3 Heating Effect of Electric Current

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1 44 UNIT 3 Heating Effect of Electric Current Learning Objectives To understand different effects of electric current through discussion and demonstration To learn applications of heating effect of electric current bydemonstration and discussion To know Joules law of heat. 3.1 Effects of Electrical Energy Resistance in electric circuits produces heat just as mechanical friction produces heat. This is called the heat of resistance. 3.2 Heating Effect of Electric Current When electric current (i.e. flow of free electrons) passes through a conductor, there is a considerable friction between the moving electrons and the molecules of the conductor. The electrical energy supplied to the conductor to overcome this electrical friction (which we refer it as resistance) is converted into heat. This is known as heating effect of electric current. The heating effect of electric current is utilised in manufacturing of many heating appliances such as electric heater, electric kettle, electric toster, soldering iron etc. The basic principle of these appliances is the same. Electric current is

2 Paper - I Elements of Electrical Engineering 45 passed through a high resistance(called heating element), thus producing the required heat. The heating element may be either nichrome wire or ribbon wound on some insulating material that is able to withstand heat. 3.3 Joule s Law According to Joules law, the heat produced in a current carrying conductor is directly proportional to the square of the current and to the resistance of the conductor and to the time of flow of current. Heat producted ( H ) I 2 Rt H = I 2 Rt / J calories H = Heat produced in calories I = Current in Amperes R = Resistance of the conductor in ohms. T = Time for flow of current in seconds and J = Joules mechanical equivalent of heat which is a constant. = 4.2 Joules / calories. i.e. 1 calories of heat = 4.2 Joules. 1 Joules = 0.24 calories Joules = 4.2 x calories Calories = 0.24 x Joules Calories is the amount of heat required to rise the temporature of 1 gm of water through 1 o c Heat Produced by flow of current When electricity is passed through a wire, heat is produced due to the collision of electrons in the wire. The amount of heat (friction) produced depends on square of current, resistance of the wire material and time for which Current has passed through the wire as explained in Joules law. (a) Filament or Incandescent Lamp Fig: 3.1 represent a metal filament lamp. It consist a fine wire or filament made of tungsten and is placed in a glass-bulb containing inert gas. The lamp emits when current passes through the filament.

3 46 Pins of Bayonet Cap Brass or Aluminium Cap Exhaust Tube Support Wires Heat Reflectior Glass Stem Lead in Wires Molybdenum Glass Support Filament(Tungsten) Gas Glasscover Fig. 3.1 Metal Filament Lamp (b) Fluorescent Lamp It mainly consists of fluorescent tube, choke (ballast) and a starter as shown in Fig The fluorescent tube coated with fluorescence powder and consists of mercury vapour plus small amount of argons gas. The choke steps up the supply voltage to start the fluorescent tube and regulates the current afterwards. Starter provides voltages kick for ignition. Starter Fluorescence Coating Choke Supply Fig. 3.2 Fluorescent Lamp (c) Electric Kettle Electric kettles are used to boil water, milk, tea etc. Electric kettles may be of saucepan type or immersion type as shown in Fig.3.3. In some type of kettle an ejector type safety device is incorporated to prevent low quality of liquid in it.

4 Paper - I Elements of Electrical Engineering 47 Inner Sealing Washier M.I. Heating Element Kettle heating element (a) Saucepan Type 1. Bottom cover, nut and water 2. Heating element 3. Central bolt of the body. 4. Pressure plate 5. Absestos sheet 6. Bottom cover 7. Handle 8. Top cover 9. Ebonite leg 10. Outlet socket (b) Immerse Type (d) Electric Cooker Fig. 3.3 Electric Kettles It is a simple form of heat producing appliance for cooking. It mainly consists of a heating elements and heater plate as shown in Fig 3.4. The heating element is made of nichrome wire and it may be either exposed type or enclosed type. The heater plates are usually made of porcelain material with grooves on one side to accommodate the supply, heat will be produced in it. 3.4 Electric Cooker

5 48 The basic principle of electric iron is to convert electrical energy into heat energy, with which the clothes will be pressed. Fig. 3.5 shown the various parts of an electric iron. (i). Sole Plate or Base Plate: It is a chromimum coated flat bottom surface. It receives heat from the heating element and is used to press or iron the electric clothes. Power Cord Sleeve Handle Top Cover Heel Plate Pressure Plate Asbestos Sheet Heating Element Electrical Insulation Sole Plate Fig. 3.5 Electric Iron

6 Paper - I Elements of Electrical Engineering 49 (ii) Heating element: It is made of nichrome strip or wire placed in between two mica sheets. It is placed over the base-plate to transfer the heat developed to it. (iii) Asbestos Plate: It is placed over the heating elements to prevent the heat to the sole-plate. (iv) Pressure Plate: It is made of cast iron and it clamps the heating elements to the sole-plate. An electric iron also consists a heal plate to fix the terminals and a top cover to hide all parts and at the same time serves the decoration. The handle is usually made of ebonite (or wood). An automatic electric iron consists a thermostat which controls the temperature is required. (e) Space Heater It is basically consists of a heating element and a heat reflector as shown in Fig.3.6. The heating element is made of nichrome and is wound on a ceramic or china clay tubes. The reflector is in the form of semi-circle or concave and the inner surface is chromium plated or epoxy powder painted galvanised steel to reflect the heat. Space heaters also known as room heaters. These heaters are suitable for continuous or intermittent use. These heaters are ideal for use in locations where direct heat is required for personnel without affecting the ambient room temperature and provide solar type zone hating with the efficiency if radiant heaters. Instant visible heat, absolute silent operation, long range heat projection, light weight and easy to install are some of the features of these heaters. The heaters are available with or without fan assistance for additional heating. Stand Handle Polished Metal Body Reflector Ends Connector Porcelian Cleat Porcelian or china clay rod with element of Nichrome wire Fig. 3.6 Space or Room Heaters

7 50 (f) Geysers Geysers provide hot water in a house at different points such as bathrooms, kitchens, wash-basins etc. It mainly consists of immersion element operates in direct contact with water. Geysers are more efficient water heaters. Fusible Plug Vent Pipe 12 mm Water Level Heating Element (w) Glass Wool Outer Case Inner Tank Drain Pipe Cover Outlet 12 mm Inlet 12 mm Thermostat Pressure Release valve (g) Infrared Lamp Fig 3.7 Geyser It is nothing but incandescent lamps covered with a red coloured infrared filter made of silicon. The filter emits the heat radiation developed by the incandescent lamp at the frequency of infrared radiation. The infrared radiation can be focussed at a point by some means. It is used in medical fields for the treatment of muscular pains or nervous disorder of human beings Practical Applications of Heating Effects of Electric Current The heat produced in a high resistance wire is utilized in the following appliances: (1) Soldering Iron (2) Electric Iron (3) Electric Kettle (4) Electric stove (5) Immersion water heater (6) Geyser (7) Drying machine (8) Domestic oven (9) Cooking set (10) Industrial furnaces (11) Electric welding and Brazing (12) Electric lamps (13) Radiator or Room Heater (14) Incumbator etc.

8 Paper - I Elements of Electrical Engineering Simple Problems (1) A wire of resistance10 is kept in water in a tin and a current of 2 amperes is passed through this wire for half an hour. Calculate the heat received by water. Heat produced H = 0.24 I 2 Rt calories. = 0.24 x 2 2 x 10 x 30 x 60 Calories. = 1728 Calories. = Kilo - Calories. (2) The mass of water in a calorimeter was 250 gms. A coil of resistance 15 ohms was immersed in water and a current of 2 amperes is passed through this wire for 15 minites. Calculate the final temparature of water, if its initial temparature was 25 o C. neglect all losses. Heat produced in Calories = H = 0.24 I 2 R t But in physics heat = 0.24 x (2) 2 x 15 x 15 x 60 = 12, 960 Calories. = Mass x Specific heat x change of temp in o C. = Mst Calories and Where t = (t 2 ) o C 12,960 = 250 x 1 x (t 2 ) t = 12,960 / 250 x 1 = o C (t 2 ) = o C (3) An immersion water heater of resistance 25 ohms is kept in awater bucket and connected to 230 volts supply mains. Mass of water is 10 kg. Initial temparature is 25 o C. Find the time taken for the water to reach boiling point. neglect all the losses. Heat produced H = 0.24 V 2 / R x t = M x s x (t 2 ) t = m x s x (t 2 )x R / 0.24V 2 t = 10,000 x 1 x (100-25)x25 / 0.24 x (230) 2 = sec. or t = / 60 x 60 = 0.41 hours. = 0.41 x 60 = 24.6 minutes.

9 52 4) On the name plate of electric kettle, it is written as 750 watts and 220 volts, Determine the thermal efficiency of the kettle, If it takes 20 minutes to raise the temparature of one kg. of water from 20 o C to boiling point. Thermal efficiency of kettle = Output in calories x 100 / input calories = Caolires actually utilized by water x 100 / Intake calories by the kettle. = m x s x (t 2 )x 100/0.24x I 2 R x t = 1000x1x(100-20) / 0.24 x 750 x 20 x 60 = 1000 / 27 = % Key Concept Heating effect of electric current Application of joules law of heat Simple problems to strengthen the theoretical understanding Activity Collect technical broachers of appliances available in market using heating effect. Find out heat produced by Electric Iron/Room Heater/Hot plate/geyser by using different instruments like voltmeter, ammeter, watt meter and calorie meter. Observe how the temperature is proportional to Power rating heating appliance. Short Answer Type Questions (1) State Joules law (2) Give the names of 5 electrical Appliances where heating effectis utilised. Project Oriented / OJT related Questions (1) List out the appliances working on heating effect of electric current. (2) Find out how Water Heater / Guyser, Microwave Oven are working.