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4 - THERMAL AND CHEMICAL EFFECTS OF ELECTRIC CURRENT Page 1 4.1 Heating Effect of Electric Current: Joule’s Law The electric current in a conductor is due to the motion of electrons. During their motion, electrons collide with the oscillating positive ions in the conductor and impart part of their energy to them. Ions oscillate faster and their increased energy is manifested as heat. The heat energy released in a conductor on passing an electric current is called the “Joule heat” and effect is called the ‘Joule effect”. The potential difference of V volt applied between two ends of a conductor means that V joule of electrical energy is utilized and converted into heat when one coulomb charge passes through the conductor. If Q coulomb charge passes through the conductor in t seconds resulting in current Ι , the heat energy produced is W = V Q joule = V Ι t = Ι ΙΙ 2 R t ( Q V = Ι R according to Ohm’s law ) = ( V 2 / R ) t The electric power, i. e., the electrical energy supplied per unit time or converted into heat energy per unit time in a resistance R, is P = V Ι joule / // second ( = watt ) = Ι ΙΙ 2 R = ( V 2 / R ) Thus, mechanical unit of energy, joule = watt. second which is an electrical unit of energy. This being too small, kilowatt-hour ( kwh ) = 3.6 × 10 6 joule is used as a practical unit of electrical energy. R is the Ohmic resistance of the conductor value of which does not depend upon V or Ι . Considering R as a constant, P Ι ΙΙ 2 or P V 2 In fact, all electrical appliances are rated to operate for a given potential difference and hence in household wiring, they are connected in parallel. Therefore, V is same for all whereas Ι varies. Hence, it is more convenient to use the formula P = ( V 2 / R ). Joule’s Law: “The heat produced per unit time, on passing electric current through a conductor at a given temperature, is directly proportional to the square of the electric current”. To express heat produced in calories, the following relation given by Joule is used. W = JH, where W is mechanical energy in joule, H is heat energy in calorie and J = 4.2 joule / calorie is Joule’s constant or mechanical equivalent of heat. H ( cal ) = J ) joule ( t R 2 Ι
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4 - THERMAL AND CHEMICAL EFFECTS OF ELECTRIC CURRENT Page 2 4.2 Practical Applications of Joule Heating Joule heat is used in domestic appliances such as electric iron, toaster, oven, kettle, room heater, etc. It is also used in electric bulbs to produce light. The temperature of the filament of the bulb rises considerably when current flows through it and it emits light. Hence it should be made of a metal of high melting point such as tungsten whose melting point is 3380 ° C. Also it should be thermally isolated from the surrounding as far as possible. Only a small fraction of electrical power supplied to the bulb is converted into light. Normally a bulb emits 1 candela of light per watt of electrical power consumed. Another
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This note was uploaded on 11/28/2011 for the course PHYSICS 300 taught by Professor Smith during the Spring '06 term at ITT Tech Flint.

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