What is displacement current? Antenna and electromagnetic books talk of
displacement current flowing in an insulator or dielectric and providing a
magnetic field like a real current. They make out that the magnetic field is
distributed the same way as if a real current was flowing along a
conductor. Displacement current is called a virtual current. Books refer to
a real current flowing in a conductor, and being converted to a displacement
current where the conductor stops. Wikepedia says that displacement current
is proportional to the time derivative of the changing electric field, where
a changing electric field induces a changing magnetic field. Diagrams show
displacement currents flowing through the air in the near field of the
antenna. Some scientists believe that displacement current is formed by
virtual photons. It appears that virtual photons are hard to investigate in
particle accelerators.

Displacement current is usually introduced as the virtual current that flows
through the dielectric of a capacitor. Does such a current really exist?

David wrote:
Displacement current is usually introduced as the virtual current that flows
through the dielectric of a capacitor. Does such a current really exist?

When one discharges a charged capacitor through a resistor,
the excess charge from one plate of the capacitor flows
through the resistor to the other plate of the capacitor
in order to equalize the charge on both plates. Since the
current is supposed to be the same all around a single
series circuit, it is *as if* current is flowing through
the capacitor's dielectric somewhat like current flows
through acid inside a car battery. Since actual current
flowing through the insulating dielectric would destroy
it, of course that current is only virtual and is called
displacement current. It can usually be treated *as if*
it were real.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:
David wrote:
Displacement current is usually introduced as the virtual current that
flows through the dielectric of a capacitor. Does such a current
really exist?

When one discharges a charged capacitor through a resistor,
the excess charge from one plate of the capacitor flows
through the resistor to the other plate of the capacitor
in order to equalize the charge on both plates. Since the
current is supposed to be the same all around a single
series circuit, it is *as if* current is flowing through
the capacitor's dielectric somewhat like current flows
through acid inside a car battery. Since actual current
flowing through the insulating dielectric would destroy
it, of course that current is only virtual and is called
displacement current. It can usually be treated *as if*
it were real.

Here's a fascinating (and, at the time, controversial) take on Maxwell's
displacement current, originally published in Wireless World:

http://www.ivorcatt.org/icrwiworld78dec1.htm
Ivor Catt Archive | Dinosaur Computers - Electronics World June 2003 p47

Chuck

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On 16 Mar, 12:04, Cecil Moore wrote:
David wrote:
Displacement current is usually introduced as the virtual current that flows
through the dielectric of a capacitor. Does such a current really exist?

When one discharges a charged capacitor through a resistor,
the excess charge from one plate of the capacitor flows
through the resistor to the other plate of the capacitor
in order to equalize the charge on both plates. Since the
current is supposed to be the same all around a single
series circuit, it is *as if* current is flowing through
the capacitor's dielectric somewhat like current flows
through acid inside a car battery. Since actual current
flowing through the insulating dielectric would destroy
it, of course that current is only virtual and is called
displacement current. It can usually be treated *as if*
it were real.
--
73, Cecil http://www.w5dxp.com

I seem to remember a capacitor defying gravity via displacement
current but I can't remember the details. Do you recall that Cecil?
Art
Art

David wrote:
What is displacement current? Antenna and electromagnetic books talk of
displacement current flowing in an insulator or dielectric and providing a
magnetic field like a real current. They make out that the magnetic field is
distributed the same way as if a real current was flowing along a
conductor. Displacement current is called a virtual current. Books refer to
a real current flowing in a conductor, and being converted to a displacement
current where the conductor stops. Wikepedia says that displacement current
is proportional to the time derivative of the changing electric field, where
a changing electric field induces a changing magnetic field. Diagrams show
displacement currents flowing through the air in the near field of the
antenna. Some scientists believe that displacement current is formed by
virtual photons. It appears that virtual photons are hard to investigate in
particle accelerators.

Displacement current is usually introduced as the virtual current that flows
through the dielectric of a capacitor. Does such a current really exist?

"The concept of displacement current, or displacement current density,
was introduced by James Clerk Maxwell to account for the production of
magnetic fields in empty space. Here the conduction current is zero, and
the magnetic fields are due entirely to displacement currents." - Kraus,
_Electromagnetics_

There are two kinds of current, both arguably "real". One is
conventional or conduction current. This is the flow of charge from one
one point to another. The conventional current is the rate of charge
flow -- one ampere of current equals one coulomb of charge per second.(*)

The other kind of current is displacement current. This is the rate of
change of electric (field) flux passing through a surface. James Maxwell
is usually credited with connecting the two; his equations show that a
time-varying conduction current gives rise to a displacement current,
and vice-versa. Conduction current consisting of charge moving onto and
off a capacitor's plates creates displacement current within the
capacitor's dielectric, which then creates conduction current in the
plates on the other side. The effect is just as though charges are
moving right through the dielectric, although they don't. Only fields
move through the dielectric, constituting the displacement current.

(*) Although "conduction current" usually refers to charge flowing in a
conductor, charge can also flow through space or non-conducting
materials, as for example the electron beam of a CRT. This is a
conventional or conduction current, but sometimes called a "convection
current" when in space.

Roy Lewallen, W7EL

On Mar 16, 1:57 pm, Roy Lewallen wrote:
David wrote:
What is displacement current? Antenna and electromagnetic books talk of
displacement current flowing in an insulator or dielectric and providing a
magnetic field like a real current. They make out that the magnetic field is
distributed the same way as if a real current was flowing along a
conductor. Displacement current is called a virtual current. Books refer to
a real current flowing in a conductor, and being converted to a displacement
current where the conductor stops. Wikepedia says that displacement current
is proportional to the time derivative of the changing electric field, where
a changing electric field induces a changing magnetic field. Diagrams show
displacement currents flowing through the air in the near field of the
antenna. Some scientists believe that displacement current is formed by
virtual photons. It appears that virtual photons are hard to investigate in
particle accelerators.

Displacement current is usually introduced as the virtual current that flows
through the dielectric of a capacitor. Does such a current really exist?

"The concept of displacement current, or displacement current density,
was introduced by James Clerk Maxwell to account for the production of
magnetic fields in empty space. Here the conduction current is zero, and
the magnetic fields are due entirely to displacement currents." - Kraus,
_Electromagnetics_

There are two kinds of current, both arguably "real". One is
conventional or conduction current. This is the flow of charge from one
one point to another. The conventional current is the rate of charge
flow -- one ampere of current equals one coulomb of charge per second.(*)

The other kind of current is displacement current. This is the rate of
change of electric (field) flux passing through a surface. James Maxwell
is usually credited with connecting the two; his equations show that a
time-varying conduction current gives rise to a displacement current,
and vice-versa. Conduction current consisting of charge moving onto and
off a capacitor's plates creates displacement current within the
capacitor's dielectric, which then creates conduction current in the
plates on the other side. The effect is just as though charges are
moving right through the dielectric, although they don't. Only fields
move through the dielectric, constituting the displacement current.

(*) Although "conduction current" usually refers to charge flowing in a
conductor, charge can also flow through space or non-conducting
materials, as for example the electron beam of a CRT. This is a
conventional or conduction current, but sometimes called a "convection
current" when in space.

Roy Lewallen, W7EL


Hi Roy,

With regard to "James Maxwell is usually credited with connecting
the two; his equations show that a time-varying conduction current
gives rise to a displacement current, and vice-versa," need it be
time-varying? I haven't given this a lot of thought in the past, but
it appears to me that the laws cover the case of a constant current as
well. 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.

Cheers,
Tom

K7ITM wrote:

With regard to "James Maxwell is usually credited with connecting
the two; his equations show that a time-varying conduction current
gives rise to a displacement current, and vice-versa," need it be
time-varying? I haven't given this a lot of thought in the past, but
it appears to me that the laws cover the case of a constant current as
well. 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.

I'll be glad to defer to you on this. But it's important to see that the
static field produced by a constant conduction current won't induce a
conduction current in another conductor. So there's no static equivalent
of what happens in a capacitor, which behaves as though the charge
seemingly flows through the dielectric.

Roy Lewallen, W7EL

K7ITM wrote:
On Mar 16, 1:57 pm, Roy Lewallen wrote:
David wrote:
What is displacement current? Antenna and electromagnetic books talk of
displacement current flowing in an insulator or dielectric and providing a
magnetic field like a real current. They make out that the magnetic field is
distributed the same way as if a real current was flowing along a
conductor. Displacement current is called a virtual current. Books refer to
a real current flowing in a conductor, and being converted to a displacement
current where the conductor stops. Wikepedia says that displacement current
is proportional to the time derivative of the changing electric field, where
a changing electric field induces a changing magnetic field. Diagrams show
displacement currents flowing through the air in the near field of the
antenna. Some scientists believe that displacement current is formed by
virtual photons. It appears that virtual photons are hard to investigate in
particle accelerators.
Displacement current is usually introduced as the virtual current that flows
through the dielectric of a capacitor. Does such a current really exist?
"The concept of displacement current, or displacement current density,
was introduced by James Clerk Maxwell to account for the production of
magnetic fields in empty space. Here the conduction current is zero, and
the magnetic fields are due entirely to displacement currents." - Kraus,
_Electromagnetics_

There are two kinds of current, both arguably "real". One is
conventional or conduction current. This is the flow of charge from one
one point to another. The conventional current is the rate of charge
flow -- one ampere of current equals one coulomb of charge per second.(*)

The other kind of current is displacement current. This is the rate of
change of electric (field) flux passing through a surface. James Maxwell
is usually credited with connecting the two; his equations show that a
time-varying conduction current gives rise to a displacement current,
and vice-versa. Conduction current consisting of charge moving onto and
off a capacitor's plates creates displacement current within the
capacitor's dielectric, which then creates conduction current in the
plates on the other side. The effect is just as though charges are
moving right through the dielectric, although they don't. Only fields
move through the dielectric, constituting the displacement current.

(*) Although "conduction current" usually refers to charge flowing in a
conductor, charge can also flow through space or non-conducting
materials, as for example the electron beam of a CRT. This is a
conventional or conduction current, but sometimes called a "convection
current" when in space.

Roy Lewallen, W7EL


Hi Roy,

With regard to "James Maxwell is usually credited with connecting
the two; his equations show that a time-varying conduction current
gives rise to a displacement current, and vice-versa," need it be
time-varying? I haven't given this a lot of thought in the past, but
it appears to me that the laws cover the case of a constant current as
well. 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.

Cheers,
Tom


Del cross H = j + dD/dt (pretend the lower case "d"s are the Greek letters
for partial differentiation). "dD/dt" is supposed to represent the
displacement current. Some people don't like the term "displacement
current" (Feynman), so be careful where you use it.
73,
Tom Donaly, KA6RUH

Well, Feynman might have been impressed with the 'displacement'
currents we are currently generating across the variable cap of a
parallel tuned circuit at 14 megacycles... Acrylic will droop and melt
in seconds and glass heats so rapidly it shatters almost instantly
when the RF is keyed... Yes, I can be convinced that there is no real
'current' flowing, in that no electrons are passing through the
dielectric (i.e. we don't get an arc until the dielectric fails)...
But, the varying electric field across the dielectric as the cap
plates load and unload electrons is certainly pumping the outer
electrons in the dielectric rapidly enough that they dump off
considerable heat as they change state...
Did you know that you can heat an acrylic plate of 0.125" thickness so
fast and so hard that the acrylic boils in the center of the plate,
leaving frozen cavitation bubbles behind when the RF is cut off, while
the faces of the plate remain smooth and shiny? Amazing to look at...


Meanwhile, back to the drawing board...

denny / k8do

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