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Qualitative Questions on Thermal Physics1. State why the thermodynamics scale of temperature is called an absolute scale. ...
11. Explain why the total change in the internal energy of the gas during a complete cycle must bezero. N88/II/1012. Expla...
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Thermal Physics qualitative questions

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Some qualitative questions on Thermal Physics

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Thermal Physics qualitative questions

  1. 1. Qualitative Questions on Thermal Physics1. State why the thermodynamics scale of temperature is called an absolute scale. [2] J90/II/62. State two pieces of evidence for believing that matter is made up of molecules.The assumptions of the simple kinetic theory of gases include:• gases are made up of many molecules moving at random;• the collisions of the molecules with the walls of the container are elastic.What experimental evidence is there that these assumptions are reasonable? N84/I/173. In the upper atmosphere, a large fraction of the hydrogen molecules travel so fast that theyare able to escape from the Earth. There may also be some loss of helium, but for all otherheavier molecules escape is negligible. Explain:• Fast molecules are able to escape from the Earth’s atmosphere but slow molecules arenot;• Molecules of smaller mass are able to escape whereas more massive ones are not;• Some molecules escape even though the speed required for escape is very muchgreater than their root-mean-square speed;• Large quantities of hydrogen have remained indefinitely on the Earth as a constituent ofwater. [7] J89/II/104. If two sufficiently energetic sodium atoms collide, an electron in one of them may be raised toa higher energy level.• Explain why such a collision is called an inelastic collision.• Briefly explain why such inelastic collisions are not consistent with basic assumptions ofthe kinetic theory of gases. J90/III/35. An ideal gas is allowed to expand suddenly with no thermal energy entering or leaving thegas. The temperature of the gas is observed to change.• Explain, by reference to the first law of thermodynamics, what happens to the internalenergy of the gas.• Explain why the change in the internal energy of the gas will give rise to a change in theroot-mean-square speed of the molecules of the gas.• Hence explain whether the temperature of the gas will rise or fall as a result of thisexpansion. [8] N95/III/56. When water changes to steam at 100 oC, heat (the latent heat of vaporisation) must besupplied to the system. Is work done on or by the system as the water vaporises? Taking intoaccount the fact that the temperature remains constant during the change of phase, statewhether or not the internal energy changes, giving the reason for your answer.[6]N85/II/5, N97/III/5, J00/III/57. An adiabatic change is one in which no heat is supplied or extracted. When a gas expandsadiabatically, what can you deduce about the temperature of the gas? Explain. N89/II/48. Explain the meaning of internal energy. Explain why a rise in the temperature of an ideal gastakes place during an adiabatic (no heat exchange) compression. [2] J92/II/69. Explain what is meant by an isothermal change. How may an isothermal compression of a gasbe achieved in practice? Explain why, in the isothermal compression of an ideal gas, theinternal energy is unchanged even though mechanical work is done. N78/I/1510. The densities of water and steam are 1000 kg m-3and 0.590 kg m-3respectively. 1 kg of waterevaporates at 100 oC at atmospheric pressure.• State and explain the internal energy change of the molecules of water.• State, with a reason, the increase in the potential energy of the molecules. N92/II/5
  2. 2. 11. Explain why the total change in the internal energy of the gas during a complete cycle must bezero. N88/II/1012. Explain, in terms of the energies of atoms, conditions under which it is possible to increase thetotal energy of the atoms of a substance without any changes of the temperature of thatsubstance. J94/III/613. State qualitatively and explain in molecular terms, what happens to the internal energy of afixed mass of an ideal gas when, separately,• the temperature of the gas is raised,• the volume is decreased at constant temperature,• the gas as a whole is moving at a certain speed. [6] J95/III/514. In a space, such as a swimming pool enclosure, water at 30 oC and water vapour, also at30 oC, coexist.• Compare the pattern of movement and the speed of molecules in water and watervapour at the same temperature. [4]• Compare the internal energy per unit mass of water and water vapour at the sametemperature. [3]• Explain, in terms of internal energy, why the specific latent heat of vaporisation of asubstance is greater than its specific latent heat of fusion. [2] N06/III/215. An ideal gas is contained in a cylinder by means of a piston. The cylinder and piston are madeof a thermal insulator. An atom of the gas collides with the piston.• State, with a reason, whether the momentum of the atom is conserved in the collision.[2]• The piston is lowered so that the volume of the gas is reduced.o State what change, if any, occurs in the speed of a gas atom during an elasticcollision with the moving piston. [1]o Use the kinetic theory of gases to explain why the temperature of the gas rises. [2]N03/II/3

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