At room temperature, no free electrons can ever leave the surface of the metal. A free electron cannot escape the coulomb (electric) attractive forces presented by the positive metal ions in the lattice. (We'll see later that under special conditions using unique mechanisms, it is possible for electrons to escape)

Source: http://www.amazon.ca/Practical-Electronics-Inventors-Third-Edition/dp/0071771336

I don't understand why the electrons can't leave the surface. Even though there is an attractive force from the positive ions there is an almost equal repulsive force from the other free electrons. So the net force should be almost zero, so even a small voltage should be enough to get the free electrons to leave the surface right? What am I missing?

Please keep answers relatively simple. I only have a basic understanding of physics and chemistry.

  • \$\begingroup\$ Are we talking in air or vacuum, flat surface or pointed? \$\endgroup\$ – JIm Dearden Feb 9 '14 at 11:35
  • \$\begingroup\$ The claim in the book is interesting. I wonder then how would book's author explain various spark sources that are used in for example spark-plugs and gas igniters. \$\endgroup\$ – AndrejaKo Feb 9 '14 at 13:22
  • \$\begingroup\$ I think more context is needed. What metal is the author talking about, in what setup or conditions? What are the unique mechanisms the author talks about later? \$\endgroup\$ – Passerby Feb 9 '14 at 19:09
  • \$\begingroup\$ I think the correct answer is here : physics.stackexchange.com/q/147939/87652 \$\endgroup\$ – avl_sweden Aug 7 '15 at 15:58
  • \$\begingroup\$ Or more precisely: physics.stackexchange.com/a/147966/87652 \$\endgroup\$ – avl_sweden Aug 7 '15 at 15:59

Well, what happens if you try to remove an enormous amount of electrons from the wire? You'll leave behind the positive metal ions, so the wire will become charged positive. This pulls strongly on the electrons you're trying to remove.

The same thing happens with just one electron. It will be attracted to the wire by the positive charge left behind. And this attraction is much stronger when the electron is close to the wire surface.

Details: pull an electron a little way outside the metal surface. Now this individual un-canceled electron can repel the mobile electrons in the wire, leaving positive charges exposed. The positive charges attract the removed electron. But if we pull the electron farther away, the patch of positive charges becomes wider and overall weaker. The attraction is strong when the electron is first removed, and less when it's pulled farther from the metal surface. There's a physics rule-of-thumb to cover this: the positive charge in the metal behaves just like a single negative electron, and it acts like a mirror-image of the electron being removed. (Look up "image charge.")

And what the book probably discusses later is... hot filaments. If the wire is white hot, some of the mobile electrons are zooming around fast enough that they can fly a large distance away from the wire before being pulled back. The hot filament in a light bulb or vacuum tube is surrounded by an electron cloud.

  • \$\begingroup\$ Great answer, got a couple of questions though. You said that removing an electron repels the mobile electrons in the wire which exposes positive charges which attract the removed electron. But why would the electron being removed repel the mobile electrons? When the electron was in the conductor the other electrons weren't repeled away from it, so why would they be repelled away when you try to remove it? \$\endgroup\$ – dfg Feb 9 '14 at 19:21
  • \$\begingroup\$ DOH, old comment. Electrons in metal do repel their neighbors and experience forces. But when deep in bulk metal the distribution of neighboring electrons is roughly spherical, so the usual repulsion-of-unlike-charges won't create a net force in a single direction. At the metal surface this is no longer true. The barrier of surface potential isn't a new force, instead it's an existing force which has become asymmetrical. \$\endgroup\$ – wbeaty Mar 16 '14 at 3:29

I think the author has just not used the word "never" in a very loose way. Because an escaping electron would leave behind a net positive charge in the metal, it requires an energy input to separate the electron from the material. This creates an "energy barrier" (called the work function of the material) that must be overcome before electrons can escape. Therefore electrons do not spontaneously leave a metal surface --- something must provide energy for them to do so. There are in fact several ways we can overcome the energy barrier and induce electrons to leave the surface of a metal.

In a hot filament, the metal is simply heated to a temperature where a fraction of the electrons have sufficient energy to escape the metal. This is called thermionic emission, and is the operating principle of many vacuum tubes.

In the photo-electric effect, an applied electromagnetic wave transfers energy to the electrons, allowing them to escape the metal. This effect is used in devices like photo-multiplier tubes.

Finally, simply applying a sufficient electric field, as in a spark gap, can allow electrons to escape. As some comments alluded to, a pointed shape to an electrode will cause electric field to concentrate at the point, and is often used to create a spark gap with a lower breakdown voltage.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.