Q & A: How can a battery work?

I am trying to understand! Why does Volta's Pile work. For especially in UK metals get covered with a monolayer of water. So the voltaic couple cu/water monolayer/Zn generates volts equal and opposite to Zn/wet paper/Cu So no volts. This brings up the fascinating question "How THICK does an electrolyte have to be?" And what is an "ohmic contact" Do the Fermi levels of Zn and CU match the volts of the cu/zn cell? If we try to draw energy from the ZN/Cu cell we use copper wire. So Why and HOW does the volts of the electrolytic Zn/Cu couple overcome the back volts of the "dry" zinc electrode to copper wire junction?
- John (age 70)
Surrey, UK

That's a great subtle question. Here's a first try at answering.

You're absolutely right that if you just stacked layers of Zn and Cu, you couldn't get any steady current to flow. Basically, again as you suggest, there would be an initial slight shift of electrons from one metal to the other so that their electrochemical potentials match. The electrical potential difference would be just enough to make up for the difference in Fermi levels.

So what's the water doing? The water layers are thick enough to prevent electrons from flowing through them. Only ions can flow in them. So there's no requirement that the electrons' net electrochemical potential be level across a water gap, and of course it isn't. The key is the different electrochemical properties of the Zn⁺⁺ and Cu⁺⁺ ions, described on other battery questions on this site. So the solution properties of the ions,  not just the Fermi levels of the electrons, enters into the battery potential.

Now we get to the last major issue. If there's a monolayer of water between the nominally touching Zn and Cu layers, why doesn't that wreck the battery? A monolayer of water is just not enough to stop electron flow. The electron wave-functions don't abruptly fall to zero. They leak out even into a vacuum. This tail of the wave-functions can sustain a current of electrons. The process is dubbed "quantum tunneling", since the electrons get through regions where the potential is too high for them to cross classically. So you don't need very much water to stop electron flow and shut down the battery, but a monolayer is ok.  I haven't calculated precisely but I bet a layer of water only 100nm thick would be more than enough to block the current. The monlayer is less than 1 nm thick.

As for ohmic contacts, that just means contact for which, over the range of currents you're interested in, V=IR. In things like diodes, the range of currents for which that's true is very small. In a metal film resistor, it's larger.

Mike W.

(published on 05/16/2011)

What am amazingly good answer, Mike! So it seems all ohmic contacts are quantum tunneling! OK, I go for that - it explains why they conduct both ways! Now when they taught me PN junctions I asked "But what about the copper leads - how do THEY not form rectifier junctions too. (I was told to shut up "So we can finish the syllabus") OK the first rectifier I built was a lead plate in a aluminium cup of bicsrb of soda. Then I spent all my pocket money (65 years ago) on a copper oxide rectifier They were copper coated with red oxide CuO. To the copper you join the copper wire from the a c supply To the OXIDE you squeeze a lead washer (Soft to give contact?) and join the a c to that lump of lead So why does the lead washer make it rectify? It seems Cu/CuO/Cu could not!
- John (age 70)
Surrey, UK

I'm not saying that all ohmic contacts are via quantum tunneling. If the contact is tight enough, there may be no barrier there to tunnel through. The wave-functions can directly overlap without going through that tunneling stage.

On those old junctions:

Cu/CuO/Cu can't rectify by symmetry! Turn it around and you have the same thing. That's not true for Cu/CuO/Pb, so you'd generally expect it to rectify somewhat.  Somebody more serious about these things could tell you why it rectifies so well. Pb is good for another reason too. It's so soft that pressing it on shouldn't break the CuO layer.

Mike W.

(published on 05/17/11)