Title THERMAL PHYSICS.pdf ✅

 Digital www.allendigital.in [ 3 ] Temperature and Thermal Equilibrium A branch of science which deals with the measurement of temperature of a substance is known as thermometry. Temperature Temperature is relative measure or indication of degree of hotness or coldness of a body. It is a scalar physical quantity with S.I. unit Kelvin (K). Example: Normal temperature of human body is 310. 15 K = 37°C = 98.6°F NTP or STP implies 273.15K = 0°C = 32°F Heat : When two bodies of different temperature are made in contact, the energy will flow from higher temperature body to lower temperature body, which is known as heat. Note : (i) For heat transfer, there must be temperature difference between bodies in contact. (ii) Heat is a energy in transit. (iii) Unit : SI unit : joule (J) ; CGS :- erg ; Practical unit :- calorie (iv) Unit Conversion : 1 Joule = 107 erg 1 calorie = 4.186 Joule  4.2 Joule Thermal equilibrium Two bodies are said to be in thermal equilibrium, when no (net) heat flows from one body to the other i.e. when both the bodies are at the same temperature. If TA = TB, then net heat flow between them is zero. Zeroth law of thermodynamics If objects A and B are separately in thermal equilibrium with a third object C (say thermometer), then objects A and B are in thermal equilibrium with each other. Zeroth law of thermodynamics introduces the concept of temperature. Two objects (or systems) are said to be in thermal equilibrium if their temperatures are same. In measuring the temperature of a body, it is important that the thermometer should be in thermal equilibrium with the body whose temperature is to be measured. Illustration 1: If two bodies have same temperature but the amount of heat energy in both of them is different. Will there be a flow of heat energy between them if both bodies are placed in contact? Solution: As the temperature of the two bodies are same they are in thermal equilibrium which means there would be no transfer of heat energy between them. Flow of heat energy takes place when there is temperature difference between the bodies which are in contact with each other. Thermal Properties of Matter C A B 01 Thermal Physics
NEET : Physics [ 4 ] www.allendigital.in  Digital Temperature Scale A thermometer is a device used to measure the temperature of a body. Property of a material which changes with the change in temperature is called Thermometric Property like expansion of liquid or gas. Different Scales of temperature A thermometer can be graduated into following scales : (a) The Centigrade or Celsius scale (°C) (b) The Fahrenheit scale (°F) (c) Kelvin scale (K) Scale Symbol Lower fixed point (LFP) Upper fixed point (UFP) Number of divisions on the scale Celsius °C 0°C 100°C 100 Fahrenheit °F 32°F 212°F 180 Kelvin K 273.15K 373.15K 100 Relation between different scale : • Temperature on one scale can be converted into other scale by using the following identity. Reading on any scale LFP UFP LFP − − = Constant for all scales • All these temperatures are related to each other by the following relationship C 0 F 32 K 273 A LFP 100 212 32 373 273 UFP LFP − − − − = = = − − − C F 32 K 273 A LFP 5 9 5 UFP LFP − − − = = = − Note: (i) To calculate difference in scale Difference in temperature: T H C F K B.P. M.P. 100 180 100      === − C F K 5 9 5    = = (ii) The Celsius and Kelvin scales have different zero points but the same size degrees. Therefore, any temperature difference is the same on the Celsius and Kelvin scales. (T2 – T1)°C = (T2 – T1) K OR T°C = TK (iii) Reading of one division  1 Total number of divisions If number of divisions increases → Reading of one division decreases Order of one division reading 1°F < 1°K = 1°C 100 divisions 180 divisions 100 divisions U.F.P. L.F.P. Celsius Scale Fahrenheit Scale Kelvin Scale °C °F K
Thermal Physics  Digital www.allendigital.in [ 5 ] (iv) Value of temperatures in Celsius and Fahrenheit scale may be same. Value of temperature in Fahrenheit and kelvin may be same. But Value of temperature in Celsius scale and kelvin scale can never be same. C K 273 5 5 − =  K = °C + 273 Scale of Temperature Graph between of temperature of a body in degree Celsius and degree Fahrenheit C F 32 5 9 − = 5 160 C F 9 9 = − ...(i) y = mx + c ...(ii) From equations (i) and (ii) Slope = m = tan  = 5 9 y intercept = c = 160 9 Absolute Zero Equals to –273.15°C or 0 Kelvin At this temperature kinetic energy of gas molecules becomes zero. Negative temperature is not possible in kelvin scale. Illustration 2: Temperature of a patient is 40°C. Find the temperature on Fahrenheit scale? Solution: F 32 40 0 F 104F 180 100 − − =  = Illustration 3: At what temperature is the Fahrenheit scale reading equal to twice of Celsius? Solution: F 32 C 0 2x 32 x 0 x 160 180 100 180 100 − − − − =  =  = Illustration 4: The lower and upper fixed points of a faulty thermometer are 5 W and 105 W. If the thermometer reads 25 W, what is the actual temperature in Celsius scale? Solution: 25 5 C–0 C 20 C 100 100 − =  =  Illustration 5: A thermometer with an arbitrary scale has the ice point at –20° and the steam point at 180°. When the thermometer reads 5°, a Centigrade thermometer will read Solution: C 0 t ( 20) 100 0 180 ( 20) − − − = − − − (Here t = 5°) C 5 20 C 12.5C 100 200 +  =  = Illustration 6: The temperature of an iron piece is raised from 30°C to 90°C. What is the change in its temperature on the Fahrenheit scale and on the Kelvin scale? 100°C 32° F 212°F (°F) (°C) 160 9
NEET : Physics [ 6 ] www.allendigital.in  Digital Solution: C = 90° – 30° = 60°C Temperature difference on Fahrenheit Scale ( ) 9 9 F C 60 C 108 F 5 5  =  =  =  Temperature difference on Kelvin Scale  =  = K C 60K Thermal Expansion When matter is heated without any change in its state, it usually expands. According to atomic theory of matter, asymmetry in potential energy curve is responsible for thermal expansion. As with rise in temperature the amplitude of vibration increases and hence energy of atoms increases, hence the average distance between the atom increases. So, the matter as a whole expands. According to atomic theory of matter, asymmetry in potential energy curve is responsible for thermal expansion as with rise in temperature say from T1 to T2 the amplitude of vibration and hence energy of atoms increases from E1 to E2 and hence the average distance between the from r1 to r2. Due to this increase in distance between atoms, the matter as a whole expands which is known as thermal expansion. Had the potential energy curve been symmetrical, no thermal expansion would have taken place in spite of heating • Thermal expansion is minimum in case of solids but maximum in case of gases because intermolecular force is maximum in solids but minimum in gases. • Solids can expand in one dimension (linear expansion), two dimension (superficial expansion) and three dimensions (volume expansion) while liquids and gases usually suffers change in volume only Linear Expansion: When a solid is heated and its length increases, then the expansion is called linear expansion. d  dT and d  0 d = 0dT d = 0dT If  is constant • Change in length  = 0T (0 = Original length, T = Temperature change) • Co-efficient of linear expansion, 0 T   =  If 0 = 1 and T = 1°C ; Then,  =  •  – 0 = 0T Final length  = 0 + 0T • Unit of  is °C-1 or K–1. It’s dimension is [ −1  ] r2 min. r1 avg. r2 avg. U E2 E1 r1 min. r1 max. r2 max. r l0 ሺT + ΔTሻ°C On heating l = l0 + Δl