Galvanic elements: the principle of action and varieties
Today, the galvanic cells are aloneof the most common chemical sources of current. Despite their shortcomings, they are actively used in electrical engineering and are constantly being improved.
The simplest example of a galvanicelement looks like this. In a glass jar with an aqueous solution of sulfuric acid, two plates are immersed: one - copper, the second - zinc. They become the positive and negative poles of the element. If these poles are connected by a conductor, the simplest electric circuit will be obtained. Inside the cell, the current will flow from the zinc plate, which has a negative charge, to the copper plate, which is positively charged. In the outer chain, the motion of charged particles will occur in the opposite direction.
Under the action of current ions of hydrogen and acidthe remainder of sulfuric acid will move in different directions. Hydrogen will give its charges to the copper plate, and the acid residue - zinc. So the terminals of the element will be supported by voltage. At the same time, hydrogen bubbles will settle on the surface of the copper plate, which will weaken the action of the galvanic cell. Hydrogen creates an additional voltage together with the metal of the plate, which is called the electromotive force of polarization. The charge direction of this EMF is opposite to the charge direction of the EMF of the galvanic cell. Bubbles themselves create additional resistance in the element.
The element considered by us is a classicexample. In reality, such galvanic cells are simply not used because of the large polarization. In order that it does not occur, in the manufacture of elements, a special substance that absorbs hydrogen atoms, called the depolarizer, is introduced into their composition. As a rule, these are preparations containing oxygen or chlorine.
Advantages and disadvantages of modern galvanic cells
Modern galvanic cellsare made of different materials. The most common and familiar type is the coal-zinc elements used in finger-powered batteries. To their pluses, you can relate relative cheapness, to cons - a short shelf life and low power.
A more convenient option is alkalinegalvanic cells. They are also called manganese-zinc. Here, the electrolyte is not a dry substance such as coal, but an alkaline solution. Being discharged, such elements practically do not emit gas, due to what they can be made hermetically. The shelf life of such elements is higher than that of coal-zinc.
Mercury elements are similar in construction toalkaline. Mercury oxide is used here. Such current sources are used, for example, for medical equipment. Their advantages - resistance to high temperatures (up to +50, and in some models up to +70 ° C), stable voltage, high mechanical strength. The disadvantage is the toxic properties of mercury, due to which it is necessary to handle very carefully with the elements that have worked out their time and send them for processing.
In some elements, silver oxide is used forthe manufacture of cathodes, but because of the high cost of metal, their use is economically unprofitable. More common elements with lithium anodes. They also have a high cost, but they have the greatest tension among all the types of galvanic cells considered.
Another type of galvanic cells isconcentration galvanic cells. In them, the process of particle motion can proceed with and without transfer of ions. The first type is an element in which two identical electrodes are immersed in solutions of electrolyte of different concentration, separated by a semipermeable partition. In such elements, the emf is due to the fact that the ions are transferred to a solution with a lower concentration. In the elements of the second type, the electrodes are made of different metals, and the concentration is equalized by the chemical processes that occur on each of the electrodes. The electromotive force of these elements is higher than that of the elements of the first type.