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The back emf is produced because the changing current in the inductor causes a changing magnetic field around it and the changing magnetic field causes,in turn, an emf to be induced back into the inductor. This process is called self inductance.
now my question is why does the changing magnetic field necessarily causes an emf to be induced back? smiley-confuse
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You can generate voltage across a coil either by
(A). moving a coil through a stationary magnetic field,
(B). moving a magnetic field through a stationary coil.

Example: If you have 2 coils near each other, and one coil is running alternating current (which causes changes in magnetic field), you'll generate a voltage across the 2nd coil ... which is basically how transformers work. (B)

Another example: a coil moves/passes/spins through a stationary magnetic field, generating voltage across the coil... which is how a generator work. (A)
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Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.
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A changing magnetic field (from whatever cause) through a circuit always causes a voltage to be induced in it, it's a fundamental law of nature. For the details, google Faraday's Law.
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I understood that part...but similarly
"The induced voltage creates a current in the coil that flows either with or against the main coil current,depending on whether the coil current is incresing or decresing.if the coil current is incresing,the additional current flows against the main coil current but when the coil current is decresing, the additional current flows with the main coil current"
and my qsn is why does it happen so?why the additional current flows against when the main coil current when the coil current is increasing and similarly flows with the main coil current if the coil current is decrasing??
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The key bit is that the induced emf always opposes the increase or decrease of current in the coil. As Faraday's Law says, "The induced electromotive force in any closed circuit is equal to the negative of the time rate of change of the magnetic flux through the circuit." Think of an inductor as a device that likes to have a constant (or zero) current flowing through it, and that argues with you whenever you try to change that current.
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Electromagnetic theory explains why a changing magnetic field causes current to flow.
Current flowing in a wire creates a magnetic field.
A wire moving thru a magnetic field create a current.
A changing magnetic field creates a current in a stationary wire.

When current flows in a wire, a magnetic field is created. When current stops flowing, the magnetic field collapses; the changing magnetic field creates a current.
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The key bit is that the induced emf always opposes the increase or decrease of current in the coil. As Faraday's Law says, "The induced electromotive force in any closed circuit is equal to the negative of the time rate of change of the magnetic flux through the circuit." Think of an inductor as a device that likes to have a constant (or zero) current flowing through it, and that argues with you whenever you try to change that current.
but why does the additional current flows with the main coil current when the main coil current decreases?Still why it doesn't flow against the main coil current like the one when the main coil current is increasing?
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but why does the additional current flows with the main coil current when the main coil current decreases?Still why it doesn't flow against the main coil current like the one when the main coil current is increasing?

Because the induced emf is 'trying' to oppose the decrease in current.
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It's just a fundamental law of physics.

You may well ask why is the speed of light what it is?

Why is Planks constant what it is?

If you can answer those kind of questions then you'd be a lot more knowledgable than a lot of Quantum Phyisists.

We just know that some things ARE. We don't necessarily know the WHY of everything though.

It's related to the fact that the relationship between current, magnetic field and force in Flemings laws is dependent on whether current is applied directly to a coil (motor principle), or inducted into a coil (generator principle). Self inductance is the coil trying to act on both Fleming's left hand AND right hand, rules at the same time.

You can see this if you hold both hands up in the left hand and right hand rule position. Motion (the direction of change in magnitude of the current) is common. Current direction is common. Field direction is opposite.
« Last Edit: January 17, 2013, 07:27:32 pm by tack » Logged

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I understood that part...but similarly
"The induced voltage creates a current in the coil that flows either with or against the main coil current,depending on whether the coil current is incresing or decresing.if the coil current is incresing,the additional current flows against the main coil current but when the coil current is decresing, the additional current flows with the main coil current"
and my qsn is why does it happen so?why the additional current flows against when the main coil current when the coil current is increasing and similarly flows with the main coil current if the coil current is decrasing??

Erm, that text is pretty poorly phrased/misleading.

The total EMF in the circuit, combined with the various elements of the circuit (resistances, whatever) determine the current.  But
the total EMF includes the induced EMF which itself is determined by the rate-of-change of current.  Thus there is a circular
interdependence between current and voltage in the circuit.

You typically have to solve a differential equation to predict the result evolving in time...

Energy is conserved in an inductor so you won't see an increase in current causing an increase in current (ad infinitem)...

If the magnitude of the current is increasing then energy is flowing into the magnetic field, so is being taken from
the rest of the circuit, which is why the back EMF (as its called) always seems to oppose the change in current
(the external circuit has to work harder to pump energy into the magnetic field if the field is increasing).

When the field is decreasing the EMF will seem to want to preserve the field (actually it is the means that energy is put back into
the circuit from the field.)   Attempting to stop the current in an inductor very quickly will induce a very large EMF (which
allows the field energy to force its way back into the circuit quickly).  Thus a low voltage circuit containing an inductor
can generate upto thousands of volts (limited mainly by the switching device used to arrest current flow).
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