Violencia intrafamiliar

energy. The chemical reaction in the primary cell is irreversible. The examples of primary cells are Voltaic cell, Daniel cell, Leclanche cell, Dry cell, etc.

The secondary cells are those in which the electrical energy is first stored up as the chemical energy. When current is required to drawn from the secondary cell, then the chemical energy is reconverted into the electrical energy. The chemical reaction in the secondary cell is reversible. The examples of secondary cells are Lead- acid accumulators, alkali accumulators or Edison cell.

The initial cost of a primary cell is low as compared to the secondary cell. But, the running cost of a secondary cell is low as compared to the primary cell.

the Primary cells

Voltaic cell: Voltaic cell was invented by Allexandro de Volta in 1800. It consist of two rods (called electrodes) one of copper and another of zinc, partly immersed in dilute sulphuric acid (called electrolyte) contained in a glass vessel (Fig. 1.19). The copper rod acts as positive electrode and zinc rod acts as negative electrode.

When the electrodes are connected to an external resistor, the circuit is completed.

There will be flow of electrons from the negatively charged zinc rod to the positively charged copper rod through the external resistor. Now the conven tional electric current is said to flow from copper to zinc.

Daniel cell: It consists of a copper vessel containing saturated copper sulphate solution.

The copper vessel itself acts as the positive electrode or anode. A porous pot containing 10% dilute sulphuric acid (called electrolyte) and amalgamated zinc rod (called cathode),

Fig. 1.19: Voltaic cell

is placed in the copper vessel and is partly immersed in a copper sulphate solution. The porous pot prevents the solution from mixing, but allows the hydrogen ions to pass through it. A perforated shelf contain ing the copper sulphate crystal is placed at the top of the vessel in order to keep the concentration of the copper sulphate solution same (Fig. 1.20).

In this cell, as the reaction continues, the concentration of copper sulphate solution decreases. Some CuSO₄ crystals get dissolved immediately from the perforated shelf into CuSO₄ solution. Thus, the concentration of CuSO₄ is maintained. As the concentration of the copper sulphate solution remains constant, when Daniel cell is in use, therefore, its emf remains constant.

Lechlanche cell: A Lechlanche cell consists of a vessel of glass containing strong solution of ammonium chloride which acts as electrolyte. An amalgamated zinc rod dipping in ammonium chloride acts as negative electrode or cathode. A porous pot is placed inside the glass vessel. The carbon rod placed inside the porous pot acts as positive electrode or anode. The space in the porous pot is filled with manganese dioxide and charcoal powder (Fig. 1.21). The charcoal powder makes the manganese dioxide electrically conducting and manganese dioxide acts as depolarizer. The inner side of glass vessel near the open end is coated with black paint which works as reflector for the ammonium chloride crystal as they have the tendency to creap along the glass wall. This helps in maintaining the proper concentration of ammonium chloride solution. The electrons released are collected by zinc rod, making it at negative potential with respect to electrolyte. The ammonia gas so produced escapes. The hydrogen ions diffuse through the porous pot and interact with manganese dioxide.

The positive charge is transferred to the carbon rod which attains the positive potential with respect to electrolyte. The depolarizer (MnO₂) in Leclanche cell is in solid form and is slow in action. Therefore, when the current is drawn from the Leclanche cell, the hydrogen is liberated quickly than MnO₂ can use it up. So, after some time, a partial polarization sets due to accumulation of hydrogen on anode and thereby, the current falls off. When the circuit is switched off, the hydrogen gas escapes. The cell regains its original emf and is again ready for use.

Thus, Lechlanche cell is useful in those experiments where intermittent supply of current is needed.

Fig. 1.20: Daniel cell

Fig. 1.21: Lechlanche cell

The emf of Lechlanche cell is 1.45 V and its internal resistance can vary from 0.1 Ohm to 10 Ohm.

Dry cell: A dry cell is a portable form of Lechlanche cell. It consists of zinc vessel which acts as a negative electrode or cathode. The vessel contains a moist paste of sawdust saturated with a solution of ammonium chloride and zinc chloride. The ammonium chloride acts as an electrolyte and the purpose of zinc chloride is to maintain the moistness of the paste being highly hygroscopic. The carbon rod covered with the brass cap is placed in the middle of the vessel. It acts as positive electrode or anode. It is surrounded by a closely packed mixture of charcoal and manganese dioxide (MnO₂) in a muslin bag. Here MnO₂ acts as a depola rizer. The zinc vessel is sealed at the top with pitch or shellac. A small hole is provided in it to allow the gases formed by the chemical action to escape (Fig. 1.22).

The emf of dry cell is 1.5V. If this cell is used continuously, the polarization defect may develop in this cell but it regains its emf if allowed to rest for a while.

Fig. 1.22: Dry cell