# Copper block minimum conduction voltage?

Discussion in 'General Electronics Chat' started by Ruptor, Jul 2, 2009.

1. ### Ruptor Thread Starter Active Member

Apr 26, 2009
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Hi everybody
Physics of devices was not my strongest subject hence could someone help me with this question. If I have a copper bar and make connections to the ends what is the minimum voltage necessary to cause a current to flow through the bar. I don't have a clue what parameter to search for on the internet. Is it something to do with electron orbits?
Thanks.

2. ### R!f@@ AAC Fanatic!

Apr 2, 2009
9,647
1,111
You were wrong when you said physics is not your strongest subject.
In fact physics is not even your weakest of subjects.
Copper is like one of the best conductor in nature. Even a micro voltage is enuf to force a current through copper.

Did you know that copper is used as the main conducting medium is anything that uses a voltage to operate

Rifaa

3. ### Ruptor Thread Starter Active Member

Apr 26, 2009
44
0
If you are saying 1uV then the voltage required is > 1uV and 0.5uV will not get current to flow in a copper bar even if it is 1 metre diameter or bigger. Can you tell me where I can read more about this? I would like to know what the exact bottom limit is and the theoretical reason.
Thanks.

4. ### beenthere Retired Moderator

Apr 20, 2004
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Actually, you can apply Ohms' law - I = E/R, where I is current, E is voltage, and R is resistance. You may find that any applied voltage, no matter how small, will cause current flow.

But you can get the bulk resistivity of copper and actually figure R for your copper block, and obtain a reasonably exact figure for the current.

5. ### Ruptor Thread Starter Active Member

Apr 26, 2009
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Yes but I am interested in the point where conduction doesn't happen. Consider a block of infinite size or just very big so the resistance will be exceedingly small. I suspect that at that point a quantum level must be the limit I would like to know what it is. The electrons are less attached to the copper nuclei but there must still be a voltage to overcome to get one moving. Perhaps I should have posted this question under the physics heading.
Thanks.

6. ### R!f@@ AAC Fanatic!

Apr 2, 2009
9,647
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I couldn't have said any better than this.
Thanks beenthere

Rifaa

7. ### R!f@@ AAC Fanatic!

Apr 2, 2009
9,647
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I think you should do a search in conductivity of a medium and also it's chemical properties to find the number of free electrons.
By the way electrons and holes are responsible for current through a meduim.

Rifaa

8. ### russ_hensel AAC Fanatic!

Jan 11, 2009
825
57
When you get down to low voltages all sorts of effects can come up including thermoelectric effects if the junctions are of dis-similar materials and the temperature is not uniform. Is there a magnetic field, is its rate of change exactly 0?. If the bar is at room temperature I believe that the electrons are in a sea, and that any energy to make them move ( net, they are moving all time time ) is very close to 0. If the electron were bound it would take some quanta to move it.

9. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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Unlike semiconductors, true metals have no "forward conduction" voltage...they will begin conducting from the most minuscule forward potential. In this sense, they are TRUE ohm's law materials...from any voltage from 0 to infinity.

ERIC

10. ### Ruptor Thread Starter Active Member

Apr 26, 2009
44
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Hi Guys
From what I have read so far an energy of 7eV is required to free the electrons from the copper nuclei and pop them in to the conduction band. This is small but is still not zero and is a quatum step below which conduction will not occur. I am still trying to understand how this level relates to a very small voltage applied to a copper bar in the real world.

11. ### russ_hensel AAC Fanatic!

Jan 11, 2009
825
57
lots of 7ev hanging around at room temperature. Might be a problem if very cold, unless resistance drops to 0.

12. ### Wendy Moderator

Mar 24, 2008
21,838
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Actually, in any metal, there are already free floating electrons. It is one of the defining characteristics of metal, any metal. It also indirectly causes metal to be shiny, and a reflector. The free electrons absorb the photon, then re-emit it back the way it came.

13. ### ispymalik Member

Apr 7, 2008
18
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A metal such as Cu has its Conductive band and Valance band overlapping.
So, a band gap does not technically exists. - if im not mistaken.

So in the case of current there is Diffusion and Drift.

Copper has a work function of 4.7 eV so any light with hv > the w.f.
could create some photocurrent.
so .26 micro meters is around ultra violet?

As far as Drift current. Cu has about ~1.777 micro ohms /cm
So calculating the area and resistance you can find some voltage to satisfy your current.

lol dont listen to me im just thinking here.

Last edited: Jul 3, 2009
14. ### Ruptor Thread Starter Active Member

Apr 26, 2009
44
0
Hi Guys
I have just been reading about the Fermi-Dirac statistics of energy distribution where it says the 7eV mentioned earlier is the Fermi level for copper. In copper the Fermi level is at the top of the valence band and is the energy an electron has to have to enter the conduction band. The conduction and valence bands are adjacent in metals and almost all the free electrons at the top of the valence band can easily gain enough energy to enter the conduction band. I guess the need for electrons to gain energy in order to reach the conduction band might be why copper has a resistance and super conductors don't but don't take that as gospel since I don't know any about this stuff and I'm still reading.

15. ### ispymalik Member

Apr 7, 2008
18
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7ev is alot for a fermi level
sillicons entire band gap is 1.1ev

conductivty is related to electron mobility concentration of electrons and the charge of an electron.

A fermi level is the probabilty an electron can be occupied at an avaliabe state it's only a 50/50 chance. Furthoremore, since it's a metal with a "sea" od electrons I don't see how it could have a fermi level.