Some questions on electromagnetism

  • #1

Murad A.Omar

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
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  • #2
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 length surves the purpose I guess...:wink:

Live long and prosper.
  • #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.
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