POLARISATION

If an ammeter is included in the external circuit of the simple cell, it indicates a gradual decrease in the current flowing. After some time the current may cease altogether. The decrease is due to the collection of hydrogen ubbles on the surface of Cu plate. The effect of this layer of hydrogen is two-fold.

It acts as an insulator, thus reducing the effective area of the Cu plate and thereby increasing the internal resistance of the cell.

The sticking layer of positive hydrogen ions on the Cu plate exerts a repulsive force on other hydrogen ions which are approaching the copper plate. Hence, the current is reduced. This phenomenon is called polarization and the cell which is in this condition is said to be polarized.

Moreover, the hydrogen and zinc set up between them an e.m.f. which is opposite to that set up between zinc and copper. This e.m.f. delivered by the cell diminishes to a very low value after a short time and hence the cell becomes useless for all practical purposes. The essential difference between various primary cells lies chiefly in the different methods employed to overcome this defect. The most widely used method is to surround the cathode by a solid or liquid depolarize which oxidizes the hydrogen as soon as it is liberated.

                                                              LOCAL ACTION

It is found that even when the voltaic cell is not supplying any load current, zinc goes on continuously dissolving in the electrolyte. This is due to the fact that some traces of impurities like iron and lead in the commercial zinc form tiny local cells which are short-circuited by the main body of zinc. The action of these parasitic cells cannot be controlled so that there is some wastage of zinc. This phenomenon is known as local action and can be prevented by amalgamating the zinc plate by rubbing mercury over the zinc plate. Mercury is supposed to cover the impurities and maintain a film of zinc dissolved in mercury.

THE SIMPLE VOLTAIC CELL

                                                     THE SIMPLE VOLTAIC CELL

Cell consists of a copper plate and zinc rod placed in dilute sulfuric acid. When terminals of the cell are joined by a wire, then hydrogen is given off at the cu plate and a current is found to flow from Cu to Zn in the external circuit and from Zn to Cu within the cell. Cu plate forms the positive electrode anode and zinc the negative electrode cathode. While the current is flowing, some of the zinc is found to dissolve in the acid.

The production of the e.m.f. can be explained with the help of contact difference of potential. Whenever a metal plate is brought in contact with an electrolyte, there always develops a tendency either for some of the metal to go into the solution in the form of positive ions, thus leaving the plate negatively charged or for some of the positive ions in the solution to be deposited on the plate which consequently becomes positively charged. In the case of a simple voltaic cell, when Zn is immersed in dilute H2SO4, some of the Zn enters the electrolyte in the form of positive ions which further combine with the negative sulphions in the electrolyte to form zinc sulfate. However, this action stops when the contact potential between Zn and electrolytic solution reaches the value of 0.62 V, Zinc being at the lower potential negative with respect to the solution film adjacent to it.

Similarly when Cu plate is placed in contact with the electrolyte, then the positive hydrogen ions in the solution have a tendency to get deposited on it until its potential rises nearly to 0.46 V above that of the solution. Hence, a total potential difference of 0.62-(-0.46)= 1.08 V is developed between the two electrodes.

As more and more of positive zinc ions pass into solution, an increasing number of electrons becomes available at the zinc electrode. These electrons run towards the copper anode via the external wire. There these electrons combine with the positive hydrogen ions, thus converting them into electrically neutral atoms. These atoms then combine in pairs to form molecules of hydrogen which bubble off from the copper plate. The chemical action taking place may be represented by the following equation

Zn+H2SO4+H2

CELL AND BATTERY

                                    CELL AND BATTERY

The word cell means one unit or a combination of materials for converting chemical energy into electrical energy. A battery means a combination of these units or cells.

                          E.M.F AND TERMINAL POTENTIAL DIFFERENCE

The e.m.f. of the cell, as said earlier, is the total potential difference established within the cell between the two electrodes when the cell is not supplying any current. The e.m.f. can be measured by connecting a suitable voltmeter across the electrodes. But the terminal potential difference is equal to the e.m.f minus the internal voltage drop. In other words,  the T.P.D is the potential difference available for external circuit. If 'i' is the current applied by the cell and 'r' its internal resistance, then Terminal potential difference V= E.M.F.- ir or  V= E-ir where V= terminal potential difference E=E.M.F. generated within the cell.

It should be noted that whereas e.m.f. E is constant, the terminal potential difference V is not, as it depends on load current supplied by the cell.

Also the term e.m.f. has local significance i.e. it is spoken of with reference to the generator itself, but potential difference is distributive. For example, we cannot say that an e.m.f. of 220 volts exists across an electric bulb. It is the potential difference of 220 V. In other words, p.d. can be distributed away from its secure of generation.

DIVISION OF CURRENT

                                     PRIMARY CELL

It essentially consists of two dissimilar conducting electrodes immersed in a liquid called electrolyte which acts chemically on one of the two electrodes more readily than on the other. By using the energy released by chemical action, electrons are shifted from one electrode to another thereby creating a potential difference between the two electrodes. The value of total potential difference created between the electrodes, when the cell is not connected to an external circuit, is known as its electromotive force.

Now, every cell has some internal resistance which depends upon the construction and condition of the cell. The internal resistance depends on the area of electrodes, the distance between the electrodes, the temperature, strength and density of the electrolyte. When the cell supplies current to an external circuit, there is always some internal voltage drop due to this internal resistance. Hence, the voltage available for external circuit is decreased by this amount. The net voltage available at the terminals for external circuit is known as the terminal potential difference.

RESISTANCE

It may be defined as the property of a substance. due to which it opposes the flow of electricity through it.

Metals acids and salt solutions are good conductors of electricity. Among st pure metals, silver, copper and aluminum are very good conductors in the given order. This, as discussed earlier, is due to the presence of a large number of free or loosely attached electrons on their atoms. These vagrant electrons assume a directed motion on the application of an electric potential difference. These electrons while flowing pass through the molecules or the atoms of the conductor, collide with other atoms and electrons, thereby producing heat.

Those substances which offer relatively greater difficulty or hindrance to the passage of these electrons are said to be relatively poor conductors of electricity like Bakelite, mica, glass, rubber, p.v.c

It is helpful to remember that electric friction is similar to friction in mechanics.


                                            THE UNIT OF RESISTANCE

The practical unit of resistance is ohm. A conductor is said to have a resistance of one ohm if it permits one ampere current to flow through it when one volt is impressed across its terminals.

For insulators whose resistances are very high, a much bigger unit is used megohm=10 to the power 6 ohm or kilohm= 10 to the power 3 ohm. In the case of very small resistances, smaller units like milliohm= 10 to the power -3 ohm or microhm= 10 to the power -6 ohm are used.

THE IDEA OF ELECTRIC POTENTIAL

In the picture is shown a simple voltaic cell. It consists of a copper plate and a zinc rod immersed in dilute sulfuric acid contained in a suitable vessel. The chemical action taking place within the cell causes the electrons to be removed from Cu plate and to be deposited on the zinc rod. This transfer of electrons is accomplished through the agency of the diluted H2SO4 which is known as an electrolyte. The result is that zinc rod becomes negative due to the deposition of electrons on it and the Cu plate becomes positive due to the departure of electrons from it. The large number of electrons collected on the zinc rod is being attracted by anode, but is prevented from returning to it by the force set up by the chemical action within the cell. But if the two electrodes are joined by a wire externally, then electrons rush to the anode, thereby equalizing

 the charges of the two electrodes. However due to the continuity of chemical action, a continuous difference in the number of electrons on the two electrodes is maintained which keeps up a continuous flow of current through the external circuit. The action of an electric cell is similar to that of a water pump which, while working, maintains a continuous flow of water water current through the pipe.

It should be particularly noted that the direction of electronic current is from zinc to copper in the external circuit. However, the direction of conventional current is from Cu to zinc. In the present case, there is no flow of positive charge as such from one electrode to another. But we can look upon the arrival of electrons on copper plate as equivalent to an actual departure of positive charge from it.

When zinc is negatively charged it is said to be at negative potential with respect to the electrolyte, whereas anode is said to be at positive potential relative to the electrolyte. Between themselves, Cu plate is assumed to be at a higher potential than the zinc rod. This difference in potential is continuously maintained by the chemical action going on in the cell which supplies energy to establish this potential difference.

Electric Current and Ohm's Law

                               Nature of Electricity

It is comparatively easy to describe what electricity can do than to give a simple and direct answer to the question: What exactly is meant by electricity ?  Electricity has become such a universal medium for transmission and utilization of energy that almost every one is familiar with its innumerable uses right from the earliest childhood. Electric energy is variously utilized as for lighting, transportation, communication, for operating electric furnaces, elevators and for driving various kinds of machine tools etc. It can be easily stored and concentrated to produce extremely high temperatures as in welding and electric furnaces, are lights and spark plugs etc.

Turning back to the question regarding the nature of electricity, it may be noted that ancient Greeks were the first to observe that when amber is rubbed against a piece of silk cloth, it attracts light objects like small pieces of paper etc. The agency that endowed this attracting property to amber was given the name of electricity by Gilbert in 1600 A.D. The name electricity is derived from the word electron which is the Greek name of amber.

To explain the few observed phenomenon about electricity, Benjamin Franklin advanced, in 1749, what was called on Fluid Theory of electricity. According to this theory, electricity was assumed to be a sort of an invisible and intangible fluid which was associated with matter in different degrees. The normal state of matter was associated with a certain definite amount of this fluid. Any disturbance of this assumed normal distribution of fluid was supposed to result in the body being either positively or negatively charged corresponding to an excess or deficit of this normal amount of fluid. This theory was successful in explaining the production of positive and negative electricity when two bodies are rubbed together. Because, according to this theory, rubbing of two bodies together led to an unequal redistribution of their fluids whose total sum was supposed to remain constant. One body which got more of this fluid became positively charged and the other which got less became negatively charged.

It will be noted that this theory does not assume two types of fluids, but simply two states of electrification resulting from an excess or deficit of the same fluid. This theory, however, did not hold ground for long since it could not satisfactorily explain many electrical phenomena including induction.

In the year 1735, Dufay advanced the Two Fluid Theory which was later modified and restated in a more satisfactory manner by Symmer in 1759. According to this theory, two different types of fluids resided simultaneously in matter corresponding to two states of electrification positive and negative. The type and degree of electrification was measured by the excess of one type of fluid over the other. The normal non-electrical state of matter was assumed to be due to the presence of equal quantities of these two fluids which neutralized each other.

This theory was successful for some time, but was, later on, discarded because it was based on the untenable assumption of two invisible fluids present in the matter and it was found inadequate to fully explain many of the facts about electricity discovered afterwards.