1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electron vacuum behaviour

  1. Oct 12, 2014 #1
    Good day, from what I read we can make a vacuum in an enclosed space and we can inject or keep electrons in that vacuum , I hope that far im right.Vacuum is a good , the book says the best insulator of electrical current, since it has basically no particles or medium for the current to run through,

    so if I have a box with enclosed vacuum inside and two metal contacts at each side , when applying a voltage across them no current flows , now I wonder what would happen if i had the vacuum box but this time with a certain amount of electrons in that box , would it then act as a conductor depending on the amount of electrons in that space or no? I know a metal conducts because of free electrons in it's lattice atoms and air conducts when ionized when electrons are stripped off of neutral atoms but how do the electrons act in vacuum would they form a current path ?
    also if a current path would form between the contacts in the sides of my box , could i disrupt the current by pushing the electron beam or path away from the contacts with a magnetic field or by electrosatic means , like adding a positive plate outside the box away from the contacts so dragging the electrons close to a side with no contacts on it ?
    Last edited: Oct 12, 2014
  2. jcsd
  3. Oct 12, 2014 #2
    You need something to replace the electrons. The electrons will all be swept to the positive plate, but then current stops unless you can produce free electrons at the negative plate, for example by a heated cathode as in a vacuum tube, or by using really high voltage (field emission)
  4. Oct 12, 2014 #3


    User Avatar
    Science Advisor
    Gold Member

    Sure ... I do this all the time. For example, you can generate electrons via thermal emission (boil them off of a point), field emission (apply a high voltage to a point), or photo emission (strike the metal cathode with an energy above the work function, eg, UV for many metals); there are also RF techniques, but let's stick to DC.

    The source of the electrons will usually have a negative DC voltage supply attached to it; any electrons lost will be replaced by the power supply, and the negative voltage will push the emitted electrons away. They will then be drawn towards a positively charged plate, though ground is "positive" enough if they start from a negative voltage.

    The electron pulses or beam can be steered via electric fields, or with magnetic fields.

    For more details look into the design and operating principles of the transmission electron microscope:

    Or the CRT/TV tube:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook