MadSci Network: Physics
Query:

Re: How does electric current work in a vacuum

Date: Wed Sep 22 07:00:35 1999
Posted By: Georg Hager, Grad student, Theoretical Particle Physics
Area of science: Physics
ID: 935463752.Ph
Message:

Dear Mike!

You want to know something about the mechanism by which electric curent can be made to flow in a vacuum. I will concentrate on the method which is used in vacuum tubes and CRTs.

First of all you have to realize that you need moving electric charge to let current flow. In most cases that will be electrons (there is also the possibility of ions contributing to current flow, for example in salty solutions). In an ordinary cunductor like a metal wire electrons can move more or less freely through the atomic lattice, following any electric field that might be present. There is in principle no difficulty in doing the same without any material around, i.e. electrons can move freely in a vacuum as well. The problem is: How does one get the electrons out of the metal and into the vacuum in the first place?

There are two factors involved in achieving this goal:

  1. Heat. Heating a wire (usually by sending a current through it till it glows, or heating it indirectly) makes it easier for electrons to escape from the material. The reason is that the hotter the metal gets, the faster its constituents (ions and electrons) move, and it becomes more likely that an electron has enough energy to escape from the atomic lattice. Thus there is a negatively charged `electron cloud' around the heated wire. Unfortunately, those electrons cannot get far away, because they leave behind a positively charged material which in turn attracts them.
  2. This problem can be circumvented by providing a steady flow of `new' electrons into the wire. The electrons which have already escaped can now easily be moved by an external electric field and will flow wherever the field guides them.
These principles are applied in usual vacuum tubes, which were the most important electronic components before the advent of semiconductor technology. A voltage is applied between the so-called cathode (the glowing wire) and the anode, which is more or less just a cold metallic conductor. If the negative pole of the voltage source is connected to the cathode, the electrons which have escaped the wire due to heat are repelled from there and attracted by the positively-charged anode: a current is flowing through the vacuum. If one reverses the polarity, no current can flow because the anode is cold and no electrons can get out of there. This electronic device is called a diode; it lets current flow in one direction, but not the other. By adding more sophisticated devices to the diode, the functionality can be expanded. You can find out more about this at The Vacuum Tube FAQ.

Hope that helps,
Georg.


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