Understanding Charge and Electric Current in a Circuit

[Peter G.]

Some websites say electrons in a wire are charge carriers.

I need to know: Charge, electric current, are the electrons from the battery chemical reaction or also the delocalized electrons in the wire, all attracted to the positive terminal of the battery?

Thanks

 

[collinsmark]

Some websites say electrons in a wire are charge carriers.

Yes, that's right. But keep in mind that a conducting wire in a simple circuit doesn't end up with any net charge on it. The wire has just as many positive protons in it as it does negative electrons. But the protons, within the atoms making up with wire, stay put. The electrons move from one atom to the next. Thus the electrons in the wire are the charge "carriers."

I need to know: Charge, electric current, are the electrons from the battery chemical reaction or also the delocalized electrons in the wire, all attracted to the positive terminal of the battery?

Similarly, batteries don't have any net charge either. There are as many positive ions (cations) as there are negative ions (anions). The difference is that inside a battery, both anions and cations move from one electrode to the other. So there is more than one type of charge carrier inside the battery.

Let me give you a few definitions that might help you out:

Cathode: positive terminal of the battery.
Anode: negative terminal of the battery.

Cation: positively charged ion.
Anion: negatively charged ion.

A mnemonic for cations is to view the ‘t’ as a plus sign: ca+ion. A mnemonic for anions is similar to an acronym: A Negative ION = ANION.

Now here is something that you should commit to memory. It it a convention developed historically, before scientists understood electricity as well (it doesn't necessarily make a lot of sense on its own, so you should just memorize it):

The cathode electrode is called a "cathode" because it attracts cations inside [a normally operating] battery.
The anode electrode is called an "anode" because it attracts anions inside [a normally operating] battery.

And the above is important. The cathode is not called a cathode because it is the same polarity as a cation. Rather, a cathode is called a cathode because it attracts cations (inside the battery).

[Edits in square brackets.]

I'll let you put together how everything works out from here.

 

[Peter G.]

First and foremost: Thanks a lot for the detailed explanation! I have a couple of doubts:

Isnt the anode the positive electrode?

But most importantly: The electrons are the charge. What I understand from charge then is the anions in the battery and the delocalozed electrons. Correct me if I am wrong. So my doubt ia what are the carriers if they are no different than the anions. And what do the carriers do?

Thanks once more!

 

[collinsmark]

Isnt the anode the positive electrode?

For most things, yes. For example, with a forward biased diode or LED, the positive terminal is the anode and the negative terminal is the cathode.

That's because (positive convention) current flows into the anode and current flows out of the cathode. In that respect, are the anode and cathode the same as a battery. Current flows out of the cathode and into the anode (electron flow, being negative is opposite that).

But batteries are different in terms of their +/- polarities. The +/- signs represent electrical potential. The potential is larger at the positive terminal than it is at the negative terminal (meaning for a given positive charge q, the potential energy is larger at the positive terminal than the negative). And that's also true for both batteries in normal operation, and forward biased diodes.

The original convention of the cathode attracting cations and the anode attracting anions in a battery in normal use, goes back a long way, and is really just a convention.

But most importantly: The electrons are the charge. What I understand from charge then is the anions in the battery and the delocalozed electrons. Correct me if I am wrong. So my doubt ia what are the carriers if they are no different than the anions. And what do the carriers do?

Inside the battery (generally speaking), both cations and anions can be charge carriers. In a battery operating normally, the cations move positive charge from the anode to the cathode; and anions move negative charge from the cathode to the anode. Depending on the technology of the battery, one type of charge carrier might be dominant. But both types can be present, generally speaking. It all depends upon the compositions of the electrodes and the electrolyte solution.

Inside the battery (ignoring the electrodes themselves), charge is carried in the form of ions (Na⁺ and Cl^- are examples of ions). There's not really any free electrons to speak of.

 

[Peter G.]

Sorry, I think I did not word my question correctly. I understand your explanation regarding conventional current and the real electron flow in a circuit. And how there are both positive and negative charge (anions and cations, the anions, being negative move the negative charge from the negative terminal (cathode) to the positive terminal (anode).

I just needed a very basic explanation of the function of the battery. I know by means of chemical reactions (involving electron flow as a more reactive metal gives out electrons, losing negative charge, thus, becoming positive and a less reactive metal takes those electrons, negative charge, becoming the negative terminal) I wanted to know if this "terminal set up" sets up an electrical field that is responsible for attracting the delocalized electrons in the wire, the charge. I also need to know that they give the electrons electrical potential energy as a result of their stored chemical energy transformation.

But my main doubt lies in the wire:

I wanted to know what is charge in the wire? I know one coulomb of charge is 6.25 x 10 ^18 electrons. My doubt was, are these electrons, that make up the one coulomb of charge the delocalized electrons, the charge carries (which I guess are charge, and are called carriers because they move) or the electrons moving from the positive side of the battery (e.g.: Zinc giving out two electrons forming a Zn 2+ cation)

I really don't want to take up a lot of your time and I hope my confusion/"ignorance" does not annoy you. :uhh:

Thanks once again

 

[collinsmark]

I just needed a very basic explanation of the function of the battery. I know by means of chemical reactions (involving electron flow as a more reactive metal gives out electrons, losing negative charge, thus, becoming positive and a less reactive metal takes those electrons, negative charge, becoming the negative terminal) I wanted to know if this "terminal set up" sets up an electrical field that is responsible for attracting the delocalized electrons in the wire, the charge.

Essentially, yes! (but be careful.)

The cathode (positive terminal) of the cell (a.k.a. "battery") develops a very small positive charge on it, attracting electrons from the wire. Likewise, the anode develops a small negative charge repelling electrons away from it, into the wire.

I've attached the diagram below to make sure we're both on the same page with what we are talking about.

Part of your question about the cell, but let's discuss the other parts of the circuit first (get the low-hanging fruit).

Notice I added a resistor to the circuit. This is important. If we were to model the cell as an ideal voltage source and connect the terminals together directly with an ideal wire, we'd end up with infinite current. In a real-world situation, the current wouldn't be infinite, but it would still be very large and possible cause your circuit to melt or start fire. The reason it wouldn't be infinite is because the cell and wires themselves actually have a small amount of resistance associated with them that would limit the current. But we're not trying to model those things here, so I added an external resistor to keep things simple.

//// Wire AB

• The wire is a conductor. The electric field inside a conductor is zero. However, electrons are able to move through the conductor (and in a typical circuit like this the generally move very slowly on average -- maybe a centimeter or less per second). But for every electron leaving to the left into the cell, there is another electron entering from the right from the resistor.
• The total charge on the wire is essentially zero. Even though the electrons are moving, for every electron in the wire there is also a proton in the wire. So there is no net charge on the wire. (There might be a very, very small net positive charge on wire AB. This is because wires AB and CD act like a very, very weak capacitor. But for our purposes we can ignore this very small net charge.)

//// Resistor

• As the electrons move from D to B, they lose potential energy. Pretend you and your friends are electrons, and you all fall down a very steep hill. Also imagine that there are many, many other people in front of you that have just fallen down the hill, and many others behind you about to fall down the hill. Gravity is the force that accelerates you down the hill, but don't accelerate down the hill uniformly. Instead you bounce off rocks and boulders, not to mention smashing into your friends repeatedly. Oh, the humanity! The end result is that your initial potential energy gets converted into thermal energy from all the crashing and bumping.
• There is an electric field in the resistor, and thus a potential (voltage) across the resistor. This is equal to the electric potential of the cell.

//// Wire CD

• See wire AB above, for the same concept, just reverse the direction. The only real difference is that electrons on wire CD will have a higher potential energy than those on wire AB. It's analogous to wire CD being on a level plateau near the top of steep hill (the resistor) and wire AB being a level plateau at the bottom of the steep hill.

//// Cell (battery)

• This is where things get a little complicated.
• Ions exist in the electrolyte solution from the beginning. The reason the ions don't combine and neutralize with themselves is a matter of chemistry.
• The ions in the electrolyte solution chemically react with the anode and cathode. This causes new compounds to form at one or both of the electrodes.
• These new compounds can often "plate" the electrode causing its exterior to be something different than it was previously.
• Depending on the cell composition, the surface of an electrode might even dissolve in the process, becoming part of the solution in ion form.
• Cations move to the cathode and anions to the anode mainly through the process of diffusion. As soon a few ions of a given type react with the corresponding electrode, they are no longer ions. Other ions move into take their place via natural diffusion.
• Reactions at the catode leave a leftover positive charge. Reactions at the anode leave a leftover negative charge. Since the electrodes are conductors, electrons are able to leave the anode (removing the net negative charge on the anode); or in the case of the cathode, be neutralized by an entering electron.

Now look what happens when we make the resistor very large -- or even better, let's just remove it from the circuit.

In the above circuit, a few chemical reactions take place leaving behind the leftover charges on the electrodes. This does cause an electric field. This field inhibits the cations from reaching the cathode, and the anions from reaching the anode. The end result is that no more chemical reactions take place (well, almost no more), and the cell reaches a state of equilibrium.

I also need to know that they give the electrons electrical potential energy as a result of their stored chemical energy transformation.

The chemical reactions involved so far are chemically spontaneous. The results of the reactions produce energy, in the form of electrical potential energy of the electrons at the anode (and in wire CD).

Now you might be asking, "why don't the reactions spontaneously happen in reverse? Energy is conserved either way, so why does it only happen one way spontaneously?" The answer has to do with entropy and the second law of thermodynamics. Only the way described above causes the overall entropy to increase.

But my main doubt lies in the wire:

I wanted to know what is charge in the wire?

The net charge on the wire is essentially zero*. Yes, the wire has a bunch of conducting electrons within it. But also has essentially the same amount of stationary protons too.

*(There may be a tiny excess charge on the wire, but as I mentioned before we're going to ignore this.)

You'll need quantum mechanics (or at least solid state physics to really show this). Atoms in a conducting material contain a nucleus with a positively charged nucleus, which contains protons and neutrons. Most of the electrons in the material are bound to the atoms and cannot move around. But some of the electrons are shared between atoms. And can move from one atom to the next. This is known as the conduction band. A couple of points:

• Not all of the electrons in the conductor are in the conduction band. Most of them are bound to the atoms and are not free to move around.
• The electrons that are in the conduction band do not result in excess charge. The total number of electrons in the material (both bound electrons and electrons in the conduction band taken together) are essentially the same in number as the number of protons in the material. The net charge is essentially zero.

I know one coulomb of charge is 6.25 x 10 ^18 electrons. My doubt was, are these electrons, that make up the one coulomb of charge the delocalized electrons, the charge carries (which I guess are charge, and are called carriers because they move) or the electrons moving from the positive side of the battery (e.g.: Zinc giving out two electrons forming a Zn 2+ cation)

Both (if I'm understanding your question). For every electron leaving the anode of the battery, there is another one entering the cathode. The total charge of the battery remains constant (zero). The total charge of the wires/resistor remains constant (zero).

Let's just look at the cathode in isolation for a moment. Inside the battery, the cathode can gain positive charge by either dissolving, giving off part of its mass in the form of anions, or plating, increasing its mass by combining with cations. (Which one actually happens, or both, depends on the makeup of the battery). Whatever the case, the cathode can gain positive charge via chemical reactions. The important point is this: in steady state, the increase in positive charge due to chemical reaction is exactly counterbalanced by the increase in negative charge caused by electrons flowing in from wire AB. Thus in steady-state, its net charge remains constant.

Good luck!