What is an electric cell
An electric cell is a device that converts chemical energy into electrical energy and maintains a constant flow of charge in a circuit. It consists of two rods of different metals called electrodes or plates.
It is immersed in a liquid called electrolyte. This liquid has this property that when the plates are immersed in it, one plate becomes positively charged and the other becomes negatively charged.
When both the plates are connected with a wire, the charge starts flowing in the wire. Such a chemical reaction takes place in the electrolyte inside the cell, due to which the charges on the plates keep on filling and the charge flow in the wire remains.
In this way the cell keeps on converting chemical energy into electrical energy.
E. M. F. of a Cell :-
To maintain the continuous flow of charge in an electric circuit, some work has to be done. This work is done by the cell. The energy that is liberated in the chemical reactions taking place in the cell is what drives the charge in the circuit.
In this way the cell converts the chemical energy of its electrodes and electrolytes into electrical energy.
The energy given by the cell in moving a unit charge through the whole circuit is called Electromotive force (EMF) of the cell.
EMF is a characteristic of each cell, which depends on the nature of the plates and electrolytes used in the cell. The amount of electrolyte and the size of the plates or the distance between them does not affect it.
If the energy given by the cell in passing a q coulomb charge in a circuit is W joul, then the EMF of the cell
E = W / q joule / coulomb
The unit of EMF is joule / coulomb which is called Volt. If the energy given by the cell on passing 1 coulomb of charge in a circuit is 1 joule, then the EMF of the cell is 1 Volt.
The direction of the current sent by the cell in the circuit is from the ‘inside of the cell’ negative electrode to the positive electrode.
In other words, the positive charge inside the cell flows from the negative electrode to the higher potential. Therefore, the EMF of the cell is directed from the negative electrode to the positive electrode inside the cell.
Terminal Potential difference :
Let the EMF of a cell (E) connected in an electric circuit and (W) is the energy given by the cell when (q) charge flows through the circuit. Then
E = W / q
The charge q would be the same in all parts of the circuit. If the energies spent in different parts of the circuit are W1, W2, W3, ….. then their sum will always be W. Therefore
E = W / q = (W1 + W2 + W3 + …..) / q = W1 / q + W2 / q + W3 / q + ……
It is clear that V1, V2, V3 are the energies spent in different parts of the circuit in carrying a unit charge. They are called terminal potential difference of different parts of the circuit.
Thus, if W’ joule of work is to be done (W’ joule is the work expended) on passing a q coulmn charge between two points of a valid circuit, then the terminal potential difference between those points will be V = W’ / q volt. The potential difference is measured with a voltmeter.
Internal Resistance of a Cell :
When we connect a cell with a wire, then the electronic current in the wire flows from the positive plate of the cell to the negative plate, and in its solution inside the cell, from the negative plate to the positive plate.