see url Voltage, Potential Difference and EMF
According to the structure of an atom, we know that there are two types of charges namely positive charge and negative charge. A force of attraction exists between these two charges because of the opposite polarity of these charges (positive charge and negative charge). A certain amount of energy is required to overcome this force and move the charges through a specific distance. Positive and Negative charge posses a certain amount of energy because of the separation between them. The difference in potential energy of the charges is called the potential difference.
In electrical terminology the potential difference is also called voltage. In closed circuit current will flow only and only if there is a potential difference exist between two points of the circuit. If the potential difference between two points become equal then current will not flow through the circuit.
In Simple words, we can say that, voltage is called electric pressure that causes flow of electric current through any section of a wire
Voltage is expressed in terms of energy (W) per unit charge (Q).
Where W is energy in Joule
Q is charge in Coulomb
dw is small change in energy in joule
dq is small change in charge in coulomb
The Unit of voltage is here volt.
source site E.M.F (Electromotive force)
The force that causes flow of electric current through a circuit is called E.M.F.
But above definition is wrong because the emf is not a force.
We can define the emf is this way, the emf of a source is a measure of the energy that it gives to each coulomb of charge
E.M.F. is also measured in volt.
The instrument which is used to measure the voltage or E.M.F or potential difference in the circuit is called Voltmeter. Voltmeter always connected in parallel with the circuit whose voltage is to be measured.