How can voltage be positive or negative? I understand what voltage is, but I don't get the "sign" issue. When do we use a - or a + sign?
Voltage is the difference in electrical pottential between two points. It could be signed accondingly to the charge that is creating the difference. If you have a lot of negative charges, it will be a negative pottential; and positive charges will create positive pottential.
there is two terminal of a domestic supply
+ve is a live terminal and return path is -ve terminal because it comes from neutral
current flows through +ve terminal and completes the circuit through -ve terminal
The best way to explain it is by the 'Sine' wave voltage of 'Alternating Current'.
Taking 110V as the power supply, the Sine Wave shows that the voltage is alternating between 55 volts Positive (+) (the upper half of the wave) and 55 volts Negative (-), (the lower half), giving the total of 110V.
In the middle the voltage is Zero.
In an Electrolysis Cell, (DC. power), the 'Anode' carries a positive (+) voltage to attract Negative 'Anions' and the Cathode a negative (-) that attracts the Positive 'Cations'.
Whether something is positive or negative is to do with the flow of charge. Remember, something only has a positive or nagative charge compared to something else. Normally we compare it to the charge of the Earth (which is considered to be neutral).
If something has a positive charge this means that electrons (which are considered to be negative) will be attracted to it. If something has a negative charge electrons are repelled from it.
Why electrons flow from one body to another is due to their relative electron to proton ratios. Let's say we have two bodies, Body A has more protons than electrons, for Body B the reverse is true. Thus if we connect them with a conductor (such as copper wire) electrons will flow from Body B to Body A until their charges are equal. Body B is negative in this case and Body A is positive.
So, from this we can say that you call something negavite when the electrons are moving away from it, and positive when electrons move towards it.
These principles are used in some very clever ways to create batteries and other such items.
I would like to say that Bramble's answer is also very good. The concpt of voltage as an electrical 'pressure' is a nice one.
Voltage is actually a very difficult concept. It is basically charges and not voltages which are either positive or negative. If there are excess positive charges in a body then the body will be at a positive potential (or voltage) and vice versa if there are excess negative charges. Voltage might therefore be "understood" as an electrical "pressure" between two points that can cause a current to flow between these points. Such a point out of which a voltage drives (or could drive) a positive current is the positive point (often referred to as terminal) and the point into which this positive current flows is the negative point - positive current entering being equivalent to negative current leaving. Current therefore flows from positive to negative. Basically the need to distinguish positive and negative comes from the fact that electricity deals with both positive and negative charges and voltages are "pressures" which can move these charges around.
Don't be confused by answerers referring to ac supplies. There the potential is changing around 100 times a second and the terminals are "live" and "neutral" which has nothing directly to do with positive and negative.
Note also that the presence of positive or negative charges is very real and a neutral level between them is not arbitrary but corresponds to a real balance in their numbers. The earth is close to neutral but if you can charge yourself up with either positive or negative charges your hair will stand on end. It would do this all the time if the earth had an excess of either type of charge.
+ sign meant for ions
- sign meant for electrons.
Now,why to make this different signs.
because in a circuit for an example 110 V D.C lamp,it will glow only when current passes through its filament.How the current will flow?unless you make the difference in potential (voltage) it will not flow and your lamp will not glow.
So like the water flow from an elevated place to a lower level.For a current to flow it needs one higher potential and Lower potential.Thats why its marked as + and - signs
Voltage is a relative quantity. You have to measure it relative to something, some arbitrary zero point. We usually make this arbitrary zero point ground.
potential energy = voltage * charge
So a positive voltage means that a positive charge will have positive potential energy--more energy at that point than it would at ground. So it will flow from the positive voltage to ground. Forces push particles from high potential energy to low. Just like gravity pushes things down from high potential energy to low. A negative charge will have less energy at that point than it would at ground. So it will flow from ground to the positive voltage.
For negative voltage, switch that up. A positve charge has less energy there than at ground, so it flows from ground to negative voltage. A negative charge has more energy there than at ground, so it flows from negative voltage to ground.