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-   -   Induced electric field (http://www.physicsforums.com/showthread.php?t=658323)

dev70 Dec11-12 01:13 PM

Induced electric field
 
Can electric field be induced at a point near a time varying uniform magnetic field? "Near" means not the in the place where magnetic field exist. But at a point outside the field's presence.

mfb Dec11-12 05:03 PM

Re: Induced electric field
 
You can induce electric fields everywhere. Why do you expect that it would not be possible somewhere?

Meir Achuz Dec12-12 12:12 PM

Re: Induced electric field
 
Quote:

Quote by dev70 (Post 4193278)
Can electric field be induced at a point near a time varying uniform magnetic field? "Near" means not the in the place where magnetic field exist. But at a point outside the field's presence.

You probably meant 'by a magnetic field, but not in the place where the magnetic field exists.

A time varying magnetic field will have time varying vector potential
[tex]\frac{\partial{\bf A}}{\partial t}[/tex] that can exist beyond the field, and induce an E field. This is like the 'Aharonov-Bohm' effect.

elfmotat Dec12-12 12:42 PM

Re: Induced electric field
 
Yes. Say, for example, there's a long solenoid with a time-varying current I(t) running through it. The resulting magnetic field is nonzero only inside the solenoid. However, (assuming ∂B/∂t isn't zero) the electric field induced will also be nonzero outside of the solenoid.

mfb Dec12-12 02:04 PM

Re: Induced electric field
 
Quote:

Quote by Meir Achuz (Post 4194654)
A time varying magnetic field will have time varying vector potential
[tex]\frac{\partial{\bf A}}{\partial t}[/tex] that can exist beyond the field, and induce an E field.

Only in areas where there is a changing magnetic field.

Quote:

Quote by elfmotat (Post 4194704)
However, (assuming ∂B/∂t isn't zero) the electric field induced will also be nonzero outside of the solenoid.

∂B/∂t ≠ 0 implies that there is a magnetic field (apart from some specific points in time maybe).

andrien Dec13-12 01:08 AM

Re: Induced electric field
 
Take a circular area beyond the region of changing magnetic field,but it should include changing magnetic field area then
E.2∏R=-∏r2.∂B/∂t,E is induced in region beyond WHERE B changes.

Meir Achuz Dec13-12 03:53 PM

Re: Induced electric field
 
Quote:

Quote by mfb (Post 4194817)
Only in areas where there is a changing magnetic field.

B= curl A. Apply Stokes' theorem for a B field in a solenoid.
This gives an A outside the solenoid, where there is no B.

mfb Dec13-12 03:59 PM

Re: Induced electric field
 
I don't see how your quote and your post are related. You can get a non-zero A everywhere if you like - even in a perfect vacuum, as you have gauge freedom. But you do not get an electric field without a changing magnetic field or some charge distribution.

elfmotat Dec13-12 04:24 PM

Re: Induced electric field
 
Quote:

Quote by mfb (Post 4194817)
∂B/∂t ≠ 0 implies that there is a magnetic field (apart from some specific points in time maybe).

Yes, but only inside the solenoid. The electric field it produces also "exists" (is nonzero) outside the solenoid where B=0.

mfb Dec13-12 04:47 PM

Re: Induced electric field
 
Quote:

Quote by elfmotat (Post 4195596)
The electric field it produces also "exists" (is nonzero) outside the solenoid where B=0.

Sorry, but what you want just violates the laws of physics.

$$curl(B)=\frac{1}{c}\frac{\partial E}{\partial t} + \frac{4\pi}{c} j$$
You do not want currents and no magnetic field? => electric field is time-invariant. You cannot switch it on or off.

This means that a time-independent charge distribution (which might consist of moving charges) is the only relevant option for a source of an electric field.

elfmotat Dec13-12 05:56 PM

Re: Induced electric field
 
Quote:

Quote by mfb (Post 4195621)
Sorry, but what you want just violates the laws of physics.

$$curl(B)=\frac{1}{c}\frac{\partial E}{\partial t} + \frac{4\pi}{c} j$$
You do not want currents and no magnetic field? => electric field is time-invariant. You cannot switch it on or off.

This means that a time-independent charge distribution (which might consist of moving charges) is the only relevant option for a source of an electric field.

No, it certainly doesn't. If there's a long solenoid of radius a and turn density n with a current I(t) running through it, it will induce a magnetic field B(t)=μ0nI(t) inside the solenoid. Outside of the solenoid B=0 everywhere.

Evaluating the integral ∫E∙ds=-∂/∂t ∫B∙dA ⇔ E=-μ0na2 I'(t) / 2r

Even though B=0 outside the solenoid, it still produces a nonzero E outside the solenoid.

K^2 Dec13-12 06:30 PM

Re: Induced electric field
 
Quote:

Quote by mfb (Post 4195621)
Sorry, but what you want just violates the laws of physics.

Transformers violate laws of physics? You learn something new every day!

Sorry, I shouldn't be mean about it. It is a bit counter-intuitive. But yeah, if you take an infinitely-long solenoid, the magnetic field is ONLY present inside the solenoid. Yet you can wrap another solenoid around it, and induce a current on it by time-varying the current on the inner-solenoid. The B-field outside remains zero, but E-field is non-zero.

This all has to do with curl of the electric field being governed by ∂B/∂t. Outside of the solenoid, both curl and divergence of E is zero, but it doesn't mean that the field itself is zero. Feel free to verify that circular E field with 1/R intensity satisfies conditions of both curl and divergence being zero. (In other words for [itex]E = E_0\frac{\hat{\phi}}{r}[/itex], [itex]\nabla \cdot E = 0[/itex] and [itex]\nabla \times E = 0[/itex] everywhere except r=0.)

andrien Dec14-12 02:06 AM

Re: Induced electric field
 
I have shown in post no.6 that even outside a solenoid if one take a circular area and if it encloses the region of changing magnetic field then electric field will be induced at far distances also.

mfb Dec14-12 07:50 AM

Re: Induced electric field
 
Ah ok, you are right. So we need a coil of infinite length, where B(t) changes linear in time. This gives a constant (in time), circular E(t) and no magnetic field outside.

dev70 Dec20-12 09:35 AM

Re: Induced electric field
 
then..how will a time varying electric field induce magnetic field and where?


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