VOLTAGE OR ELECTRIC POTENTIAL DIFFERENCE: It is the work-done for moving a unit positive charge i.e, 1 coulomb from one point to another in an electric field.
This is a scalar quantity and measured in Joules/Coulomb.

Suppose a potential charge Q, placed in air at A point. Another positive charge at B point is infinity distance away from A point. Now force is practically zero. A work is required to bring Q positive charge from point A to point C, as same charges repeal each other.
If work done is 1 joule and unit positive charge is 1 coulomb, then potential of that point is 1 volt, name as per Alessandro Volta. V = Joule/coulomb.

So potential difference of 1 volt means 1 joule work is done to move 1 coulomb charge from one point to another. When a voltage is generated, it is some times called an electromotive force or E.M.F.

If work is done to move the charge from point B to A, the voltage of A is higher than B, it indicated as Vab with arrow. So Vba means -Vab.

ZERO POTENTIAL: In electric field infinity is the theoretical place of zero potential. But for practical electrical circuit the earth or ground potential is usually taken to be zero, because earth is a so large conductor that its potential remains practically constant and everything is referred to the earth.
E.M.F OR ELECTROMOTIVE FORCE: E.M.F is the voltage that generates from a battery or magnetic force, denoted as E and measured in volt. The device which convert chemical, mechanical and other form of energy into electrical energy is called emf source.

EQUIPOTENTIAL SURFACES: It is that surface where all points have the same electric potential or voltage that is, in an equipotential surface, the potential all over the surface is constant. That means potential difference between two points inside the surface is zero.
The electric field is always perpendicular to an equipotential surface.

POTENTIAL GRADIENT: In the direction of electric force, the rate of change of potential with respect to distance is called potential gradient. Its unit is volt/meter.

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