G10 Chapter 6 Heat and Temperature PDF

CHAPTER 6 HEAT AND TEMPERATURE The concept of temperature is very important for the physical and biological sciences. This is because the temperature of an object is directly related to the energies of molecules composing the object. Natural processes often involve energy changes...

CHAPTER 6 HEAT AND TEMPERATURE The concept of temperature is very important for the physical and biological sciences. This is because the temperature of an object is directly related to the energies of molecules composing the object. Natural processes often involve energy changes and the temperature is an indicator for these changes. Learni ng Outcom es It is expe cted that stude nts will identif y that ther mal energy is an intern al energy of a matte r. expla in why heat is  consid ered to be a form of energ y. ⚫ distin guish betwe en heat and tempe rature. examine thermometric properties of substances and differentiate thermometric properties of mercury and alcohol. examine linear, area and volume expansion. explain heat as the energy transferred between substances that are at different temperature s. apply basic knowledge and skill of thermal physics to daily-lif e phenom enon such as thermal expansion. 6.1 HEAT AND TEMPERATURE The sensations of hotness, warmness and coldness can be experienced by touching objects. Temperature is the quantity that  determines how cold (or) how hot the object is. The temperature of a hot body is higher than that of a cold body. To measure temperature accurately, we use instruments called thermometers. There is a relation between heat and temperature. The energy exchanged between an object and its surrounding due to different temperatures is defined as heat. Heat is the energy in transit. The unit of heat is the same as units of energy. Heat and temperature are different quantities. When a body at a higher temperature is in contact  with a body at a lower temperature, heat flows from the first to the second body. The motions and positions of molecules in matter result in the kinetic energy and potential energy. The total energy, that is, the sum of the potential energy and the kinetic energy, of molecules in matter is in fact the internal energy of that matter. Temperature is related to that internal energy. Temperature is a measure of the internal energy of molecules.  Key Words: internal energy, energy exchange 57 Reviewed Exercise Disting uish betwee n heat and temper ature. 6.2 TYPE S OF THER MOM ETER SMET Every thermomete r uses a physical property that varies with temperature. This property is referred to as the thermometr ic property of the thermomete r. For example, the thermometr ic property of a liquid-in-gla ss  thermometer is the thermal expansion of the liquid. Liquid-in-Glass Thermometer The liquid-in-glass thermometer consists of a thin glass bulb joined to a capillary tube with a narrow bore which is sealed at its other end. The liquid fills the bulb and the adjoining section of the capillary tube (Figure 6.1). When the bulb becomes warmer: the liquid in it expands more than the bulb so some of the liquid in the bulb is forced into the capillary tube. the threa d of liquid in the capill ary tube increa ses in length. the thinner the bulb wall is, the faster the response of the thermometer will be when the temperatu re changes. The liquid used usually contains mercury (or) coloured alcohol. Alcohol has a lower freezing points than mercury so it is more suitable for low-tem perature measur  ements. bulb capillary tube Figure 6.1 A liquid-in-glass thermometer Thermocouple Thermometer Thermocouple thermometers are electrical thermometers which make use of the voltage that develops when two different metals are in contact. This voltage varies with temperature. An iron wire and two copper wires may be used to make a thermocouple thermometer, as shown in Figure 6.2. One of the junctions is maintained at 0 °C and the other junction is used as the temperature probe. The voltmeter can be calibrated directly in °C. Because of the small size of a thermocouple junction, thermocouple thermometers are used to measure rapidly changing temperatures. In addition, they can be used to measure much higher temperatures than liquid-in-glass thermometers. Also, the voltage of a thermocouple can be measured and recorded automatically. 58 Textbook Physics Copper wire + Burning match mV Copper wire. Iron wire ice 1 Figure 6.2 A thermocouple thermometer Resistance Thermometer Resistance thermometer uses the fact that the electrical resistance of a metal (e.g platinum) wire increases with temperature. A resistance thermometer can measure temperatures accurately in the range -200 °C to 1200 °C and best for steady temperature s, but it is bulky. Thermometric substances can be solids, liquids (or) gases. They have physical properties that vary  continuously and linearly with temperature. These properties are called thermometric properties. Reviewed Exercise 1. State the physica l propert y that varies with tempera ture in (a) liquid-in -glass thermo meter, (b) thermoc ouple thermo meter. 2. Why the temperature range of a clinical thermometer is from 35 °C to 42 °C? Key Words: thermometric properties, temperature difference, voltage, electrical resistance offionaron BL 6.3 UNITS  OF TEMPE RATUR E (OR) TEMPE RATUR E SCALE S Temperature units depend on the scale used. The temperature scales most widely used today are Celsius (Centigrade), Fahrenheit and Kelvin scales. The SI unit of temperature is kelvin (K). To calibrate a thermometer, two reference points are chosen and the interval between these points is subdivided into a number of equal parts. The freezing point and boiling point of water under normal atmospheric pressure are chosen as reference points which are marked on the thermometer. The interval between these two points is divided into one hundred equal parts for the Celsius scale. If the freezing point of water (or) ice point is marked 0°C and the boiling point of water (or) steam point is marked 100 °C, the thermometer scale is the Celsius scale.  On the Celsiusscale, the ice point is 0°C and the steam point is 100 °C. On the Fahrenheit scale the ice point is 32° F and the steam point is 212°F. The interval between these two points is divided into 180 equal parts for the fahrenheit scale. On the Kelvin scale the ice point is 273 K and the steam point is 373 K. * Tk= Te +243 Tc = Temperature Scales 1 Celsius (Centigrade) (2) (TF-32) Fahrenheit (F) 50 Grade 10 Physics Textbook The relationship between the Celsius temperature T, and the Fahrenheit temperature T, is given by the equation T-(7-32) (or) 7,- 1.8 T +32 For example, normal body temperature is 98.6 "F. On the Celsius scale, this is (6.1) T.--(7,-32) - (98.6 - 32) =37.0 °C The relationship between the Celsius temperature T and kelvin temperature T, is given by (6.2) Tc+273 = Tx. Tc=27 °C Example (1) The room temperature is found to be 27 °C. What is the temperature in kelvin? Tx-T +273/  =27+27 3 = 300 K Example (2) The lowest air temperature recorded in the world is 184 K. This temperature was measured in Antarctica in 1983. What is the temperature in degree Celsius? T=184 K Tx-TC+273 To-TK-273 -184-27 3 --89°C Reviewed Exercise What temperature on the celsius scale corresponding to 104 "F, the body temperature of the person who is gravely ill? Key Words: body temperature, room temperature, freezing point, boiling point. 6.4 THERMAL EXPANSION OF SUBSTANCES When a substance is heated, its volume usually increases. The dimensions of the substance increase correspondingly. This increase in size can be explained in terms of the increased kinetic energy of the molecules. The additional kinetic energy results in each molecule colliding more forcefully with its neighbours. Therefore, the molecules push each other further apart and the substance which is heated increases in size. 60 Textbook Physics Grade 10 Increasing the temperature of a gas at constant pressure cause the volume of the gas to increase. This increase occurs not only for gases, but also for liquids and solids. In general, if the temperature of a substance increases, so does its volume. This phenomenon is known as thermal expansion. You may have noticed that the concrete roadway segments of a sidewalk are separated by gaps. This is necessary because concrete expands with increasing temperature. Without these gaps, thermal expansion would cause the segments to push  against each other, and they would eventually buckle and break apart. Linear Expansion Although two different metal bars of the same length are heated such that the increase in temperature is the same, the magnitudes of their expansion may not be the same. For example, the expansion of copper is one and a half times that of steel. Aluminium expands twice as much as steel does. The dependence of the change in length of an object on its original length and change in temperature is AlxlAT Al=al AT =AL LAT ΔΙ 1 a= X -7 1 AT l' = 1(1+αAT) (6.3) where Al = change in length AT = change in temper ature a= coeffici ent of linear expansi on 1= original length of the object l'= length of the object at T+ AT The coefficient of linear expansion is the change in  length per unit length for one degree change in temperatur e. The unit of a is per K, which can be written as K1. The value of a for some materials are given in Table 6.1. Table 6.1 The value of a for some materials per kelvin (K) Material a (K-1) Celluloid 1.09 × 104 Steel 1.27 X 10-5 Copper 1.70 x 10-5 Aluminium 2.30 X 10-5 Diamond 1.00 × 10-6 Glass 8.30 × 10-6 Platinum 8.90 × 10-6 Grade 10 Physics Textbook The following example (3) illustrate s the  importan ce of linear expansi on. Example (3) One roadbed of a steel bridge is 12.80 m long. If the temperature varies from 25 °C to 35 °C during a day, what is the difference in lengths at those temperatures? The road is supported by steel girders. For steel, a = 1.27 × 10-5 K-1. 1= 12.80 m, a = 1.27 x 10-5 K-1 AT = 35 °C -25 °C 1 = 10°C = 10 K Here we write AT = 10°C = 10 K, since AT (35+273)-(25+273) K=10K ΔΙ = al AT = 1.27×1 05x 12.80 × 10 = 0.001 62 m or 0.162 cm 69.21=698 Hence, the change in the roadbed length due to linear expansion must be allowed for the design of the bridge so as not to damage the bridge. Example (4) The length of a metal bar having coefficient of linear expansion a is l at the  temperature T. What is the length of that metal bar at the temperature T + AT? The change in length due to the temperature change AT is ΔΙ = αΙΔΤ Therefore, the length of the metal bar at T + AT is 1.22 10 1' = 1 + Al =1+al AT Area Expansion and Volume Expansion = 1 (1+α AT) The relations analogous to the one which gives the increase in length Al for the increase in temperature AT can be derived for the area expansion and volume expansion. The relations obtained are and bon AA BAAT (for area expansion) AV=y VAT (for volume expansion) ΔΑ AAT (6.4) (6.5) In these equations, ẞ is the coefficie nt of area expansio n and y is the coefficie nt of volume expansion. The coefficient of area expansion of a substance is the change in area per unit area for one degree change in  temperatur e. The coefficient of volume expansion of a substance is the change in volume per unit volume for one degree change in temperature. Unit of dB, Y=> Per Anomalous Expansion of Water 1223 Lelvin (K) ဆင်အ 0=Av VAT Generally all substances expand on heating and contract on cooling. But in the case of water the behavior is different, when water at 0°C is heated its volume decreases up to 4 °C and density increases. At 4°C the density becomes maximum and beyond this temperature volume start to increase. This unusual expansion of water is called anomalous expansion of water. 62 62 Ice Water at O'C Ic 2°C 3°C 4°C Figure 6.3 Temperature in an ice covered lake 1000 999 998 Density (kg/m3) 997 996 995 45 0 10 15 20 25 30 Temperature (°C) Figure 6.4 The change in density of water with temperature ‫سکرائی۔‬ ‫کرده‬ Example (5) The area of a metal plate is A, at the temperature T and A, at T2. If T,>T,, obtain 2 the relation between A, and A,. The coefficient of area expansion of metal is ß. AA = A2- A AT = T2-T1 Since AA=B AAT A-A-B A, (T2-T) Reviewed Exercise A2 = A, [1+B (T2- T)] ‫ין‬ Obtain the relations hip between the coefficie nt of linear expansi on and the coefficie nt of area expansion of a substance. Key Wo rds: ther mal exp  ansi on, ano mal ous exp ansi on 63 rade Tu SUM Tempe rature is a measur e of hotness (or) coldne ss of a body. Heat is a form of energy. It is the energy exchanged between an object and its surrounding due to different temperatur es. Coefficient of linear expansion is defined as the change in length (of a substance) per unit length for one degree change in temperatur e. Coefficient of area expansion is defined as the change in surface area (of a substance) per unit area for one degree change in temperatur e. Coefficient of volume expansion is defined as the change in volume (of a substance) per unit volume for one  degree change in temperatu re. EXERCISES 1. Complete the sentences below using words from the following list. expans ion, temper ature, voltage (a) The degree of hotnes s of an object is a meas ure of its (b) The liquid- in-gla ss therm omete r makes use of the changes. (c) In a thermocouple thermometer, a change of of the liquid when its causes its to change. 2. (a) A liquid-in-glass clinical thermometer is marked with a scale from 32°C to 42°C which covers a distance of 80 mm. A  liquid-in-glass laboratory thermometer is marked with a scale from 0 °C to 100 °C, which covers a distance of 160 mm. State and explain which thermometer; (i) has the greater range, (ii) is more sensitive. (b) The following results were obtained when the voltage of a thermoco uple thermome ter was measured at different temperatur es. Voltage (millivolt) 0 1.4 2.8 4.3 5.7 6.9 Temperature (°C) 0 20 40 60 80 100 (i) Plot a graph of the voltage on the y-axis against temperatur e in °C on  the x-axis. (ii) Use the graph to determine the voltage at 50°C and the temperat ure at 5.0 mV. 3. A solid expan ds when heated. What happen s to its (a) mass, (b) volume , (c) density ? 4. If the unit of the coefficien t of linear expansio n is changed from per K to per 'F, does the numerical  value of that coefficien t change? 5. If the temperat ure of a room is found to be 30 °C, what is the room temperat ure as measure d on the Kelvin scale? 6. The coefficien t of volume expansion of Pyrex glass is one-third that of ordinary glass. Which glass can stand more thermal strain? 7. At what temper atures are the reading s on the Fahren heit and Celsius scales the same? 9007 AL=# 8. A steel railroad track is 20 m long at 20 °C. How much longer is it at 40 °C? (The coeffi cient of linear expans ion of steel is 1.27 x 10 K) 9. A steel railroad track is 30 m long at 0 °C. How much shorter is it at -20 °C? (The coeffic ient of linear  expans ion of steel is 1.27 x 10-5 K1) 538 Srade TU 10. A heat-resistant glass at 15 °C is fully filled with 250 cm3 of glycerine. If the temperature increases to 25 °C how much glycerine overflows? The coefficient of volume expansion of glycerine is 5.1 x 10 K and that of heat-resistant glass is 0.09× 10 K! CONCEPT MAP energy exchanged Heat Temperature due to different temperatures when substance is heated measured by Thermometer Thermal expansion requires requires Scales (units) Celsius (C) requires Fixed point Ice point Kelvin (K) Steam point Fahrenheit (F) Linear expansion expansion Area Volume expansion Thermometric substance that has a physical properties which vary with temperature such as such as such as Volume of fixed mass of liquid Electrical resistance Voltage Liquid in glass thermometer Resistance thermometer Thermocouple thermometer 65 Lacie