Why do electric rockets need to be in a vaccum?

[epicbattle]

Yeah, why do they need to place plasma rockets in a vacuum? Temperatures? Air resistance from the particles? Is there any thrust at all in an atmosphere?

 

[MATLABdude]

Yeah, why do they need to place plasma rockets in a vacuum? Temperatures? Air resistance from the particles? Is there any thrust at all in an atmosphere?

If you mean an ion drive, it is because most of them produce so very little thrust:
http://en.wikipedia.org/wiki/Ion_drive

The whole philosophy is that slow and steady wins the race. Whereas a normal chemical rocket will exhaust its fuel in a matter of a few minutes (if going at full throttle), the ion drive can keep on accelerating for a very long time (a little bit at a time).

If you look at the Wikipedia link above, you'll see that the NSTAR (developed to power interplanetary probes, and used in the Deep Space 1 probe launched in 1998) has a thrust of 92 milliNewtons (mN):
http://en.wikipedia.org/wiki/Deep_Space_1

A letter-sized piece of 20 lb. paper (grammage is 75 g / m^2, letter is 8.5 x 11 inches) has a mass of 4.52 grams. Its weight (how much gravity pulls on it) is then 44.4 milliNewtons (using 9.81 N / g). The NSTAR then has a thrust equal to the weight of two pieces of paper!

Clearly, you won't be going anywhere (in an atmosphere and/or subject to earth-ish gravity) on that much thrust. But do it in outer space, with minimal particles to run into, and working against microgravity, and do it for a few years? You can get up to quite a decent speed (Deep Space 1, though it accelerated for 2 years, was intended to stay in the solar system, rather than going where no man, woman, or probe has gone before).
http://nmp.nasa.gov/ds1/arch/mrlog69.html