Displacement Current

The issue of Displacement Current is a bit complicated, but is associated with one of the biggest errors in modern Electromagnetism. Unfortunately, many Scientists and Engineers still believe in the fallacy discussed below.

During the 19th century, Scientists and Experimenters became fascinated by the properties of electricity. Among their studies, they learned that:

1. When a wire was connected between the poles of a battery, it gets HOT (sometimes disastrously so)

2. When a wire is connected between the poles of a battery, a compass will be attracted to the wire. This effect is increased if the wire is coiled up

3. If the wires are connected a set of metal plates that are close to each other, thus setting up an Electric Field between the plates, the above two phenomena are seen for a very short time when the wires are connected OR disconnected to the battery. Further experiments showed that if the battery voltage is varied up and down, the same results are seen for as long as the voltage is changing.

4. If a magnet is moved near a coil of wire, thus applying a changing Magnetic Field to the coil, electricity is generated in the wire.

There were many other discoveries, of course, but the above were among the most important. The Scientists and Experimenters also noticed that there were specific relationships between the voltage supplied by the battery and the current flowing in the wire. From these relationships, they deduced a number of equations for describing what happened when the above interconnections were made.

Unfortunately, none of these individual equations “worked” under all the conditions. The primary “sticking point” was the equations’ inability to describe how current could flow through the OPEN CIRCUIT metal plates described above.

In the late 1800s, the British Scientist, James Clerk Maxwell developed equations that included a postulated DISPLACEMENT CURRENT that “flowed” through the gap between the metal plates.

Not only did Displacement Current complete the equation set, it PREDICTED that RADIATION was possible from an electrical circuit! Not long after the equations were published, the German Scientist Hertz demonstrated the existence of RADIO!

So far, so good. :-)

But a number of Scientists were fascinated by the implied relationship between discovery 3 and discovery 4. If a moving magnet “causes” an electric current, then doesn’t it follow that a moving Displacement Current “causes” a magnetic field.

For over a century, textbooks and have contained statements more or less to this effect.

However, despite many attempts, including ultra-sophisticated experiments utilizing superconducting elements, NO ONE HAS EVER SUCCEEDED IN MEASURING THE MAGNETIC FIELD “CAUSED” BY DISPLACEMENT CURRENT!

In the late 80s, Dr. Oleg Jefimenko (now Professor Emeritus in physics at the University of West Virginia) looked at this issue from the standpoint of CAUSALITY.

The principle of Causality is very simple. If “A” causes “B” then “A” MUST precede “B” in time.

But all of Maxwell’s Equations contain terms that are occurring AT THE SAME TIME. Therefore, Jefimenko reasoned, Displacement Current cannot possibly be causing a magnetic field. Similarly, he noted, changing Magnetism cannot be causing an electric current, since they also happen at the same time.

Instead, he learned that charges and the motion of charges caused both the Electric Field and the Magnetic Field. The equations associated with this finding are called “Jefimenko’s Equations” and can be viewed online at: Jefimenko's Equations

They were also developed independently by Panofsky and Phillips in the 1960s but appear to have been largely ignored by the dogmatic physics community. It is not clear if Jefimenko was aware of them.

So, contrary to what is taught in today’s classes, a magnetic field does not cause an electric field/current AND Displacement Current does not cause a magnetic field.

There is more to this and will be covered in a later post.