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Name: Nathan U.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 8/4/2004


Question:
I am reviewing some materials for the electricity unit in my physics classes. I have come across similar passages in different texts as follows:

"A conductor in both the solid state and liquid state can carry a current to complete an electrical circuit. However, a solid conductor involves the movement of electrons through the atomic lattice, whereas a liquid conductor transports ionic charges through the solution." (Northwestern University Materials World Modules, 2002).

I am puzzled by this generalization of liquids. How does liquid mercury or sodium (nuclear reactor design) conduct electricity? I am not aware of the requirement of ions.


Replies:
It is too bad; those passages seem to have over-generalized, quite detrimentally to the reader's understanding.

Every metal, whether liquid or solid, has easily-moving electrons in an atomic lattice. Virtually everything that is silver-gray colored and electrically conductive is in this category. So you were correct to notice that mercury or molten sodium are metals and electron-conductors, rather than ionic conductors. Metallic (Electron) conductors range in color from bright silvery to dark silvery gray to black (such as graphite). Only a few have non-neutral colors (examples copper, gold, titanium-nitride).

Salt-water is the prototypical example of an iconically conducting liquid. There are ionic conductors that are solid. Examples are Zirconium oxides, many Lithium-containing mixed oxides, and proton-conducting oxides. They get used in fuel-cells and some batteries and in every car's smog-sensor. They tend to be roughly white and ceramic-brittle, not silvery and not malleable. Admittedly they do not conduct electricity very well compared with metals or the best ionic liquids. It takes many square meters or high temperatures to get an ampere of useful current through them. But iconically conductive solids and liquids share this correlation: ionic conductors are not metal-colored. They tend to be clear or white or off-white, usually not including black. They might easily have non-neutral colors such as red, orange, yellow, green, or blue tint.

True, when one imagines electrically conductive liquids, water-related ionic liquids are the most commonly experienced. It is equally predominant in common experience that conductive solids are metals.

However, some word of qualification: "typically" or "usually" or "commonly", is missing from the passage you quoted. This makes it sound like there is a scientific understanding which requires solids to conduct electronically and liquids iconically. This is the core misrepresentation in the passage.

It might be more scientifically illuminating to the reader, to start from "electronic" and "ionic" conduction, and work towards solid and liquid. A conductor must have mobile carriers of charge. Possible charged particles include electrons, protons, and heavier positive and negative ions. If the mobile charges are tiny, slippery electrons, the remaining bulk of the mass is likely to be able to sustain the structure of a solid.

But such a lattice can easily melt, and if it does the free electron conductivity will not be greatly impaired. If the charges are large ions, the bulk will likely need to be a liquid to let it move a useful amount.

However some ions are small and some solids have large passages, allowing some solids to conduct iconically.

At higher temperatures, say 1500C, there are many melted metals and melted salts, and there will be about as many metallic conductive liquids as ionic conductive liquids. Above 2500C, there are few insulating solids. The few solids remaining are all either electronically conducting or iconically conducting. Again, similar in number.

At cryogenic temperatures, atom-sized particles with strong charges or polar groups will always stick together. So there will be no ionic conductors, liquid or solid. Only electronic conductivity can work there. And insulators.



Jim Swenson



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