Wednesday, August 10, 2016

EMF and Internal Resistance of a Cell

Charge or current flows between two points only when there is a potential difference between the given two points. The simplest way of generating this potential difference is to provide a battery. This battery generates the required potential difference between given two points. Using some chemical process, and two plates it can do it. The driving potential difference between two plates of the battery is called as electro motive force. It is not a force, rather it is a potential difference between two plates of the battery.

The simple difference between potential difference and EMF is, we can find the potential difference between two points only when they are connected in a circuit with a battery. But EMF will be there in between the two plates even when they are not connected in a circuit. Simply EMF can exists in a open circuit where as potential difference can be found only in a closed circuit.

EMF drives the charges in the circuit. During this process of passing current in a circuit, there is some opposition to the flow inside the cell and it is called internal resistance. The internal resistance acts like a series resistance in the circuit and there will be some potential drop across it. It means all EMF given by the cell is not available in the circuit rather only the remaining part after the drop across the internal resistance. Thus the portion of the voltage available in the circuit after the drop across the internal resistance is called terminal voltage or terminal potential difference.

We can express the terminal voltage and internal resistance in terms of EMF and terminal voltage as shown in the page below.

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