Tiêu đề Med-RM_Phy_SP-2_Ch-11-Thermal Properties of Matter.pdf ✅

Chapter Contents Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 TEMPERATURE AND HEAT Temperature is a relative measure, or indication of hotness or coldness. The SI unit of temperature is kelvin (K), whereas degree celsius (°C) is a commonly used unit of temperature. Energy transfer that takes place solely because of a temperature difference is called heat. The SI unit of heat energy transferred is expressed in joule (J). In conventional system, the unit of heat is called calorie (cal) and 1 cal = 4.186J. A hot body has more internal energy than another identical cold body. MEASUREMENT OF TEMPERATURE The instrument used to measure temperature called thermometer. To construct a thermometer, we use any thermometric property of any substance which varies proportional to temperature. To measure the temperature we assume to fixed temperature i.e freezing point and boiling point of water at atmospheric pressure and given them number 0 and 100 respectively. The temperature difference between 0 and 100 divide in 100 equal parts each part called 1 degree. The value of degrees measured on different scales. 100°C C 0°C 373 K K 273 K 212°F F 32°F Celsius scale Kelvin scale Fahrenheit scale Measured temperature – LFP constant UFP – LFP  Temperature and Heat Measurement of Temperature Thermal Expansion Specific Heat Capacity Calorimetry Change of State Heat Transfer Newton’s Law of Cooling Black Body Spectrum Some Important Formulae Chapter 11 Thermal Properties of Matter
Aakash Educational Services Limited - Regd. Office: Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 108 Thermal Properties of Matter NEET 0 273 32 Measured temprature – LFP 100 0 373 273 212 32 UFP – LFP CK F       CK F 273 32 55 9     Example 1 : What is that temperature at which the Celsius and Fahrenheit scale give the same temperature value? Solution : Let that temperature be x, then – 0 – 32 100 – 0 212 – 32 C F ∵  Now, put C = F = x or, – 0 – 32 100 – 0 180 x x  or, x = – 40 So, at – 40°C, Fahrenheit reading is also – 40°F. Example 2 : What is that temperature at which the Fahrenheit reading is double that of the Celsius reading? Solution : Let the Celsius reading be x then the Fahrenheit reading will be 2x. So, – 0 2 – 32 100 – 0 212 – 32 x x  or, 2 – 32 5 9 x x  or, 9x = 10x – 160 or, x = 160°C So, 2x = 320°F. Example 3 : Suppose that on a temperature scale X, water boils at –60°X and freezes at –180.5°X. What would a temperature of 350 K be on the X-scale? Solution : Upper fixed point = –60 °X Lower fixed point = –180.5 °X Suppose 350 K = x °X, then 100 15.273350 )5.180(60 )5.180(    x  or, 7685.0 100 85.76 5.120 5.180  x  or, x = (120.5) (0.7685) – 180.5 = –87.9  350 K = –87.9°X
Aakash Educational Services Limited - Regd. Office: Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 NEET Thermal Properties of Matter 109 Different Types of Thermometers 1. Mercury Thermometer : Fahrenheit was the first to choose mercury as the thermometric substance on account of its many advantages. It doesn’t wet glass, can be easily obtained pure, remains liquid over a fairly wide range, has a low specific heat and high conductivity, it is opaque and its expansion is approximately uniform and regular. The range of an ordinary mercury thermometer is limited by the fact that mercury freezes at – 38.87°C and boils at 356°C but the upper limit can be realised to about 500°C by filling the top of the tube with nitrogen under pressure. If instead of mercury we use Alcohol, we can measure as low as – 111°C, and petroleum ether can measure as low as – 190°C. 2. Constant Volume Gas Thermometers : Gas thermometers are more sensitive than liquid thermometers as expansion of gases is more than that of liquids. They are most accurate thermometers. Their temperature range is between – 260°C to 1600°C. The pressure and temperature are related as Pt = P0(1 + t) 0 100 0 – – 0 – 100 – 0 P P t t P P  0 100 0 100 C P P t t P P     3. Platinum Resistance Thermometer : The platinum resistance thermometer works on the principle of Wheatstone bridge. Rt = R0 [1 + t] then the resistance and temperature are related as 0100 0 100 0 0      t RR t RR 0 100 0 100 C R R t t R R     These thermometers can measure temperature from – 182°C to 1200°C. 4. Thermoelectric Thermometers : It is based on Seebeck effect. In this thermometer two distinct metals are joined to form a closed circuit known as thermocouple. One junction is generally kept in ice and the other junction is placed on the body whose temperature is to be calculated. A current flows in the circuit given by current, I = aT + bT2 where a, b are constants and T is temperature. Such thermometers have several advantages. They can easily measure fast changing temperatures. Since temperature is in the form of electric current a computer can do the recording of changing temperature over a time interval and it can even be sent through satellites etc. from one place to other. Different parts of our body has different temperature and it can easily make temperature diagrams of our body to be shown on a computer. It has very wide application in engineering and medical world. The temperature range is (–200°C to 1600°C) 5. Pyrometers : These devices are based on Stefan’s law which says radiations received is proportional to fourth power of the absolute temperature of the body. It measures very high temperatures [pyre means fire] say that of a furnace etc. The temperature of the sun is measured by pyro-helio-meter. 6. Vapour Pressure Thermometers : Its use is based on the well-known fact that the vapour-pressure of a liquid varies uniquely with the temperature. The method consists simply in measuring the vapour pressure of a certain liquid at the required temperature.
Aakash Educational Services Limited - Regd. Office: Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 110 Thermal Properties of Matter NEET 7. Magnetic Thermometers : Temperatures below 1 K are measured by ‘magnetic thermometers’ which uses the principle of adiabatic demagnetisation. It is based on Curie-Law according to which magnetic susceptibility of a paramagnetic salt varies inversely proportional to its absolute temperature. Example 4 : The pressures of the gas filled in the bulb of a constant-volume gas thermometer are 66 cm and 88 cm of mercury column at 0°C and 100°C respectively. When its bulb is immersed in a liquid placed in a vessel, its pressure is 82.5 cm of mercury column. Calculate the temperature of the liquid. Solution : Pressure at 0°C is P0 = 66 cm, pressure at 100°C is P100 = 88 cm. Pressure at unknown temperature t is Pt = 82.5 cm. According to formula, the unknown temperature is t = 100°C ×           0100 0t PP PP = 100°C ×         6688 665.82 = 100°C × 22 16.5 = 75°C Example 5 : The following observations were recorded on a platinum resistance thermometer : Resistance at melting point of ice = 3.70 ohm, resistance at boiling point of water at normal pressure = 4.71  and resistance at t°C = 5.29 ohm. Calculate (i) Temperature coefficient of resistance of platinum, (ii) Value of temperature t. Solution : (i) Temperature coefficient of resistance is given by  = 0 100 0100   R RR = 10070.3 70.371.4   = 370 01.1 = 2.73 × 10–3 per °C (ii) For temperature t, we have t = 100°C × 0100 0t RR RR   = 100°C × 70.371.4 70.329.5   = 100°C × 01.1 59.1 = 157.4°C THERMAL EXPANSION Most of the materials expands, when their temperature increases, with few exception such as carbon. Thermal Expansion and Interatomic Energy We can understand thermal expansion qualitatively on the molecular level. Consider the interatomic forces in a solid as springs, as in figure (a). There is an analogous relationship between spring forces and interatomic