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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