Date: Thu Oct 30 14:24:55 2008 Back to Contents

Author: George Herold

Subject: Re: displacement current

Post:

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"I don't see how a toroid helps here..."
=20
From my perspective a toroid looks like the perfect pickup coil. You'd
want the toroid to fill out to the outer edge of the capacitor plates.
I'd also try and do some estimates of the size of current signal you
were going to see. And then ask how you will measure the changing
current? =20
=20
George Herold

________________________________

From: tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] On
Behalf Of Sam Sampere
Sent: Thursday, October 30, 2008 1:05 PM
To: tap-l@lists.ncsu.edu
Subject: Re: [tap-l] displacement current



You're probing for circular B-fields, so you don't want a toroid. Nor do
you want to cross the center of the plates. You want to maximize =
phi=3DBA
(really d(BA)/dt). When you turn the coil parallel to the plates, phi
--> 0, i.e. v(out) --> 0. When the loop is perp to the plates and
parallel with the radius, phi is a max so v(out) should be max. But I'm
not seeing any change, and I guess that's because the current I is
small.=20

=20

I don't see how a toroid helps here...

=20

Thanks,

=20

Sam

=20

________________________________

From: tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] On
Behalf Of chuck britton
Sent: Thursday, October 30, 2008 11:39 AM
To: tap-l@lists.ncsu.edu
Subject: Re: [tap-l] displacement current

=20

so you are saying that Dick's straightforward with clear results is
bogus??

=20

Why would a toroidal iron core be needed?

=20

If Dick's demo had faraday shielding around the plates - would it still
show results? - would it be more satisfactory to the ney-sayers?

=20

More discussion on this would be appreciated (i.e. I'm too lazy to do
the research)

=20

=20

On Oct 30, 2008, at Oct 30(Thu) 11:17 , Zani, Gerald wrote:





Read Dick Berg's page:

http://www.physics.umd.edu/deptinfo/facilities/lecdem/services/demos/dem
osk2/k2-63.htm

=20

I've never had luck with this demo for two reasons. It is tricky to do
correctly. And when I do get it working the Faculty are not
theoretically convinced that the signal is any different from an RF
antenna signal.

=20

1.) To do it the pick-up coil must be a many turns iron core Toriod.

=20

2.) The AC signal on the cap plates should be of high frequency. One
handy, free way to get a very high frequency signal is to get an old VCR
player, play a video tape and take the RF signal output out the back.
RF modulators with a 4 MHz bandwidth are commonly found built into old
VCRs


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"I =
donít see how a=20
toroid helps hereÖ"

From my =
perspective a=20
toroid looks like the perfect pickup coil. You'd want the toroid =
to fill=20
out to the outer edge of the capacitor plates. I'd also try and do =
some=20
estimates of the size of current signal you were going to see. And =
then=20
ask how you will measure the changing current? =


George=20
Herold


From: tap-l-owner@lists.ncsu.edu=20
[mailto:tap-l-owner@lists.ncsu.edu] On Behalf Of Sam=20
SampereSent: Thursday, October 30, 2008 1:05 PMTo: =

tap-l@lists.ncsu.eduSubject: Re: [tap-l] displacement=20
current


Youíre =
probing for=20
circular B-fields, so you donít want a toroid. Nor do you want to =
cross the=20
center of the plates. You want to maximize phi=3DBA (really d(BA)/dt). =
When you=20
turn the coil parallel to the plates, phi ŗ 0, i.e. =
v(out)=20
ŗ 0. When the =
loop is=20
perp to the plates and parallel with the radius, phi is a max so v(out) =
should=20
be max. But Iím not seeing any change, and I guess thatís =
because the current I=20
is small.

I donít =
see how a=20
toroid helps hereÖ

Thanks,

Sam





From:=20
tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] On Behalf Of chuck =
brittonSent: Thursday, October 30, 2008 =
11:39=20
AMTo: =
tap-l@lists.ncsu.eduSubject: Re: [tap-l] displacement =

current


so you are saying that Dick's straightforward =
with clear=20
results is bogus??



Why would a toroidal iron core be=20
needed?



If Dick's demo had faraday shielding around =
the plates -=20
would it still show results? - would it be more satisfactory to the=20
ney-sayers?



More discussion on this would be appreciated =
(i.e. I'm=20
too lazy to do the research)





On Oct 30, 2008, at Oct 30(Thu) 11:17 , Zani, =
Gerald=20
wrote:


Read=20
Dick Bergís page:

http://www.physics.umd.edu/deptinfo/facilities/lec=
dem/services/demos/demosk2/k2-63.htm



Iíve never had=20
luck with this demo for two reasons. It is tricky to do =
correctly. =20
And when I do get it working the Faculty are not theoretically convinced =
that=20
the signal is any different from an RF antenna=20
signal.



1.) To do it =
the=20
pick-up coil must be a many turns iron core=20
Toriod.



2.) The AC =
signal on=20
the cap plates should be of high frequency. One handy, free way to =
get a=20
very high frequency signal is to get an old VCR player, play a video =
tape and=20
take the RF signal output out the back. RF modulators with a =
4 MHz=20
bandwidth are commonly found built into old VCRs

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From tap-l-owner@lists.ncsu.edu Thu Oct 30 14:24:55 2008

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