# I believe breakdown resistance across 1" air gap is 10,000 volts. How about a 1" gap of a perfect va

Discussion in 'Electronic Basics' started by Gary Helfert, Dec 15, 2004.

1. ### Gary HelfertGuest

This is an academic question among my coworkers. Some believe it is
infinite, however I believe at some voltage the electrons will jump the gap.
Any ideas?

2. ### Fritz SchlunderGuest

Vacuum doesn't have infinite dielectric strength, however it can be fairly
large or really quite pitiful. I understand the shape of the electrodes
(needle points will concentrate electric field lines and decrease breakdown
strength), the temperature of the electrodes (high temperature leading to
thermionic emission which takes minimal voltage), and the specific electrode
material used (different vacuum work functions) will significantly alter the
effective dielectric strength of vacuum. Presumably cathode ray tube (CRT)
monitors and TVs use electron guns which work in vacuum but only require a
few tens of kV (but they use electrode heating) or less.

3. ### John PopelishGuest

Since vacuum contains no electrons, it cannot break down and have its
electrons start to travel under the influence of an electric field, as
matter filled dielectrics can.
http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Capacitors/Diaelectric.html

Put a pair of surfaces in a vacuum and apply a potential difference
across them, and the breakdown involves the release of electrons from
the more negative surface, a completely different process than
dielectric breakdown. But once those electrons are released, they
travel, unimpeded through vacuum.

4. ### John LarkinGuest

A vacuum can support a huge electric field, infinite except for
obscure quantum effects at incredible field levels. But real
electrodes will have surface emission effects that will rip ions out
of the metals or whatever; once a few ions get loose and whack the
opposite electrode, all hell breaks loose. It takes something like
1e8v/m to rip ions out of metal, but you can get that sort of field at
the tip of a rough spikey bit using only kilovolts. Clean high-vacuum
systems can work in the tens of megavolts per meter range, as I
recall.

As in interesting aside, look up the Farnsworth Multipactor effect.
It's cause some serious grief in satellite RF systems.

John

5. ### peterkenGuest

Don't really know for vacuum, but I assume it's a better insulator as
"normal" air...
My (practical) experience is that 10kV / cm...12kV /cm can be done in a
"normal" environment without any breakdown (unless spikey surfaces or highly
humid air are involved)