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