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Say I put some wires in a container filled with saltwater, but the wires do not touch, and I connect these wires to a battery:

schematic

simulate this circuit – Schematic created using CircuitLab

I know that the saltwater will complete the circuit, and some current will flow. But how can this happen? Do the electrons fly off one wire and jump through the saltwater to the other wire? Aren't the electrons stuck in the wire? Why would they be able to do this when there is saltwater between the wires, but not when there is air between the wires? If the electrons can't leave one wire, travel through the saltwater, and get to the other wire (can they?), then how is there a complete circuit?

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  • \$\begingroup\$ See, for example: en.wikipedia.org/wiki/Electrolysis#Process_of_electrolysis \$\endgroup\$ – Alfred Centauri Jul 8 '13 at 14:51
  • \$\begingroup\$ This probably would receive more complete treatment at chemistry.stackexchange.com \$\endgroup\$ – Chris Stratton Jul 8 '13 at 15:05
  • \$\begingroup\$ @ChrisStratton yeah, they have some good questions on the topic there, but I'm looking for something more framed in terms relevant to EE. I think the value here isn't really in current in saltwater, but rather the insight it provides into what current is, because many of the wrong mental models that people have can't explain how this works. \$\endgroup\$ – Phil Frost Jul 8 '13 at 15:15
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The salt (NaCL) water solution is an electrolyte solution which is, essentially, a conductive solution.

The conductivity of the salt water is due to the presence of both positively and negatively charged ions. These ions in solution are free to accelerate in the presence of an electric field and thus, like the free electrons in a metal conductor, are able to participate in an electric current (not to be confused with an electron current).

When there is an electric current through the salt water, there are actually two contributions: (1) the positive sodium ions drifting in the direction of the electric current and (2) the negative chlorine ions drifting the opposite direction.

While it may seem that the oppositely directed ion currents should cancel they, in fact, add. The flow of negative ions contributes to an electric current in the opposite direction due to the negative sign of their charge.

At the interface between the metal conductor and salt water, there are reactions that either remove electrons or add electrons to the conductors thus completing the path for charge to flow around the circuit.

From the online GenChem Textbook section on Electrolysis:

enter image description here

Figure 1 An electrolytic cell. The battery pumps electrons away from the anode (making it positive) and into the cathode (making it negative). The positive anode attracts anions toward it, while the negative cathode attracts cations toward it. Electrical current is carried by electrons in the wire and electrodes, but it is carried by anions and cations moving in opposite directions in the cell itself. Since the anode can accept electrons, oxidation occurs at that electrode. The cathode is an electron donor and can cause reduction to occur.

It is important to note that electric current is simply defined as the flow of electric charge and this definition does not depend on the species of charge carrier.

There is, in fact, just one electric current in the circuit while there are three (or more) different species of charge carriers along the circuit's path: (1) electrons in the metal conductors, (2) positive sodium ions in the salt water, (3) negative chlorine ions in the salt water, and (4+) if the voltage source is a battery, ions in the battery's electrolyte.

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  • \$\begingroup\$ This is very well written. Are you sure that the ions contribute/accept electrons at the electrodes? I'm not sure you are wrong (and really it's more of a chemistry question), but from what I've gleaned from people who understand it better than me, this may or may not happen, depending on the voltages, temperatures, and particular chemicals involved. I think it's possible that the ions just pile up at the electrodes (under some conditions) also, but there are just so many ions in solution relative to the current that the effect isn't noticeable on a reasonable timescale. \$\endgroup\$ – Phil Frost Jul 8 '13 at 15:55
  • \$\begingroup\$ Explanation of charge build-up, which is largely over my head: physics.stackexchange.com/a/21834/24140 \$\endgroup\$ – Phil Frost Jul 8 '13 at 15:59
  • \$\begingroup\$ @PhilFrost, I've added a quote from Wiki (not the most reliable source I know) to address your comment. \$\endgroup\$ – Alfred Centauri Jul 8 '13 at 15:59
  • \$\begingroup\$ @PhilFrost, updated answer with different source and image. \$\endgroup\$ – Alfred Centauri Jul 8 '13 at 16:40
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    \$\begingroup\$ charge build-up @PhilFrost, if there is any charge build-up, then the plates act as insulators, and the current halts within microseconds. Also, beware of a common grade-school misconception where batteries are described as capacitors, as if all the charges are stored in the plates, and where no charges pass through the electrolyte. Wrong. Many, many K6 books explain batteries this way, even giving diagrams with no electrolyte current. All are wrong. Batteries act as chemically-fueled charge-pumps. They cannot store up coulombs in each plate (that would require megavolts! Teravolts!!!) \$\endgroup\$ – wbeaty Feb 3 '17 at 7:49
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Most materials do not change when a current flows through the material. The material somehow allows charge (in most cases electrons) to pass through the material without affecting the material. Metals are very good at this because some of the electrons in a metal are very loosely bound to the atoms.

Pure water consist mostly of H20 molecules. They offer very little option for electrons to pass through the fluid.

Salt water consists (mostly) of H20 molecules, Na+ ions and Cl- ions. When two electrodes are put in such a fluid with some potential difference the Na+ ions will be attracted by the negative electrode and will move (somewhat more) to that electrode. Likewise for the Cl- ions and the positive electrode.

When the voltage (potential difference) is large enough the Cl- ions at the positive electrode will give an electron to the electrode, and recombine to Cl2 molecules (or react with the electrode). You will be able to see and/or smell this. The situation at the negative side is more complex, but at that side the net result will be that the Na+ will help the H20 to get converted to H2 and OH-. You will see bubbles of H2 gas here.

The net effect is that NaCl and water become Cl2 gas and H2 gas. This is a chemical reaction. It is quite different from normal electron conduction:

  • it will last only while the is NaCl (or rather: Cl- ions) left
  • it requires some minimum voltage

Without the NaCl a comparable process can take place, using the small amount of H2O molecules that are dissolved into H+ and OH- ions. Because this amount is very very small pure water will conduct much much less than salt water.

In some sense these electrochemical conduction is more like charging a battery than pushing current through a wire. In a wire (or other resistive material) all energy from the electrical current*voltage is converted to heat. With electrochemical conduction some energy goes into the endothermic (= energy consuming) chemical reaction. With a lot of extra work (in this case, oa. capturing the gasses and keeping the close to the electrodes) what you get is a charged battery: it can supply current when you connect its two leads.

So a very brief answer to your question could be: because salt water behaves like a (badly engineered) accu.

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Just to add, since none of the other answers have pointed it out, but electricity CAN jump through air. Or even rubber or other insulators, with the right voltage and current. Ever see a spark? A air gap between two wires can be bridged, and it happens accidentally (spark when you plug something into an outlet, when you connect jumper cables to a car, static electricity) or intentionally (car spark plugs, electric lighters, Jacob's Ladders). At a certain voltage/current, normally insulating material like rubber and glass can conduct electricity, at a break down voltage. This is the Dielectric strength of the material. Most things can be made to conduct if you juice them enough. Salt Water is just relatively easier to do.

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While it is true that there is some actual physical movement of electrons from atom to atom or molecule to molecule, the bulk of the energy is actually electrons moving to higher states of charge within an atom and in essence the material which the atoms reside within. This is electron vibration.

This energy is transferred to nearby electrons producing the "movement" of current...the electrons in the material move to higher energy states and occupy those shells with energy transferred to anything willing to accept it...a conductor. Salt water, due to its makeup, is an excellent conductor. The speed of physical electron movement is the same in each material with regard to voltage, current and frequency...which normally is only inches per hour.

The observed movement of electrons cannot be used to determine the level of charge contained in the material.

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Its all about the bulk resistivity of materials. We make wires out of low-resistivity materials because the job of a wire is to pass current and otherwise get out of the way. Saltwater happens to have a higher resistivity than copper and other "conductors", but still much less than "insulators" like glass and most plastics.

Your question is no different than asking how the electrons get from one end of a resistor to the other. Yes, the electrons do travel thru the resistive material. However, they aren't as "loose" in the resistive material as they are in the more conductive electrodes. But the principle is the same. After all, these electrodes have some resistivity too, as long as we're not talking about supercondutors. So it's just a matter or degree. If you can believe that electrons flow in copper, which has some finite resistivity, then you should be able to believe they can flow in something with twice that resistivity. By extension, you should be able to believe they can flow in something twice that resistivity. Iterate indifinitely.

Eventually you get to such high resistivities that it the flow is very small, so small that we call these material "insulators". However, that is just a matter of relative magnitude. Electrons do flow a little in insulators, but the many orders of magnitude between the resistivity of copper and glass allows us to make simplifying assumptions and call one a "conductor" and the other a "insulator".

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  • \$\begingroup\$ Yes, but where are the electrons in saltwater, and how do they get from the wire to the water, and back to the wire? Electricity is electrons, right? ;) \$\endgroup\$ – Phil Frost Jul 8 '13 at 14:27
  • \$\begingroup\$ @Phil: The same way the would get on and off a block of steel (higher resistivity than copper) held between two copper wires. The individual molecules of the material are close enough so that electrons can jump between them. Depending on the particular molecules and how they are arranged electrons are more free to jump (lower resistivity) in some materials than others. They can jump between the copper atoms of the wire and the some other material the same way. \$\endgroup\$ – Olin Lathrop Jul 8 '13 at 14:33
  • \$\begingroup\$ @PhilFrost - In copper, as one electron leaves the copper atom, another arrives from the other direction to replace it. Now I think that with brine, since one electron from each sodium atom is 'borrowed' by a chlorine atom you get sodium (Na+) ions and chloride (Cl-)ions which are attracted together. Under an electric field, the sodium ions drift towards the negative electrode where they gain an electron, then react with the water to form sodium hydroxide and hydrogen gas. The chloride ions drift the other way where they give-up an electron and become molecular chlorine gas. \$\endgroup\$ – MikeJ-UK Jul 8 '13 at 14:54
  • \$\begingroup\$ Whoever downvoted this, it would be useful to know what exactly you think is wrong, misleading, etc. I have re-read what I wrote, and still think it is correct at the level of detail presented. There is no way to know what to fix or rebut without knowing your objection. \$\endgroup\$ – Olin Lathrop Jul 8 '13 at 16:08
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    \$\begingroup\$ @Olin, I don't believe there is an "electron gas" present in saltwater the way there is in steel or copper. \$\endgroup\$ – The Photon Jul 8 '13 at 16:45

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