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Physics LibreTexts

4.1: Introduction

An electric cell consists of two different metals, or carbon and a metal, called the poles, immersed or dipped into a liquid or some sort of a wet, conducting paste, known as the electrolyte, and, because of some chemical reaction between the two poles and the electrolyte, there exists a small potential difference (typically of the order of one or two volts) between the poles.  This potential difference is much smaller than the hundreds or thousands of volts that may be obtained in typical laboratory experiments in electrostatics, and the electric field between the poles is also correspondingly small. 

Definition.  The potential difference across the poles of a cell when no current is being taken from it is called the electromotive force (EMF) of the cell. 

The circuit symbol for a cell is drawn thus:

The longer, thin line represents the positive pole and the shorter, thick line represents the negative pole.

Several cells connected together form a battery of cells.  Thus in principle a single cell should strictly be called just that – a cell – and the word battery should be restricted to a battery of several cells.  However, in practice, most people use the word battery to mean either literally a battery of several cells, or a single cell.

I shall not discuss in this chapter the detailed chemistry of why there exists such a potential difference, nor shall I discuss in detail the chemical processes that take place inside the several different varieties of cell.  I shall just mention that in the cheaper types of flashlight battery (cell), the negative pole, made of zinc, is the outer casing of the cell, while the positive pole is a central carbon rod. The rather dirty mess that is the electrolyte is a mixture that is probably known only to the manufacturer, though it probably includes manganese oxide and ammonium chloride and perhaps such goo as flour or glue and goodness knows what else. Other types have a positive pole of nickelic hydroxide and a negative pole of cadmium metal in a potassium hydroxide electrolyte.  A 12-volt car battery is typically a battery of 6 cells in series, in which the positive poles are lead oxide PbO2, the negative poles are metallic lead and the electrolyte is sulphuric acid.   In some batteries, after they are exhausted, the poles are irreversibly damaged and the battery has to be discarded.  In others, such as the nickel-cadmium or lead-acid cells, the chemical reaction is reversible, and so the cells can be recharged.  I have heard the word “accumulator” used for a rechargeable battery, particularly the lead-acid car battery, but I don’t know how general that usage is.

Obviously the purpose of a battery is to extract a current from it.  An electrolytic cell is quite the opposite.  In an electrolytic cell, an electric current is forced into it from outside.  This may be done in a laboratory, for example, to study the flow of electricity through an electrolyte, or in industrial processes such as electroplating.  In an electrolytic cell, the current is forced into the cell by two electrodes, one of which (the anode) is maintained at a higher potential than the other (the cathode).  The electrolyte contains positive ions (cations) and negative ions (anions), which can flow through the electrolyte.  Naturally, the positive ions (cations) flow towards the negative electrode (the cathode) and the negative ions (the anions) flow towards the positive electrode (the anode).

The direction of flow of electricity in an electrolytic cell is the opposite from the flow when a battery is being used to power an external circuit, and the roles of the two poles or electrodes are reversed.  Thus some writers will refer to the positive pole of a battery as its “cathode”.   It is not surprising therefore, that many a student (and, one might even guess, many a professor and textbook writer) has become confused over the words cathode and anode.  The situation is not eased by referring to negatively charged electrons in a gaseous discharge tube as “cathode rays”.

My recommendation would be:  When referring to an electrolytic cell, use the word “electrodes”; when referring to a battery, use the word “poles”.  Avoid the use of the prefixes “cat” and “an” altogether.   Thus, refer to the positive and negative electrodes of an electrolytic cell, the positive and negative poles of a battery, and the positive and negative ions of an electrolyte.  In that way your meaning will always be clear and unambiguous to yourself and to your audience or your readers.