Since the subject has turned from voltage polarity to current flow, let me clarify..
Electron current flow is from the negative toward the positive charge.
Conventional current flow is also known as "hole flow", positive to negative.
Conventional flow can be illustrated with a bunch of golf balls in a vertical tube.. fill the tube with golf balls, then take one out. Let the golf balls be considered electrons. With one missing at the top you have a "hole" (no golf ball). If you turn the tube over, where did the "hole" go? It went back up to the top of the tube. So, did the "electrons" flow down when you turned the tube over, or did the hole flow up?
The answer is, yes.
The hole is a "positive charge", but it's not a physical thing, rather, it's the lack of electrons relative to something else nearby that has an abundance of electrons. That "something else" with it's abundance of (negatively charged) electrons has a negative charge relative to the item nearby that is lacking them (which can be considered "positively charged" - lots of "holes" - if you are using the electron-rich material as your reference point).
That positively charged item has an abundance of "holes" just waiting for those electrons to come over and "fill 'em up". Or, conversely, like in our tube, the holes want to go and replace those electrons up at the top, and the only place for the electrons to go is down to where the holes used to be at the bottom before they traveled up.
Current will flow if there's a path for the electrons to get from the abundant source (the one with all the electrons) to the lacking source (the one with all the "holes") - our tube in the analogy I used. Or, conversely, if you wish, if there's a path for the holes to flow the other way in our tube.
All depends on which way you want to look at it, electrons flow, or holes flow. Whatever. In electrical terms (forget our tube and golf balls now, that worked on gravity), the current stops when there's an equal number of holes and electrons on both sides (the battery is dead, or discharged.) At that point, there's no voltage differential between the two sides, they're equal in holes and electrons.
Again, voltage polarity (negative or positive) depends on what point you want to call your reference. It's not the same as current flow (either electron or "conventional"), as you can have a voltage differential without current flow. In fact, you need a differential, and a travel path, before you can get current flow.
But back to the original question, using a digital voltmeter (analog voltmeters are constructed to show positive voltage only, usually, and will "bury" the meter indicator backwards if reversed, you don't want to damage your analog meter doing this*), put the digital meter on a charged 12V battery - negative lead to negative terminal, positive lead to positive terminal, and the meter will read +12 Volts. reverse the leads and the meter will read -12 Volts. Same battery, same voltage differential, just a different reference point (the negative meter lead is now on the positive terminal and vice versa).
(*I know someone will say there's a polarity switch on most analog meters that allows switching between negative and positive voltages - all that switch does is reverse the leads internally to compensate for you switching the leads externally. And yes, there are some analog meters that have the meter needle in the middle of the scale, "positive" voltage readings deflect the needle to the right, "negative" voltage readings deflect to the left, but those are getting rare and not likely available to someone trying the experiment.)
Oh, about alternating current (AC) that someone mentioned - back to our tube.. keep flipping the tube over and over.. from the perspecive of someone inside the tube watching, the current (holes or electrons) appears to keep switching direction, first going one way then going the other way, back and forth as the tube keeps turning over..
AC current is created using spinning motors, I'll let you ponder that coincidence..
Hope this helps..
Comments
Thanks. can you scroll down and see what you think about my negative voltage issue (with screen shot?
by Remmik on December 24th, 2011