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Magnetically Induced EMF with NO current in the path???

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LAN_403

Signification

Mon Aug 05 2019, 06:52PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

I just saw a classroom youtube video on E&M in which the professor worked a sample textbook problem which I can't, in a most annoying way, get my head wrapped around. It claims that a NON-conductive ring in which a B-Field, that is increasing through it, induces an EMF around it, but there is NO current. It is here

at about timestamp 47:30.
If this is true, then do you need a loop (or path) at all...and just consider an imaginary closed path??

Sulaiman

Tue Aug 06 2019, 01:37AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

It is a theoretical situation, the emf can not be physically measured by a measuring device (as no current can flow)
and there will be no counteracting field produced by the non-conducting loop.

Think of it like this,
if the loop is made of copper wire then it is understandable,
if the loop is made of higher resistance material then the emf generated will be the same, but less current will be available,
as the resistance of the loop approaches infinity then the current approaches zero.
It is a conceptual emf, it cannot be directly measured.
The emf is a property of the enclosed area in a changing magnetic field,
until a conductor encloses the area, it is a 'virtual' emf (voltage)

so
"If this is true, then do you need a loop (or path) at all...and just consider an imaginary closed path??
you are correct, just consider an imaginary closed path, the plastic loop is irrelevant.

Plasma

Tue Aug 06 2019, 01:57AM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268

Suleiman takes the path of magnetic, but you can also see it as a capacitor, the B field polarized the diletic making a voltage, as soon as the magnetic field stops changing, the EMF disappeared. The surface area,distance and diletic constant will govern how much current can be drawn, but its fixed in time, if you draw say 1 joule, if the field is still increasing you won't be able to draw anymore current.

Sulaiman

Tue Aug 06 2019, 03:57AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Plasma wrote ...

Suleiman takes the path of magnetic, but you can also see it as a capacitor, the B field polarized the diletic making a voltage, as soon as the magnetic field stops changing, the EMF disappeared. The surface area,distance and diletic constant will govern how much current can be drawn, but its fixed in time, if you draw say 1 joule, if the field is still increasing you won't be able to draw anymore current.

Have you a reference for a B field polarising a dielectric in the way that you described ?

and HOW could you collect energy from a non-conducting plastic ring ?

@Signification ... I advise you to ignore the response from Plasma.
(unless he can provide a reference)

2Spoons

Tue Aug 06 2019, 05:14AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Hell, i'm just impressed by the guy writing backwards on a piece of glass in that video.
Btw this is not much different to loop antenna really - an EM wave in vacuum has an electric field and a magnetic field, but no current.

Signification

Tue Aug 06 2019, 12:05PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

I think that thing is called a "Learning Glass" (or something like that). It's just a reflection. There are many videos made with this. Usually the guy has writing on his shirt--it is always backwards--Also, wedding rings are on right hands, and most are writing left-handed. Google "Learning Glass". Also, he doesn't seem to be aware , for one example, that a north pole of a magnet moving toward a current loop can induce another North pole in the loop on the same side of the loop as the magnet, if, THE NORTH POLE OF THE MAGNET IS MOVING TOWARD THE NORTH POLE OF THE LOOP AND THE MAGNET IS "SLOWING DOWN" Am I correct here?

Plasma

Tue Aug 06 2019, 06:01PM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268

The orbits of electrons in the shell move because of the EM field, they are still locked in the shell, but become polarized.

Opion I think the field has to be de/acceralting rather than steady or stead increase,decreasing.

Far enough ;)

2Spoons

Tue Aug 06 2019, 10:33PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Signification wrote ...

I think that thing is called a "Learning Glass" (or something like that). It's just a reflection.

Well, well, well - learn something new every day. I don't think I've ever seen that system before. I must confess to being mildly disappointed.

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