summary: How does electroplating move material from the positively charged depositing material to the negatively charged base?
So, I’m reading up on electroplating on wikipedia, which involved refreshing my memory on cathodes and anodes and came across the following phrase
This contradicts my understanding of electricity flow which is that electrons move through a circuit, so you’re always going from negative towards positive (since electrons are negatively charge, and they’re what move through a circuit). Now, I can wrap my head around a positive charges as a hole waiting to be filled with electrons. I can even understand electrons flowing between two objects of similar polarity but different levels (i.e. a negative charge effectively acting as a positive charge). However, this passage makes no sense to me. Is this some weird atomic seesaw going on where the arrival of an electron knocks an atom back along the electrons course?
The sentence you’ve picked out isn’t a deep mystery, it merely reflects the fact that, back when Ben Franklin first described electricity as being something which flows, he had no idea that electrons existed, let alone that they were the moving particles of interest to him and had a negative charge.
Therefore, he arbitrarily assigned positive charge to the “fluid” which he imagined flowing from place to place and causing sparks, and so all his arrows pointed the wrong way. However, if you reverse both charge and direction as regards electrical currents, the math comes out right, so this isn’t a big deal and has really never been fixed unless you actually are talking about electricity at the quantum scale. (This property is called Charge-Parity or CP symmetry, and is violated on some occasions.)
By some odd coincidence, I was just reading this blog post which explains it.
So there’s no deep mystery in the sentence you’ve quoted. It’s just Franklin having a 50:50 chance of being wrong and, well, picking the wrong end of the stick.
It’s just a convention that current flows from positive to negative. Before the discovery of the electron, they* needed to designate some direction and arbitrarily decided one terminal to be positive, the other terminal to be negative, and that the current flows from positive to negative. The math all works out regardless of the fact that electrons are negatively charged and current actually flows in the opposite direction.
*I’m not sure exactly who “they” are. Ohm? Ampere? Volta?
EDIT: Ninja’d! And it was none other than Ben Franklin that got the signs wrong.
In a normal wire, the positively charged nuclei stay fixed in place while the electrons move. We can sorta pretend that instead positively-charged holes are moving in the opposite direction, but it’s basically a ruse.
However, with electroplating, it is the nuclei themselves that move. That’s the whole point: to get atoms on one electrode to move to the other electrode. The electrons get moved through the wire hooked up to the electrodes, but a real positive current of ions moves through the electrolyte, and that’s how the material transfer happens.
So Franklin was only “wrong” in the case of electrons moving through a wire. In the case of electroplating, the charged particles move the way you expect.
Only the positively charged particles. Now of course those are the ones you care the most about, since they’re doing the plating, but there will always be negative particles moving the other way.
Some will, but the whole reason it works is that the electrons are being pumped through the wire instead of the electrolyte. One way of looking at it is that a metal atom is ionized at the anode. The electrons travel to the cathode through the wire (via the battery), while the ions travel through the electrolyte. They rejoin at the cathode and you end up with ordinary metal again.
I was wrong to write always, since in the situation you describe, with an anode of the metal you’re plating with, only the metal ions move. But you also have plating processes where both kations and anions in the bath are consumed, and then they’re both in motion.
There are a few experiments that can tell the difference between positive particles moving one way and negative particles moving the other, but only a few. For the vast majority of purposes, it doesn’t matter which you have, and so there’s no reason to change the convention Franklin adopted.
