NEET Physics Notes Electrostatics-Electric Cell
An electric cell is a device which maintains a continuous flow of charge (or electric current) in a circuit by a chemical reaction. In an electric cell, there are two rods of different metals called electrodes.
Types of Electric Cell
There are two basic types of electric cell
A cell is called primary, if it used only for discharge. The current leaves the cell at the positive (+ve) terminal goes through the external circuit and enters the cell at the negative (-ve) terminal. Examples of primary cells are daniell cell, laclanche cell and dry cell.
The most popular cell is dry cell. It is a special type of laclanche cell in which ‘ both NH4Cl are MnO2 are are prepared in the form of a paste. The paste is contained which is negative electrode. The internal resistance of a dry cell is very small generally Its emf is generally 1.5 V.
In a secondary cell, the current pass in both directions When current leaves the cell at the positive (+ve) terminal and enters the cell at the negative (-ve) terminal, the cell is discharge. This is ‘ called normal working of the cell. In this case, the chemical energy is converted into electrical energy. The most commonly used secondary cell is a lead accumulator.
A lead commulator consists of electrodes made of PbO 2 and Pb immersed is dil. H2S04. Discharging process, the ions move towards Pb electrode give negative
charge and H+ ions move to the PbO 2 electrode, given up positive charge.
The PbO so formed reacts with dil.H2S04 to get PbS0 4 and H20.
Therefore, PbS0 4 is formed at the both electrodes. In charging process, a current forced from the positive to the negative inside the cell. The H+ ions move towards the negative electrode and react with the PbS0 4
At the positive electrode, the reactions
Electromotive Force (EMF) of Cell
The potential difference between the two poles of the cell in an open circuit is called the electromotive force (emf) of the cell. It is denoted by E.
Its SI unit is volt (V) or joule coloumb-1 (JC_1)
Thus, when a current is drawn through a source, the potential difference between the terminal of the source is
V = E – ir .
This can also be shown as below E r
Following three special cases are possible
- If the current flows in opposite direction (as in case of charging of battery), then V = E + Ir
- V = E, if the current through the cell is zero.
- V = 0, if the cell is short circuited.
- This is because current in the circuit
Potential Difference and Emf of a Cell
Emf of a cell is the terminal potential difference of cell when it is in an open circuit, i.e. it is not supplying any current to the external circuit. However, when it is supplying a current to an external resistance, the voltage across the terminals of cell is called the terminal voltage or terminal potential difference.
If E be the emf and r the internal resistance of a cell and a resistance R is joined with it, then current in the
Terminal voltage is more than EMF of cell when cell is charged and it is given by V = E + Ir
Combination of Cells in Series and in Parallel
A group of cells is called a battery. Two common grouping of cells are
In series grouping, if all the cells are joined so as to supply current in the same direction, then resultant emf
Eeq =E1+E2 + E3 +…
However, if one or more cells are joined so as to supply current in the reverse direction then the EMF of that/those cells is taken as negative while calculating the equivalent emf.
The equivalent internal resistance of cell
req =r1+r2+r3 + …
If n cells, each of emf E and internal resistance r, are joined in series, then
Eeq = nE and req = nr
In parallel grouping, if positive terminals of all cells have been joined at one point and all negative terminals at another point, then