This seems to be very basic but I am confused about it. I know ground is a point whose potential is zero. Now let's take a piece of wire , and mark a point A on that wire. I said , A has potential of -15 with respect to ground, what does that exactly mean? How can a voltage be lesser than zero? Any explanation in terms of electron? For example we define positive voltage as the force at which electrons ar being pushed in a particular point. Can we have an intuitive Idea about negative voltage using this force or electron concept?
You could consider the voltage a bit like floors on a building. A numbering system used in many places in Europe defines that the ground floor is 0 or G, that floors above it are numbered positively and numbers below it are negatively. You now have the option of measuring everything relative to ground (the floor number) or measuring the difference in level between any two floors (the potential or voltage difference).
In the left image above our man is standing on Floor 2 relative to ground. The electrical analogy is that some point on the circuit is connected to ground / earth and by convention is zero volts and all voltages (heights) are measured relative to this.
An 'all above ground' building will have no negative floors. A bunker or underground car-park will have no positive floors.
If the building is launched off into space he has no ground reference and is free to number the floors any way he wishes, including have Floor 0 at any arbitrary point. This is analogous to having an electrically isolated circuit with no ground connection in that we can call any point 'ground'.
Figure 2. Two 1.5 V cells with three different reference points.
Hopefully Figure 2 makes it a bit clearer. Depending which point we assign as reference (GND) the other points' relative voltage changes.
Please consider this diagram:
You ALWAYS measure a voltage with respect to a reference point. Often that reference point is GND or GROUND. So if you measure A with respect to GND you will get 12V.
If, however, you measure B with respect to GND you will get -12V.
Voltage is like height. The amount of energy it takes to roll a ball from A to B depends on the difference in height between points A and B. Similarly, the amount of energy it takes to move an electron from A to B depends on the difference in voltage between points A and B.
Just like height, there's no real zero. You can always dig a deeper hole, or build a higher tower, and there's no reason to think that the ground level at my house is a better zero than the ground level at your house, but I'm going to measure from my ground, because that's where I am.
The height analogy works very well until moving magnets come into the picture. They can make a round trip go uphill both ways.
The electron approach is exactly correct.
Electrons are drawn towards positive voltage and away from negative voltage. Just to make it confusing, positive voltage itself is often a deficit of electrons, and negative voltage implies a surplus of electrons (roughly speaking).
Thus, if allowed (e.g., via a wire), electrons will flow away from negative voltage and towards positive voltage - where it is "less crowded".
Note also that this makes everything your are taught in modern circuit designs is completely backwards. Such diagrams are drawn top to bottom - with bottom being ground - so it seems like current flows "down" the circuit. In reality, electrons flow from ground to +VCC (which is backwards). This is how old tube circuits were drawn, as the electrons "boil off" the cathode and fly towards the anode.
The number of electrons answer is a good answer from an electrons point of view. Pranav Kumar also has an excellent answer. Here's mine.
If you've studied algebra, this explanation will make more sense to you. I'll try to keep it simple so that you might understand this even if you haven't studied algebra. If you are familiar with the metric system temperature units, then you are also aware that the temperature can drop below 0 degrees Celsius. -15V is similar to a temperature drop of 15 degrees below 0 degrees Celsius. The temperature's 0 degrees Celsius and GND (0V) in electricity are reference points by which to compare other values (temperatures or voltages). This means that the temperature (or voltage) can rise above 0 degrees Celsius (or +V), or it can drop below the 0 degree Celsius (or -V) reference points (0 degrees Celsius, GND or 0V). The positive (+) and negative (-) symbols indicate the polarity of the voltage. These symbols also tell you to which side of the reference point (0) the values belong to; the greater than or less than side of 0.
When comparing two integers one number is always going to be greater than the other. Right? So, if I say, +4 degrees > +2 degrees, you'll agree that +4 degrees is greater than +2 degrees. If I say, +4V > +2V, then you'll agree that +4V is greater than +2V. I hope that by now you grasp the similarity between temperatures and voltages. Just as the temperature can drop below 0 degrees Celsius, voltages can also drop below 0V (or the GND reference point). The 0 degrees Celsius, or GND (0V) can be thought of as a reference by which to compare warmer (+ degrees Celsius) and colder (- degrees Celsius) temperatures, or positive voltage (+V) and negative voltage (-V). If I say, +2 degrees Celsius > -4 degrees Celsius, then I hope you are able to comprehend that +2 is greater than -4. And finally, if I say that 7V > -7V, that you agree that +7V is greater than -7V.
You can find several excellent explanations of positive and negative voltages in alternating current examples all over the internet. I hope this was helpful.
The potential is not an absolute quantity.It depends on the reference chosen, for example ,here you chose the reference as ground and assigned it 0 potential. Also, your definition of voltage(potential difference) is wrong.The work done on a positive unit charge when moving it from point A to point B is the potential difference V_BA.
Having said that you should now notice that assigning potentials to different positions is basically done by assigning the potential difference with respect to the reference chosen(here it is 0 for ground).What it means for the potential to be negative is that the potential difference between that point and the reference is negative.
I think a good thing to add to all the previous answers is the concept of a voltage difference.
Generally, if you have 5V, 0V and -5V on one installation, another way to show the exact same thing is to shift the zero points, in other words 10V, 5V and 0V. The whole idea is that often we are more concerned with the "voltage difference". Of course in a real circuit we often are limited by what point we consider zero due to the fact that the idea that zero = no voltage (ground) is ideal. But if you designed your circuit differently you can use any point as a ground, given that you changed the rest of the circuit accordingly.
But when talking about -5V, you can simply also think of it as 5V, flowing in the other direction. As the voltage difference's pure value between both 5V and -5V to ground is both equal, just running in the opposite direction. A good way to think of it is velocity, where negative velocity often means the same as the positive one, except that its running in the opposite direction.
So when you have both 5V and -5V in a circuit, you can take it simply as 5V- (-5V), which means there are 10V between them, regardless of the sign. And the sign just simply shows you which direction the current will flow, as current will flow from higher potential to lower potential.
Voltage really describes the potential energy or stored energy of the electric field present.
You can make an analogy with the gravitational field, although there is no negative charge persay, or repulsion, The nature of the description of this classical field is very similar to the electric static field.
So hopefully you can see the gravitational energy, is defined relative to some point. Now merely switching the points will introduce a negative sign. So you see negative potentials can arise from this nature.
But as well now, with electricity, we have positive and negative charge. So their are two ways the negative sign can arise. From positive and negative charges and with respect to the way coordinates are measured.
**Key point ** So, there are two ways you get a negative sign with voltage, positive and negative charges, and reversing your reference points.
Hope don't mind the lazy physics type answer.
At the intuitive level, Voltage is pressure, and the idea of positive and negative pressure has been around for a few thousand years; you can suck water into a straw, or blow water out of a straw. We use terms like "positive" and "negative" to describe the direction of the applied pressure, but there is nothing magical about either one. That most electronics today run on a "positive voltage" is part tradition, part convenience, and part technological performance. There are no absolutes in how we describe and measure the pressure. In most consumer electronics today there is one power source potential. 12 V, 9 V, 6 V, 5 V, and 3.3 - 3.6 V are very common, and for all of these (whether from batteries or a wall wart), the voltage is positive with respect to (((the circuit's))) ground.
But it was not always so. The giant consumer item of the early-to-mid 60's was the transistor radio. The typical model has six transistors and ran on a single 9 V battery. BBBBBUT - the battery's anode (positive terminal) was the circuit's ground potential, and the circuit "ran off of" the -9 V.