Electric Motors
The principle of electrical Motor. Learn how current flows and power is produced.
Related
Most Viewed in Science
Generators
Electric Current
Sea Animals - Electric Fish
In a storage battery, a lead plate becomes negatively charged and a lead peroxide plate becomes positively charged when both stand in the same container of sulphuric acid.
Chemical action in a dry cell battery leaves the carbon rod positively charged and the zinc case negatively charged creating a difference in the electrical potential (voltage) which results in a flow of electricity.
Electric Batteries
Batteries generate electric currents through chemical action. A popular type, the dry cell, 
which is widely used to power flashlights, transistor radios, etc., uses a zinc case filled with sal ammoniac in the center of which is a carbon rod. Chemical action in the battery leaves the carbon rod positively charged and the zinc case negatively charged. This battery is of the Leclanche type, named after its inventor. It is one of a class of batteries that cannot be recharged - the chemical actions that generate the electricity cannot be reversed, and the battery loses its current after it has been in service for a while.
Batteries of the Leclanche type are the most widely used dry batteries because the hydrogen bubbles that form on the carbon rod when the battery is delivering current are rapidly removed and cannot quickly lower the voltage of the battery as they do with some other types. This is prevented in the Leclanche cell by surrounding the carbon rod with a chemical, manganese dioxide. The hydrogen bubbles combine with oxygen atoms in the chemical to form water, which then simply mixes with the sal ammoniac. When the battery has been on for some time, however, hydrogen bubbles are given off more quickly than they can combine with oxygen to form water. When this happens they do cause the battery voltage to fall. A flashlight will grow dim and a transistor radio will lose power. If the battery is switched off for a short time, however, all the hydrogen bubbles will have time to combine, and when the battery is switched on again the flashlight will glow brightly and the radio will play at full volume. Eventually, however, batteries of this class, which are known as primary batteries, do become fully discharged and cannot be used again.
If we want a battery that can be recharged when it has become fully discharged after prolonged use we must use one of the secondary or storage batteries. The most familiar examples of secondary batteries are those used in automobiles. These particular secondary batteries consist of a jar filled with dilute sulphuric acid in which two plates are immersed. The current supplied by the battery is drawn off from two terminals, one on the top of each plate.
When the battery is fully charged, one plate consists of spongy lead: the other one of lead peroxide. When current is taken from the battery the dilute sulphuric acid is split into its component molecules, which are hydrogen and a special kind of molecule called a sulphate ion. Complex chemical reactions then take place at the two plates between these molecules and the component molecules of the material of which the plates are composed. This results in the two plates being gradually converted into lead sulphate.
When a large part of the two plates has been so converted the battery voltage falls to a very low value and it can no longer be used. To recharge a secondary battery it is connected to a direct current power supply having a voltage slightly greater than that of the battery itself. The positive terminal of the battery is connected to the positive terminal of the direct current supply, and the negative terminal is connected to the negative terminal. The power supply then drives current through the battery in the opposite direction to that in which it flows when the battery is providing the power. This produces chemical reactions in the battery which gradually turn the negative plate back into spongy lead and the positive plate back into lead peroxide. As the process continues the battery voltage rises until at the end of the charge it is at a maximum.
Other types of secondary battery are in use or are being developed; but none has as yet won the same wide acceptance as has the lead acid battery. Nickel-cadmium batteries have some advantages. A particularly promising type of battery is the sodium-sulphur type.
