Negative Voltage: Man, this is a hard thing to explain, without getting into engineering terminology, and explaining space charge regions, etc. A negative voltage is determined by how the circuit reacts to either hole flow, or electron flow. A hole is an absense of an electron, a "home" for an electron of sorts, and it has a positive potential. An electron has a negative potential. They are drawn to each other. If you could see electrons flow in one direction, you'll see the holes (where the electron just left) flowing in the opposite direction, with new electrons jumping into them. I'm forced to use a drawn out example to explain this. Take a diode, which is a normal component you'll find in a lot of circuits. A diode allows current to flow in one direction, but not in the other direction. A diode is made up of two doped materials fused to each other side by side. A P-type material that has many more holes than it has electrons, and a N-type that has many more electrons than it has holes. The holes have a positive potential, and the electrons have a negative potential. So the diode is built like this: ++++-------, or +++diode---- . All the pluses on one side, and the negatives on the other. (Theres no way to have ALL electrons or holes on one side, but close enough to explain) Now if you put the negative side of the battery to the +++++ side of the diode, and the positive side of the battery to the -------- side of the diode, what will happen? You will have this-> Bat--- +++diode--- +++Bat. Just like a magnet.... The pluses will be drawn over to the negative side of the battery, the negatives drawn to the plus side of the battery, and a huge gap in the middle. It will look like this-> ++--- empty middle ++-- . Nothing will flow, cause there's this huge gap in the middle. There is no bridge between the two regions in the diode. WHAT YOU HAVE JUST DONE, IS APPLIED A NEGATIVE VOLTAGE TO THE DIODE. Now if you turn the same battery around, then you have this-> Bat+++ +++diode---- ---Bat. Since like a magnet, the like charges will repel each other, and they will be forced to merge in the middle of the diode. This will cause a bridge of sorts in the middle of the diode, and current can now flow relatively freely. YOU HAVE JUST APPLIED A POSITIVE VOLTAGE TO THE DIODE. Now the last two of applying a positive or negative voltage could be switched. It all depends on how a person is looking at the whole circuit. They could say the last one was applying a negative voltage due to having to reverse bias a transistor. Or maybe the diode is a zenor diode. etc. etc... This could get technical real quick. So the bottom line is, any battery, or any power supply could provide either a positive or negative voltage, just depends on which way you hook it to the circuit. The circuit is what determines the difference. But if you apply the wrong voltage potential, and say that diode wasn't suppose to allow the flow of electrons? You have a problem. The dam just broke, flooded the city and things start smoking. I hope this was clear enough. It is a hard one to explain.

Electrons are by convention negative and protons positive charges. Unlike charges attract and like charges repel. When the net negative charge (electrons) becomes more than net positive charge (protons)that body is said to be negatively charged or said to pocess negative potential (note that charge on an electron is equal and opposite to a proton). Now considering the flow of electric current (current is defined as the rate of flow of charge), the current BY CONVENTION is said to flow from positive voltage to negative voltage, i.e when electrons flow from negative potential to positive potential (due to the force that results from the attraction of charges called static force), current is said to flow in the opposite direction BY CONVENTION. A voltage is a dynamic potential (between two ends or bodies) which when maintained contant in time there is a continuous flow of electrons from negative potential to positive potential resulting in a electric current (a constant rate of flow of charges) . So a negative voltage is basically a negative potential maintained constant in time w.r.t another reference potential (ground) that can result in an electric current from the refrence potential to the negative potential when connected by an electric conductor (or a wire). Here the reference potential is said to be positive w.r.t the negative potential.

Electricity is the moving of electrons from one area to another, thus making things work. Since Electrons are negative particles, they move from the negative side of the battery (or dry cell) to the positive. HOWEVER, we say it goes round the other way (negative to positive). A negative voltage means the electrons are going the other way to the way your voltmeter things they should go, turn you voltmeter (or vomiter as i say it) round and the voltage will become positive. P.S. A.C. (alternating current)'s electrons keep going back a forth, like a yo-yo, thus the voltage will be changing alot.

A bit more simplistic answer. All voltages are measured in a positive or negative direction depending on the 'common' point from which the voltage is referenced. Voltages are given a positive or negative designation from an arbitrary circuit point normally called 'ground' or 'common'. Using a battery as a voltage source - if the negative terminal of the battery is connected to 'common' then the voltages in the circuit will all measure positive from that point of reference. If the positive terminal of the battery is connected to 'common' then the voltages in that circuit will all measure negative with reference to the 'common' point. To an engineer setting this point of 'commanality' in a circuit sets a point of reference for all other voltage measurements and this simplifies the description of electron flow in a circuit.

Operational amplifier circuits often use two separate supplies A positive supply and an equal, but opposite, negative supply. The common connection to these two supplies acts as the common 0V rail and the input and output voltages are usually measured relative to this rail If you take two 1.5V DC cells and place them in series with each other the total voltage = 3V. If you go between the centre point and the positive rail it will read +1.5V, now move the positve connection of your multimeter to the negative side of the series pack and it will read -1.5V. In other words the voltage is negative with respect, or the current is flowing round the circuit in the opposite direction

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..

The negative portion of any charge is practically the beginnig or end of a complete circuit. So think of a circle, but its not connected at 2 ends, now picture a + sign on 1 of those ends, and a - sign on the other. The - sign is the NEGATIVE part of that charge, or a ground pretty much. Just like when a bird sits on a power line, it doesnt get electricuted because theres no ground(negative charge) to complete the circuit.