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A cell is an electrochemical device consisting of two electrodes made of different materials an electrolyte. The chemical reaction between the electrodes & the electrolyte produces a voltage.
Primary cells -
The cell, in which e.m.f. is produced on account of chemical actions, is called a primary cell; Fig. 1 Cell It has two electrodes of different metals such as copper & zinc, immersed in a suitable electrolyte, such as dilute sulphuric acid. If the two electrodes are connected to a torch bulb with two pieces of wires, then a current flows from positive to negative terminal of cell.
Principle of a Primary cell -
The first cell was made by Volta, therefore, a primary cell is called a voltaic cell also. It consists of a glass container containing dilute sulphuric acid ( H2SO4) with copper & zinc rods as electrodes. When two pieces or wires to a torch bulb ( or any other load), then the current starts to flow. Sulphuric acid decomposes into hydrogen & sulphate ion. Negative ions of Sulphate travel towards the zinc rod & make it negative by forming zinc sulphate. Positive ions of hydrogen travel towards the copper rod & make it positive by forming hydrogen bubbles. In this way, the current flows from zinc to copper to zinc in the external circuit other types of primary cells are made on the above principle using different electrolysis & electrodes.
1. It is an instant e.m.f. supplying device.
2. A cell once discharged fully cannot be recharged again.
3. It is cheap in cost.
4. It is portable.
5. It is suitable for intermittent uses such as bells etc.
Defects of primary cell & their remedies -
There are two main defects in a Primary cell -
1. Local action.
1) Local action -
Normally, zinc consists of many, impurities such as copper iron etc. When a zinc rod is used as an electrode in a primary cell, its circuit becomes complete even current is set up from zinc to copper or iron particles. In this way, it causes a loss of zinc & electrical power; this action is known as local action. This action can be minimized by amalgamating the zinc rod. i.e. Coating the rod with mercury . By this remedy, the impurities are covered by amalgam of zinc & mercury & do not take part in the chemical action. Another remedy is to use a pure zinc rod.
2) Polarization -
In the working of a cell hydrogen as is produced in the form of bubble, which is collected around the copper electrode. Hydrogen is a bad conductor of electricity & thus it increases the internal resistance of the cell. Therefore, the voltage drop & power loss of the Cell is increased. Sometimes, the flow of current is completely e.m.f. which opposes the flow of current. This effect is known as Polarization. This action is minimized by using a depolarizer. The common depolarizer. The Common depolarizer is manganese dioxide (Mn O2). Which converts the hydrogen bubbles into water. Another remedy is to clean the rod with a brush every now & then, which is a difficult task. There are so many primary cells available in market but we are going to study only Dry Cell.
It is a modification of Lechlanche Cell. It consist of a cylindrical zinc pot, which works as the cathode. A carbons rod is placed in centre of the pot which is surrounded by a mixture of manganese di-oxide, carbon powder, ammonium chloride & zinc chloride mixed in a ratio fig.2 Dry cell of 10.10, 2&1. The paste is surrounded by a canvas or fabric sack. Between the sack & the zinc pot, a zinc chloride & Plaster of Paris is filled. Plaster of Paris makes the paste tough & strong. The cell is sealed with a pitch compound is shown in figure. A brass cap is fitted at the top of Carbon rod. The zinc pot is covered with cardboard etc. Ammonium chloride acts with zinc & produces hydrogen, which in turn acts with manganese di-oxide & produces water. The pitch compound has a small hole to work as an outlet for the ammonia evolved during the chemical actions. An over dried cell can be given a life by dropping a few drops of water at the outlet.
Use of Dry Cell-
It is used in torches, toys, transistor ( radio). Definitions -
The conductor or terminals through which an electric current enters or leaves is called an electrode. The electrode through which the current enters the linguine is called a positive electrode or anode through which current leaves the liquid is called a positive electrode or cathode.
The liquid or solution which undergoes a chemical changes in it on account of the passage of electric current this called an electrolyte salted water, acidic water & a basic solutions are examples of electrolyte.
The process of chemical changes due to the passage of an electric current through a liquid or a solution is called electrolysis.
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