Tiêu đề 11.Thermal Properties of matter-f.pdf ✅

1 | P a g e NEET-2022 Ultimate Crash Course PHYSICS PHYSICS-Thermal Properties of Matter
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3 | P a g e Points to Remember 1. At –40°C temperature, readings of centigrade and Farhenheit scales are the same. 2. It is more dangerous to get burnt by steam than by boiling water at same temperature. 3. Area or volume expansion of a body is independent of size and shape of the hole/cavity inside it. For example if a body expands, the hole or cavity will also expand in the same way. 4. When temperature is measured in °C, zero of the scale of temperature is taken at melting point of ice. 5. When temperature is measured in Kelvin scale, then triple point of water is taken as the zero of the scale of temperature. 6. Specific heat of a substance can have negative value. Negative specific heat means that to increase the temperature, a certain quantity of heat is to be withdrawn from the body. 7. The material having low specific heat cools more rapidly than material having high specific heat. 8. Specific heat of hydrogen is maximum among those of solids, liquids and gases. 9 Specific heat (or thermal capacity) is inversely proportional to the slope of temperature-time curve. 10. Newton’s law of cooling can be applied only if temperature difference between body and sorrounding is not very large i.e., 20 – 30°C. 11. Had the potential energy curve been symmetric, the average distance between the atoms would not have changed with a rise in temperature (though their amplitude of oscillation would have increased). Thus, thermal expansion is due only to the asymmetry of the potential energy curve. 12. Readings on the Fahrenheit and Celsius temperature scales are given as °F and °C (degree Fahrenheit or degree Celsius). Differences in the readings are given as F° or C° (Fahrenheit degree or Celsius degree). 13. The lowest temperature that can be measured with a gas thermometer is about 1 K, provided the low pressure helium is used as a gas. 14. There is no temperature at which the Celsius scale and the Kelvin scale give the same reading. 15. Regardless of the nature of the gas, all gas thermometers at the same temperature approach the same reading as the pressure of the gas approaches zero. 16. 1 kcal (i.e., I Cal) = 4 Btu. The Btu is still commonly used as a measure of the ability of an air conditioner to transfer energy (as heat) from a room to the outside environment. A typical room conditioner rated at 10,000 Btu/h can, therefore, removes about 107 joule from a room every hour and transfer it to the outside environment. 17. When metal rods or chains are used as measuring instruments, temperature should always be noted and a correction applied for any variation. 18. If there is a hole in a solid body, the volume of the hole increases when the body expands, just as if the hole were a solid of the same material as the body. This remains true even if the hole becomes so large that the surrounding body is reduced to a thin shell. Thus, the volume enclosed by a thin-walled glass flask or thermometer bulb increases as that of a solid body of glass of the same size. 19. Not all substances expand on heating. Iron at 1100 K has a slightly negative expansion coefficient. Other materials with negative expansion coefficients are tellurium, silicon, selenium and cobalt-iron- chromium alloy.
4 | P a g e 20. The unit of  is 1/degree. The numerical value of  depends on the size of degree. Since the Kelvin and Celsius degrees are 9/5 as large as the Rankine and Fahrenheit degrees, the fractional volume change per Kelvin is 9/5 as great as that per Rankine or Fahrenheit degree. Same is true for  and  21.  is insensitive to change of pressure but varies markedly with the temperature. Many experiments indicate that  decreases as the temperature is lowered and approaches zero as the Kelvin temperature approaches zero. It is also a peculiar circumstance that the higher the melting point of a metal, the lower the value of  . 22. The value of  for liquids are much larger (generally about 10 times) than that for solids e.g., 5 0 5 0 quartz alcohol 0.12 10 / C and 75 10 / C − −  =   =  . The value of  for all gases is approximately the same, i.e., 1/273 = 3.7 x 10-3 /C0 and is much greater than that for solids and liquids. 23. The expansion of a liquid due to rise in temperature is always accompanied by the expansion of the vessel. The observed expansion (called apparent expansion) is less than the real expansion. Simple arguments can lead us to the result : real expansion = apparent expansion + expansion of the containing vessel. This is equivalent to saying r a g  =  +  , where r, a and g stand for real, apparent and glass. The vessel has been assumed to be of glass. 24. The shape of the potential energy curve depends upon the bonding between the atoms. Generally speaking, stronger the atomic binding forces, the steeper and narrower is the potential energy curve and hence less pronounced is the thermal expansion. For diamond (strong covalent bond), 6 0 1.1 10 / C −  =  ; for iron (not as strong as covalent bond), 6 0 11.2 10 / C −  =  and for ice (weak molecular bond), 6 0 51 10 / C −  =  . Helium has no triple point at which solid, liquid and gas coexist in equilibrium. 25. Melting of ice under pressure and its resolidification when pressure is released is called regelation (re : again, gelare : to freeze). The melting point of ice decreases with increasing pressure. An ice skater makes use of this fact in an interesting way. His entire weight is supported by skate blades of very small area and the resulting pressure on the ice may exceed 1000 atm. The ice under the blades melts because of great pressure, creating a thin film of water that acts as an efficient lubricant. On unusually cold days even such pressures may not be sufficient to melt the ice and skating then becomes impossible. 26. The freezing point of a liquid is lowered when some other substance is dissolved in the liquid. A common example is the use of an "antifreeze" to lower the freezing point of the water in the radiator of an automobile engine. The freezing point of a saturated solution of common salt in water is about -20°C. 27. The boiling point of a liquid is also affected by dissolved substances, but may be either increased or decreased. Thus, the boiling point of water-alcohol solution is lower than that of pure water while the boiling point of water-salt solution is higher than that of pure water. 28. Since mean distance between atoms in the gas phase is much larger than in either the liquid or the solid phase, more heat is required to vaporise a given mass of a substance than to melt it. Therefore, it is not surprising that the latent heat of vaporisation is much larger than the latent heat of fusion of a given substance. 29. Thermal conduction is a transport phenomenon in which temperature difference causes the transfer of thermal energy from one region of a hot body to another region of the same body which is at a lower