11. Electric Cells

An electric cell is a device that converts stored chemical energy directly into electrical energy through a chemical reaction occurring inside it, thereby acting as a source of EMF for a circuit. Cells are broadly divided into two categories based on whether the chemical reaction inside them can be reversed.

Primary Cells

In a primary cell, the chemical reaction that produces electricity is irreversible; once the active chemical materials are consumed, the cell cannot be recharged by passing a current back through it and must simply be discarded. The most familiar example is the dry cell (commonly called a torch cell or battery cell), used in torches, remote controls, and toys. A typical dry cell has a zinc container that also serves as the negative electrode (anode of the discharging cell), a carbon rod at the centre serving as the positive electrode (cathode), and a moist paste of ammonium chloride and manganese dioxide as the electrolyte (it is called 'dry' only because the electrolyte is a paste rather than a free liquid, not because it is completely free of moisture). A standard dry cell has an EMF of about 1.5 volts. Other examples of primary cells include the alkaline cell (using potassium hydroxide as the electrolyte, giving longer life) and the mercury cell (once used in small devices such as watches and hearing aids, now largely phased out for environmental reasons).

Secondary Cells

In a secondary cell, the chemical reaction is reversible — after the cell has been discharged (used), it can be recharged by passing an external electric current through it in the opposite direction, which restores the original chemical composition of the electrodes and electrolyte and allows the cell to be used repeatedly. Secondary cells are also called storage cells or accumulators, and their combination is what we commonly call a rechargeable battery. The most important example for exam purposes is the lead–acid battery (also called the lead storage battery), which is the standard battery used to start automobile engines and in inverters/UPS systems. It consists of a lead peroxide (PbO₂) plate as the cathode, a spongy lead (Pb) plate as the anode, and dilute sulphuric acid as the electrolyte; a fully charged lead-acid cell has an EMF of about 2 volts, and six such cells connected in series give the standard 12-volt car battery. Other common secondary cells include the nickel-cadmium (Ni-Cd) cell and the now widely used lithium-ion battery, which powers mobile phones, laptops, and electric vehicles due to its high energy density and lighter weight compared to lead-acid batteries.