The most elementary concept in electrical science is 'Electric Potential Difference'. If you can grasp this simple concept, most of the modern developments and practical applications of this science, will be easier to understand. That is because, it is the causative agent of electricity or currents that drive every electrical device. Electric Potential Energy You must have learned in basic physics courses that charges come in two types: positive and negative. Like charges repel each other and unlike charges attract each other. They exert this force of attraction or repulsion through an electric field that surrounds the charge. Any charge that comes in vicinity of another charge experiences a force of repulsion or attraction. Consider a positive test charge, placed in an electric field. An attractive force is exerted on it by the electric field center. That is why, forcing this charge away from the electric field center requires work to be done, just like work needs to be done to lift anything against the gravitational force. As the charge is forced away, work is done to move it, which increases its electric potential energy. More the amount of charge, more will be the work required to force the charge away. Thus, electric potential energy of the charge increases as we go away from the charge center of the field. The potential is the energy required to transport a unit electrical charge to a specific position in an electrical field. Thus, every point in an electrical field has a different electric potential, which is a function of its distance from the charge source of the field. Definition The electric potential difference between two points, is simply the energy required to transport a unit charge between those two points. It is measured in volts. One volt is defined as one joule per coulomb. It could also be defined as the change in the potential energy that occurs due to transport of a unit charge from one point to the other. Electric current flows from a point of high electric potential, to a point which has a lower potential. It is just like the behavior of water that always seek its level. Current flows between two points that have a difference in electric potential, to balance this inequality. As the current flows between two points, every charge gains energy that is equal to the difference of potential between them. Two terminals of any battery have a potential difference between them, which is measured in volts. When you connect any circuit between the two terminals of the battery, charge flows from one terminal to the other, to equalize the charge imbalance and continues till the difference between them is equalized. How is it Calculated Calculating it is made possible by the Ohm's law. It says that the voltage between two points of a conductor is directly proportional to the current flowing through it as long as its physical properties remain constant. This is expressed in the form of the equation: V = IR where V is the potential difference, I is the current flowing, and R is the constant proportionality factor, which is known as resistance. Thus, by knowing the resistance and current flowing through a conductor, calculating the difference in potential is possible. There are devices called voltmeters, which can be used to measure the difference in electric potentials between two points. They measure the voltage by detecting the current flowing through that conductor. It is this property of electric current flow, caused by potential difference, that powers and drives the modern technological revolution.