The Electromotive Force (emf) 

 In order to create a potential difference between two terminals in a conductor, there needs to be accumulation of like charges at each of the terminals. By nature opposite charges attract each other. Hence an external force is required to push positive charges from the negative terminal of the device to the positive terminal. The force, which is necessary to do the work required to push a charge of 1C away from the negative terminal, is called the electromotive force.

In ideal cases when the device providing the emf has no internal resistance, the potential difference created between the terminals of the device is the emf. The emf is also referred to as electric potential or potential difference. The SI unit, of emf, is defined as the difference in potential between two points on a conductor carrying a constant current of one ampere when the power dissipated between the points is one watt. It is named after Alessandro Volta.
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Quiz

In cases when the device providing the emf has an internal resistance, how does the potential difference across the terminals get affected?


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