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Date: Around 1993


Question:
Can a spark travel across a vacuum? What happens to the air between two charged objects to allow a spark to jump between them?



Replies:
In the air between two highly charged objects there is a large electric field. If there happens to be a free electron in that space it will be accelerated to high speeds by the electric field. While it is being accelerated it will collide with the gas atoms and be slowed down. When the field gets so strong that it gains enough energy between collisions so that it can ionize (excite another electron from the colliding atom) the atom it hits, the number of electrons in the air can increase very quickly (called an electron avalanche) This large number of electrons increases quickly as each electron frees a new electron and the whole group makes up the spark. The spark can move in a vacuum, but the creation of the spark requires the gas atoms to be present as source of new electrons and the avalanche. The breakdown field depends on the density and how tightly held are the atomic electrons.

Sam Bowen

Up-date 1/25/2005

The previous answer describes a spark that occurs in air or another medium, and describes it well.

The question is actually fairly tricky because the word "spark" is not well defined. Usually by "spark" we mean a momentary flow of electricity through a medium that does not usually conduct electricity. We have sparks through air, but not through metal. When we think of the word "spark" we think of a bright flash. The word "arc" usually connotes a continuous flow of electricity. Like an arc welder.

A good vacuum is a very good insulator. Much better than air because there are no molecules to ionize and participate in an avalanche. However, researchers who work with high-vacuum, high-voltage equipment know that little "sparks" occur in vacuum with a few thousand volts or more. We do not know why it happens, but we think it has something to do with dirt or dust on charged surfaces. The strange thing is that these sparks are little points of light that do not apparently jump between anything, like a normal spark.

It is possible to have a stream of electrons travel in a vacuum. To do this the emitter must be sharply pointed and the applied voltage must be large. Electrons come off the sharply pointed emitter. This is the basis of the field-emission electron microscope. But these electrons do not make light, so they are not a "spark" or "arc" as commonly used. So, no, it does not seem possible to have a 'spark' in the conventional sense in a good vacuum.

There is something called a "triggered vacuum spark-gap switch." It at first seems like a switch based on a spark through a vacuum, but the current is really conducted by a tiny burst of evaporated metal atoms, which carry the current like air ions do.

Bob Erck



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