What is meant by self induction?demonstrate & prove

Question

i have a doubt that how when the current is flowed in a circuit can also produce an opposing emf by itself?

Answer 1

An inductor is an electronic circuit element that stores energy in the form of a magnetic field. Normally an inductor is constructed from a coil of wire upon a former. If a voltage is applied across the inductor a current passes through it. As a consequence of Faraday's law of induction: -

curl(E) = -∂B

................___

................∂t

A magnetic field builds up around the inductor due to the changing current flow. This magnetic field stores energy and sets up a 'back' emf or voltage that opposes the applied voltage when ever the current flowing in the inductor changes. The back emf is given by the equation: -

V = -LdI

.......___

.......dt

Where I is the current flowing in the inductor and L is its self inductance measured in the SI unit of the Henry (symbol H, after the American physicist).

The energy stored in the induced magnetic field around the inductor is given by the equation: -

E = ½LI²

Where I is the current flowing in the inductor.

There is a mnemonic which is useful when remembering the properties of an inductor in an electronic circuit. It CIVIL, or in a capacitor C the current I is ahead of the voltage V and in an inductor L the voltage V is ahead of the current. This delay in current, in an inductor, is created by the magnetic field's back emf, which holds back the flow of current (see above equation).

I hope this helps.

Answer 2

Self induction is caused in a coil only when the current in it is increasing or decreasing. Not for a constant current.

(A) Switching on.

When the current is switched on, a magnetic field is generated in the space in and around the coil.

At switch on, this magnetic field grows outwards, and in doing so, cuts the turns of the coil itself.

This, according to Farady's law, induces an e.m.f. in the coil, and by Lenzs' Law this e.m.f. (back e.m.f.) opposes the driving  (Ibattery) e.m.f.

So, when the current is growing, and the magnetic field moves outwards,  we have 2 e.m.f's ( the battery and the back e.m.f.)

The back e.m.f. reduces the current and delays its' growth.

So the current takes longer to reach its maximum value.

Note that:-

1) If the coil has an iron core, we get the same effect, but more powerful, as the magnetic field is much stronger. The current takes longer to reach maximum value.

2) If we wind half the coil "one way", and the other half the "other way", we can get rid of much of the self induction effect. This is because the magnetic fields of the two halves are in opposite directions and tend to cancel themselves out. The current can then reach its maximum value very quickly.

3) The growing magnetic field of a coil affecting the coil itself is called "self induction"  while  the effect on another separate coil is called "mutual induction"

4). In D.C. circuits, the self induction of a coil only appears at switch on and switch off. In A.C. circuits, it appears all the time as the current is always changing.  

(B) Switching off.

  It works in a similar way at switch off. The magnetic field collapses, cuts the turns of the coil, and an e.m.f is induced which tries to keep the current flowing. The decay of current is delayed, and since the collapse of the field is usually faster than its' growth, large back e.m.fs can be induced.

1) The large e.m.f. induced at switch off in inductive circuits can cause sparking (arcing) in switches, causing the contacts to become burnt.

2) If you connect a harmless voltage, like 6V to an iron cored coil, when you switch off, you may get a nasty shock. The back e.m.f at switch off can be many times greater than the battery voltage.

  

Answer 3

Sorry I don't have much time to answer.

Anyways, I was really confused on this topic myself. Basically a flowing current DOESN'T cause an EMF. What does cause self-induction is a changing current.

Imagine you have a loop of copper wire with no current going through it. Now you turn the switch and current starts running through the wire. Because there's current there is now a magnetic field. Originally you went from no magnetic field to some magnetic field - you have a changing magnetic field. As you've probably learned changing magnetic fields produce an emf in the direction so as to oppose the change in magnetic field (Lenz's Law). This causes self-induction.

The consequence of this is that there is a time lag for a wire to be running at full current. When you flip the switch the current isn't instantly at full throttle. Instead it takes some time to overcome the induced emf and build up to its maximum value.

Once again, I'm sorry with the brief answer but I gotta run.

Answer 4

Induction deals with charges,voltages.When u aquire a charge an go near a vehicle,u recieve a shock because u were not earthed or due to friction.Use a vandergraff generator to prove,to see what I mean or rub a plastic pen on ur hair an move it near bits of paper.