Chapter 4 Heat Heat and Temperature Heat is a form of energy Heat is the energy of random motion of molecules constituting the body. It flows from a hot body to a cold body. Unit of heat is joule (J) and calorie (cal). 1 cal = 4.186 J rounded to 4.2 J. Temperature is the degree of hotness or coldness of a body Temperature is the parameter which tells the thermal state of a body. It determines the direction of flow of heat when two bodies at different temperatures, are placed in contact. Unit of temperature is kelvin (K) and degree celsius ( 0 C). By adding 273 to the temperature in degree celsius unit, temperature is converted into kelvin unit. Thermal Expansion The expansion of a substance on heating is called the thermal expansion of that substance. Linear expansion,(length wise) superficial expansion (area wise) and cubical expansion (Volume wise) 10 x10 Learning TM 2 1. Heat can be transferred by (a) Conduction ( in solids, without movement of atoms ) (b) Convection ( In liquids and gases, with movement in one direction, upwards only) (c ) Radiation 10 Transfer of Heat (a) Conduction is the process of transfer of heat from hot to cold end from particle to particle of the medium, without leaving their position. In conduction, heat is transferred only through vibration of atoms, and no change in their position is involved. (c ) In Radiation, heat passes directly from one body to another without involving the medium. In vacuum, heat is transferred only through Radiation method. Heat from the Sun is the best example of radiation method. Conductors of Heat are substances that allow heat to pass through them. Silver is the best conductor, followed in decreasing sequence by copper, aluminium, zinc, brass, iron, steel, lead, constantan, water, ice, rubber, wood, asbestos, ebonite and air. All liquids, except mercury, are very poor conductors. (b) In Convection, heat is transferred through the actual movement of particles in the medium, vertically, upwards in one direction only. Convection method does not function in a vacuum. 10 x10 Learning TM 6 10 x10 Learning TM Page 1
Celsius and Kelvin: K= 273 + C or C = K -273 All three scales: C/5 = F-32/9 = K- 273 / 5 Three Scales of Temperature Celsius scale has 0 as freezing point of water to 100 degrees as boiling point of water. It has 1/100 th part of interval between the two points One centigrade degree = 9/5 fahrenheit degree and One fahrenheit degree = 5/9 centigrade degree For conversion C/5 = F-32 /9 Kelvin is called the absolute scale of temperature. Zero is the point at which molecular motion ceases and average kinetic energy flow is zero. This is also called the absolute zero. Fahrenheit scale has 32 degree as freezing point, and 212 degree as boiling point. It has 1/180 th part of interval between the two points. The size is the same 0 to 100 of the Celsius scale. Gas laws are made simpler due to Kelvin scale 10 x10 Learning TM 5 Comparative expansions of three states of matter Superficial and Cubical expansion Gases Expand most on heating Cubical expansion Linear, Superficial, and Cubical Expansions in solids Liquids Solids expand the least Expand less than Gases but more than solids 10 x10 Learning TM 3 1. State similarities and differences between the laboratory thermometer and the clinical thermometer. Answer. The difference between a clinical thermometer and a laboratory thermometer is as follows: Clinical thermometer Laboratory thermometer 1. A clinical thermometer consists of a long, narrow, uniform glass tube. It has The narrow uniform glass tube of a laboratory thermometer is much longer 10 x10 Learning TM Page 2
a bulb at one end. This bulb contains mercury. Outside the bulb, a small shining thread of mercury can be seen. 2 It uses a Celsius scale indicated by 0 C ( read degree Celsius ) 3 It reads temperature from 35 C to 42 C. 4 Between each degree are five marks that equal 1/5 = 0.2 0 C. 5 The normal temperature of human body is 37 0 C 6. For measuring body temperature, the bulb of the thermometer should be placed in the mouth below the tongue. In can also be placed under the armpit. than a clinical thermometer. Laboratory thermometer also uses a Celsius scale indicated by 0 C ( read degree Celsius ) It reads temperature from 10 0 C to 110 0 C. Between each degree are ten marks that equal 1/10 = 0.1 0 C. The normal boiling point of water at sea level is 100 0 C. For measuring temperature the thermometer should be held straight. Its bulb should be full immersed in the liquid that is being measured. 2. Give two examples each of conductors and insulators of heat. Answer. All metals are good conductors of heat. Silver is the best conductor but is very expensive. Copper is used as a conductor in electric wires. Insulators prevent or stop conduction of heat. Rubber and wood are examples of insulators. 3. Fill in the blanks : 3.1 The hotness of an object is determined by its temperature. 3.2 Temperature of boiling water cannot be measured by a clinical thermometer. 3.3 Temperature is measured in degree Celsius ( 0 C). 3.4 No medium is required for transfer of heat by the process of Radiation. 3.5 A cold steel spoon is dipped in a cup of hot milk. It transfers 10 x10 Learning TM Page 3
heat to its other end by the process of conduction. 3.6 Clothes of dark colours absorb heat better than clothes of light colours. 4. Match the following: Correct match is given here. 4.1 Land breeze blows during. Day 4.2 Sea breeze blows during. Night 4.3 Dark coloured clothes are preferred during. Winter 4.4 Light coloured clothes are preferred during. Summer 5. Discuss why wearing more layers of clothing during winter keeps us warmer than wearing just one thick piece of clothing. Answer. During winter wearing layers of clothing is warmer because air gets trapped between the layers and becomes warm because it cannot escape. If one thick cloth is worn, there is no space for trapping the air. Therefore, wearing more than one layer of cloth in winter gives more warmth than one thick cloth. 6. Answer. In Figure 4.13 heat is being transferred by conduction from the gas stove to the pan. Heat is being transferred by convection in the water inside the pan. Heat is being transferred by radiation from the flame to the air around it, from the heated water to the air above it. 7. In places of hot climate it is advised that the outer walls of houses be painted white. Explain. Answer. White colour reflects back the rays of all other colours. It does not absorb any colour or warmth. Therefore, the outer walls of houses are painted white in hot climate regions, to keep the inner rooms cool. 8. One litre of water at 30 C is mixed with one litre of water at 50 C. The temperature of the mixture will be (a) 80 C (b) more than 50 C but less than 80 C (c) 20 C (d) between 30 C and 50 C 10 x10 Learning TM Page 4
9. An iron ball at 40 C is dropped in a mug containing water at 40 C. The heat will a) flow from iron ball to water. b) not flow from iron ball to water or from water to iron ball. c) flow from water to iron ball. d) increase the temperature of both. 10. A wooden spoon is dipped in a cup of ice cream. Its other end a) becomes cold by the process of conduction. b) becomes cold by the process of convection. c) becomes cold by the process of radiation. d) does not become cold. 11. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that (a) copper bottom makes the pan more durable. (b) such pans appear colourful. (c) copper is a better conductor of heat than the stainless steel. (d) copper is easier to clean than the stainless steel. 10 x10 Learning TM Page 5