# magnetic field question?

Discussion in 'Physics' started by wes, Aug 21, 2010.

1. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
Hi, I have a question about a magnetic field.

there are 2 inductors

1. a magnetic field is created by a inductor
2. that inductor is shut off
3. The tricky part. I know that magnetic field collapses and causes a back-emf on the inductor but will the field that spread out still induce a voltage on another inductor even after the first one is completly off. say the 2nd inductor is far away but extremly sensitive. if not then why. the information that the inductor is off would not have had time to reach the furthest point yet?

this is a hypothetical question's
but if there is any math that could help then please please post it.

2. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
That is an interesting question. I suspect the answer is yes, you can (not necessarily will) get magnetic coupling between the two inductors.

On a related subject though, ever have a CRT on a monitor too close to a different piece of equipment? I've seen a lot of cases where you can have extreme interference from the second piece of equipment. Since CRTs use electron guns and magnetic fields to direct the electrons they are sensitive about such things.

3. ### Curleeee New Member

Aug 12, 2010
7
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When magnetic (or electric) flux densities change the effects are propagated at the speed of light. Of course, most practical circuits are spaced so that the time taken for interactions is negligible.

It was one of the intriguing consequences of Maxwell's equations for electric and magnetic fields. A changing magnetic field induces a changing electric field and a changing electric field induces a changing magnetic field. The effect propagates.

4. ### davebee Well-Known Member

Oct 22, 2008
539
46
Wes, your question itself really needs to be rephrased to make sense.

You don't shut off an inductor. What you can do is to shut off your external source of EMF from the inductor.

When you do that, the magnetic field does start to collapse, and the collapsing magnetic field causes the current in the inductor to continue to flow.

Since there is still a current flow through the inductor, I don't like using the expression "shut off" to describe the inductor, as it is active electrically.

The answer to question 3 is yes, a remote inductor will have an EMF induced in it just like a transformer, but not because the first inductor is "off", but rather because the magnetic field in its location is varying.

5. ### wes Thread Starter Active Member

Aug 24, 2007
242
2
But how can a current still flow if the inductor has it's source shut off.

the circuit is no longer a closed circuit, it's open and so how can there be a current flow?

6. ### Ghar Active Member

Mar 8, 2010
655
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There can be current because the inductor kicks back with large EMF's to keep it going. This is why no body likes disconnecting inductors with significant current in them...

If you physically open a relay or something you will have arcing where the current simply goes through air.

7. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
One might reason that the inductor could be completely at steady-state condition with zero current flow and that a remote circuit still detects the preceeding transient.

Ghar points out that the transient condition might give rise to an arcing condition at the point where the inductor current is interrupted. One might further reason that this arcing behaviour might include a self-resonant condition in the inductor which could lead to the propogation of an electromagnetic wave from the inductor itself.

At some time the inductor transient will have died away but the electromagnetic wave is still propagating outwards - albeit at ever diminishing magnitude. Presumably a "detector" sufficiently remote from the inductor could respond to the electromagnetic "signal" or impulse after the transient condition in the inductor has diminished to zero.

Last edited: Aug 23, 2010
8. ### Heavydoody Active Member

Jul 31, 2009
140
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This seemed like a worthwhile mental exercise before the start of my fall semester. Be forewarned; I am only a second year engineering student!

Looking at my physics textbook, I see that induced emf in a coil is proportional to the rate of change in flux (Faradays Law of Induction). Thus, if any of the magnetic flux from coil one passes through coil two, any change in that flux (i.e. when the magnetic field disappears) will necessarily result in an emf in the second coil.

As to distance, it seems to me the strength of the magnetic field plays a significant role in these calculations. Although the disappearance of the magnetic field may be delayed by distance, the time it takes for the magnetic field to disappear completely remains constant, irrespective of position. However, the amount of magnetic flux affecting the second coil decreases over distance. With a decreasing amount of flux disappearing over the same amount of time, the rate of change decreases as well, resulting in a decrease in emf.

My textbook does not address the strength of the magnetic field along side of a solenoid. However, along the solenoids centerline it gives the magnetic field as a function of distance:

where z is the distance, μ0 is the permeability constant, N is the number of turns, i the current, and A the area. So, clearly the strength of the magnetic field decreases rapidly as the distance increases. I suppose you could put some theoretical numbers in there for a better idea, but I doubt you would get much of a signal over any appreciable distance for just a single impulse. Anyone see any flaws? Or am I missing the point all together?

9. ### Curleeee New Member

Aug 12, 2010
7
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Want to try an experiment?

Get yourself an inductor - preferably one that has an "open" magnetic circuit. A solenoid with an iron (or no) core. - but a small transformer will work also. Connect to a 6v battery (try 3v or 9v if you don't have a 6v battery) with removable wires which can "wipe" across the battery contacts.
Get an AM radio and tune to a blank place on the medium waveband.
When the radio is close to the solenoid you should hear noise when the the solenoid is intermittently connected to /disconnected from the battery.
Investigate how the intensity of the noise depends on the geometry of the setup.

10. ### retched AAC Fanatic!

Dec 5, 2009
5,201
313
This is how "wireless power" works. One inductor is switched on and off and the other inductor picks up the "pulses" and can power small devices.

Mar 8, 2009
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12. ### russ_hensel Distinguished Member

Jan 11, 2009
820
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Beware that the resistance of the inductor and driver is very important. If it is a super conductor the current is maintained with 0 voltage ( assuming the driver has 0 internal reistance ). The stystem is stable for an infinite time. Then to shut it off you need only put a resistance in the driver. If the resistance is infinite the time derivative of the current is infinite so you get a big spark ( or a non calculable result ). I think you have to use maxwell for a complete explanation.

Last edited: Sep 6, 2010
13. ### sachin.tiger New Member

Sep 24, 2010
10
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hey boy, let us see the proceedings in different time intervals .
1.when the current flows at a steady rate i.e. a dc,there is no change in magnetic flux thru the inductor which is placed near the 1st inductor.
2. when we switch off the supply ,the current does not instantly becomes 0 but decreases steadily as there is opposition to the rate of decrease of current-the effect of the induced emf.
E=-dθ/dt where E is induced emf and rhs represent the rate of change of flux(faraday's law of induction)
3.outside the conductor when the current falls off then the magnetic field falls acc 2 biot savart law
dB=u°/2∏*I.dl wher I is the current.
Hence magnetic field changes with time this induces the current in the second inductor according to faraday's law described above.

14. ### lmartinez Active Member

Mar 8, 2009
224
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Lenz's Law: "The polarity of the induced EMF is such that it tends to produce a current that will create a magnetic flux to oppose the change in magnet flux through the loop." (an inductor in this case..)

In simple terms, the answer is given here..........