1. Jul 1, 2008

pzlded

Let there be three electronvolts of energy between a tube’s cathode and plate with a voltage of one volt. Let thermionic emission release one electron at the cathode. From the definition of electronvolt (the energy a point charge gains when it travels through one volt), that electron gains one electronvolt of kinetic energy on its journey to the plate and two electronvolts of voltage’s energy remain. Release of two more electrons will convert the remainder of the tube’s energy to electron kinetic energy. No voltage’s energy will remain in the tube. Perhaps this shows the difference between voltage's energy and energy due to attraction between masses?

If the electron bounces (with perfect elasticity) off the plate instead of landing on the plate, will that electron bounce until it gains all three electronvolts of energy. When the bouncing electron travels between the cathode and the plate, how does that change the point charges that cause the voltage?

2. Jul 1, 2008

cmos

I may be misinterpreting you, but if not then your line of thinking is flawed:

If there is 3eV between the cathode and your 1 V plate, then that means the cathode is at -2 V. Minus because you say the electron go from the cathode to the 1 V plate. So if you assume that electrons are being released by some mechanism from the -2 V cathode, then you have simply generated current.

Just to clear up the definition of the electron-volt:
1 eV is the potential energy that an electron has when placed in an electric potential of 1 V.

If want like to think in terms of kinetic energy:
1 eV is the kinetic energy gained by an electron when accelerated from rest, and displaced a distance of 1 m, by a uniform electric field of 1 V/m.

3. Jul 1, 2008

pzlded

Yes if you mean the energy required to bring an electron from infinity to the 1V spot. For example: a 1.5 volt battery does not have one contact point on an electron and another contact point near the electron. As with any unit of energy, electronvolts can be independent of any particular voltage.

An electronvolt of energy is U=QV. The energy required to charge a capacitor is U = ½ QV. Is there a difference in charging energy per charge in a capacitor vs acceleration energy per charge of an electronvolt?
A high capacitance capacitor charged with 1V has more energy than a low capacitance capacitor charged with 1V.

Last edited: Jul 1, 2008
4. Jul 1, 2008

pzlded

The energy unit ELECTRONVOLT differs from eV, (eV is volts or energy per charge. Wikipedia explains the energy unit 'electronvolt'.

http://en.wikipedia.org/wiki/Electronvolt

5. Jul 1, 2008

ZapperZ

Staff Emeritus

eV IS electronvolt. It is not different. It cannot be "volts" because it is a unit of energy (volts isn't a unit of energy). So that wikipedia link you gave contradicts what you said here.

Zz.

6. Jul 2, 2008

7. Jul 2, 2008

Cthugha

Ehm...who told you that eV is Volts? V is in Volts and e is the charge of an electron, which is given in Coulomb. So eV is Coulomb times Volts, which is Joule again.

8. Jul 2, 2008

ZapperZ

Staff Emeritus
"electronvolt" and "eV" are the same thing. I've been a physicist for years and have used this extensively. If you don't wish to listen to this, then there's nothing else to be said.

Zz.

Last edited: Jul 2, 2008
9. Jul 2, 2008

malawi_glenn

The electronvolt (symbol eV)

Is what I read on that wiki-article

10. Jul 2, 2008

pzlded

Sir, I think there is a problem with your units.

Electronvolts do not equal volts

$$qV \neq V$$

An ElectronVolt $$qV$$ is the amount of voltage's energy that converts to kinetic energy when an electron passes through 1 volt (V) of electric potential difference.

In a synchrotron, each Volt adds one electronvolt to either + and - charges; accelerating the charges AWAY from the voltage gradient. Volts caused by + charges will always attract electrons, instead of accelerating them away from the voltage gradient.

11. Jul 2, 2008

ZapperZ

Staff Emeritus
Who here actually said that "electronvolts equal volts"? From what I had read, it was YOU who are insisting that "eV" has units of volts (a post that you had edited but the original post is still preserved in a quoted comment). All of us here have been trying to tell you that "electronvolts" and "eV" are the SAME thing, and both are units of energy.

Why don't you sit down and do a dimensional analysis and satisfy yourself that eV is a unit of energy?

So are you still insisting that "electronvolt" is NOT the same as "eV", and that "eV" isn't a unit of energy?

There are WAY too many other places in physics to trip on, this is one of the most puzzling place to get stuck at.

Zz.

12. Jul 2, 2008

Cthugha

Are you just not reading, what others write or do you just want to mock us?

No one here thinks, that electronvolts are volts, but you are the only one, who thinks, that eV refer to Volts. eV is NOT volts and it is NOT energy per charge.

It is just your $$qV$$ mentioned above, where e, the charge of one electron, is used for q.

Just to make it clear again: the e in eV is the charge of an electron and NOT a prefix like in kV, MV, GV or mV.

13. Jul 2, 2008

pzlded

Sorry, I put in a draft version instead of the 'real' one. PF is a great forum and my desire is to contribute, not to detract.

I did not mean to include the first sentence (big mistake). The sentence was an overview to help me focus on providing a response to your post. I fully intended to remove the sentence, I used the word sir to mean me (as if you were calling me sir), to remind myself to keep this professional. I shall try to never make such a mistake again.

eV can correctly be used to mean either volts or electronvolts, but electronvolts and volts have very different meaning. I try to distinguish the two by only using 'electronvolts' to mean qV.

14. Jul 2, 2008

malawi_glenn

pzlded: Give up

eV is the symbol of electronvolt, which is an unit for energy. Just as Js is the symbol for Joule seconds.

Thats it, no less no more.

A person who uses eV as a unit/symbol for volts, is out of his mind.

15. Jul 2, 2008

ZapperZ

Staff Emeritus
Your question has been sufficiently answered many times. It is up to you to either accept or reject it, at your own risk.

Zz.