What is displacement current?
 

If I'm not mistaken, which I could well be in this case, it's a
time-varying electric field that "looks like" a current, and an
electric field whose time derivative is constant "looks like" a
constant current, at least with respect to the magnetic field to which
it gives rise.

To get an electric field with a constant time derivative, you need an
electric field that is forever slewing linearly. You need to keep
piling electrons onto one plate of the capacitor forever. So, you're
right that a constant conduction current can give rise to a constant
displacement current, but in any real capacitor with a breakdown
voltage, you can't get this to happen for very long, so it doesn't
have much applicability in any real system, and wouldn't get talked
about too much.

Dan

What is displacement current?
 

On Mar 17, 9:06 am, " wrote:
If I'm not mistaken, which I could well be in this case, it's a
time-varying electric field that "looks like" a current, and an
electric field whose time derivative is constant "looks like" a
constant current, at least with respect to the magnetic field to which
it gives rise.

To get an electric field with a constant time derivative, you need an
electric field that is forever slewing linearly. You need to keep
piling electrons onto one plate of the capacitor forever. So, you're
right that a constant conduction current can give rise to a constant
displacement current, but in any real capacitor with a breakdown
voltage, you can't get this to happen for very long, so it doesn't
have much applicability in any real system, and wouldn't get talked
about too much.

Dan

Well, when I posted the above quoted material, I actually gave that
some thought. I have some 0.1uF polypropylene caps that I've been
testing for self-discharge. Their self-discharge time constant is on
the order of 50 to 100 YEARS. Polyprop is a good dielectric, but not
the best (lowest leakage) available. So I considered that it's quite
possible to charge a 1uF capacitor with a constant current of, say,
1e-14 amps for longer than I'll live, and only reach 3 volts or so,
with negligible error from leakage currents. That comes pretty close
to DC, as far as I'm concerned. It's still not DC on a geological
time scale, but it is on a human life-span one--and for sure it's DC
with respect to my attention span for tracking such things!

Cheers,
Tom

What is displacement current?
 

K7ITM wrote:
On Mar 17, 9:06 am, " wrote:
If I'm not mistaken, which I could well be in this case, it's a
time-varying electric field that "looks like" a current, and an
electric field whose time derivative is constant "looks like" a
constant current, at least with respect to the magnetic field to which
it gives rise.
To get an electric field with a constant time derivative, you need an
electric field that is forever slewing linearly. You need to keep
piling electrons onto one plate of the capacitor forever. So, you're
right that a constant conduction current can give rise to a constant
displacement current, but in any real capacitor with a breakdown
voltage, you can't get this to happen for very long, so it doesn't
have much applicability in any real system, and wouldn't get talked
about too much.

Dan

Well, when I posted the above quoted material, I actually gave that
some thought. I have some 0.1uF polypropylene caps that I've been
testing for self-discharge. Their self-discharge time constant is on
the order of 50 to 100 YEARS. Polyprop is a good dielectric, but not
the best (lowest leakage) available. So I considered that it's quite
possible to charge a 1uF capacitor with a constant current of, say,
1e-14 amps for longer than I'll live, and only reach 3 volts or so,
with negligible error from leakage currents. That comes pretty close
to DC, as far as I'm concerned. It's still not DC on a geological
time scale, but it is on a human life-span one--and for sure it's DC
with respect to my attention span for tracking such things!

Cheers,
Tom


Tom, I read your description of that experiment on the web some time ago
and found it fascinating. As I recall, you used a 3456A for your
measurements. You might want to post a link to your writeup.

73,

Chuck NT3G

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