to be at the "zero" point.
    Why ?
    Because in order to keep it suspended at an angle
    or keep a potential difference (or rate of change in
    it) and a current (or rate of change in it)
    we need to supply (additional) energy.

    If we were to stop supplying it then the
    ball would drop - the circuit will emmit the magnetic
    flux energy fluctuation "back outside", because it
    can not stay in this excited state without
    energy being supplied to maintain it.

    So, the inducing magnetic flux creates an induced
    current which in turn creates an induced magnetic
    flux in the opposite direction which tries to
    "relax" the circuit - let the ball drop.

    Why ?
    If you boil water and then turn of the fire
    then the water will "relax" back to room
    temprature. The "room temprature" or "normal"
    energy level of a closed circuit loop in our
    case is the absense(/constant rate of change)
    of the magnetic flux. A change in that absense
    or rate that does not supply the energy to
    make it a constant change but rather is a
    fluctuation then results in "relaxation" which
    is the same reason the water cools down to
    room temprature - the second law of
    thermodynamics - entropy increase or - "energy
    density decrease".

    A bit long isn't it...
    Well, just a hobbyist trying to make it
    clear on a "popular" level. As long as it's clear
    the lenght surves the purpose I guess...:wink:

    Live long and prosper.
  4. Apr 8, 2003 #3
    Once again, Murad to answer #1, initially as voltage is applied to the R/L circuit it tries to go instantly from 0 volts to applied volts, say 12 VDC. Because of Back EMF that we talked about before being induced in the ciruits it can't go instantly to 12 VDC. It is the change in voltage causing a change in current flow that creates the changing electromagnetic field. The changing magnetic field induces Back emf in the coil. As time goes on the voltage rises to the applied voltage and as it nears that voltage the change in voltage over a given amount of time reduces. This reduces the change in current flow which reduces the change in the magnetic field. The reduced rate change in the magnetic field reduces the actual induced back emf even more allowing the voltage to approach the applied voltage even closer. Eventually the voltage reached applied voltage and there is no more change, i.e. the circuit stabalizes. Since there is no more change in voltage, there is no more change in current. No more change in current, there is no more change in the electromagnetic field. If the field does not change there is no induced emf. Got it? Its all about change. Once there is no more change there is no more induced emf. There is a field about the coil due to the current flowing through it but like the voltage and current it is stable and therefore cannot induce any voltage/emf.
    Last edited: Apr 8, 2003
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