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 If a current I flows through a potential drop V, the energy lost per second by the drifting charges is VI. In a resistor of resistance R, the loss of energy appears as heat. The rate of heat production P is P = VI = I2R = V2/R and is independent of the direction of current |
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Electrolysis of CuSO4: |
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| Cu2+ ions drift to the cathode and are neutralized by the electrons flowing in from the negative terminal of the external source. The reduction reaction at the cathode is: |
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| Cu dissolves into the solution producing Cu2+ and 2e-, the latter flowing to the positive terminal of the source. The oxidation relation at the anode is: |
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| In effect, copper is dissolved off the anode and deposited at the cathode. In this process one copper atom is deposited at the cathode for each pair of electrons flowing through the connecting wires. |
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Electrolysis of AgNO3: |
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| Ag+ ions move towards the cathode and are neutralized by the electron of the metal, flowing in from the negative end of the external source. The reduction reaction at cathode is: |
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| The oxidation reaction at anode is |
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| Ag dissolves into the solution and the released electron flow towards the positive end of the external source. In this process one electron circulates for the deposition of every silver atom on the cathode. |
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Faraday's laws of electrolysis: |
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| (a) The mass of substance (m) deposited at the cathode (or dissolved off the anode) in electrylysis is proportional to the total charge Q passing through the circuit. If the current I is constant and flows a time t, q = I t and m = ZIt, |
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| where Z is a constant of proportionality called electro chemical equivalent of the substance. |
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| (b) The masses of different substances produced in electrolysis by the same quantity of charge are proportional to their equivalent masses (or chemical equivalents). |
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| These laws are easy to understand in the atomic view. For each atom of valency p to be deposited, a charge pe must pass through the circuit. Therefore, for a mode of the substance deposited i.e., for m = M, where M is the relative atomic mass, the total charge flowing through the circuit is NA pe, where NA is the Avogadro's number. Thus, |
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| M = Z NA pe or Z = M/NAep |
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| Where M/P is the equivalent mass of the substance. Thus |
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Faraday's second law of electrolysis has important implications. It suggests that valency and electrical charge are connected, and that there is an elementary unit of charge e common to all matter so that all charges are integral multiples of e. The fundamental constant NAe is called Faraday constant (or Faraday) and denoted by F. From the measured values of F = 96,487 C/mol (from experiments on electrolysis) and of NA = 6.02 x 1023/mol (from experiments on Brownian motion), e is estimated to be 1.6 x 10-19C. |
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In electrochemical cells, chemical reactions are a source of electrical energy. Examples of such cells are the Daniel cell and the carbon-zinc dry cell. IN a secondary cell or accumulator such as the lead sulphuric acid cell, chemical processes that occurred at the electrodes during discharge are reversed by passing a current through it in the reverse direction, and the cell gets recharged i.e., recovers its original state. |
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(a) Seebeck effect : If two junctions of dissimilar conductors in a circuit are held at different temperatures, an emf develops causing a current to flow in the circuit. The thermo-emf VAB for two materials A and B is the difference between the emfs of two thermocouples AC and BC, provided the junctions are held at the same temperatures, i.e., VAB = VAC - VBC . A thermocouple provides an accurate and convenient device for measuring temperature. The copper constantan thermocouple is very suitable in the range 50 K - 400 K. It develops a voltage of the order of 40 mV per K. This effect is also used in thermoelectric generators. |
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| (b) Peltier effect: when an electric current is passed through a junction of two dissimilar conductors, heat is either absorbed or released at the junction, depending on the direction of the current. The effect finds application in thermoelectric refrigerators. |
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| (c) Thomson effect: This refers to the emf that develops between two parts of a single metal when they are at different temperatures. |
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