PHAS0006 Thermal Physics and the Properties of Matter Exam 2021
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PHAS0006
Thermal Physics and the Properties of Matter
Exam 2021
1. (a) Two thermally isolated objects of mass m1 and m2 are initially at temperatures T1 and T2 . Each mass has specific heat capacity c1 and c2 .
i. Find an expression for the final temperature Tf when the two objects are brought into thermal contact and have come to thermal equilibrium.
ii. Calculate the final temperature at equilibrium when a 100 kg block of ice, at a temperature of 200 K, is brought into contact with a 50 kg block of concrete at a temperature of 180 K. Round your answer to the nearest K. The specific heat capacity of ice is 1.34 kJ kg− 1 K− 1 , and for concrete is 1.84 kJ kg− 1 K− 1 . Assume that the two blocks are thermally isolated and that the specific heat capacities do not change with temperature.
iii. Determine the change in entropy when the ice and concrete have come to equilibrium using the values given in (ii).
(b) A thermally isolated container consists of a cylinder with a frictionless piston as shown in the diagram below. The container and piston has a total mass m1 , specific heat capacity c1 and is initially at a temperature T1 . n moles of an ideal, monatomic gas, initially at a temperature of T2 , are introduced into the empty container via an adiabatic free expansion.
i. Find an expression for the final equilibrium temperature of the container and piston, and the gas inside it at fixed volume. Assume that they are thermally isolated from the environment.
ii. Using your result from part (i), or otherwise, determine the final equilibrium temperature when the container and piston has a combined mass of 10 kg and a specific heat capacity of 2 kJ kg− 1 K− 1 . The container and piston are initially at a temperature of 300 K and there are 10 moles of gas initially at a temperature of 200 K.
iii. Calculate the gas pressure when the container volume is 0.01 m3 .
iv. The gas is now allowed to expand and does work. The volume increases by 0.01 m3 . Explain why this process can be well approximated by an isothermal expansion?
v. Assuming the process considered in part (iv) is isothermal, calculate the work done by the gas and the change in its entropy.
2. a) Considering both Carnot and Otto engine cycles: Describe
with the aid of a pV diagram, the thermodynamic processes that occur in each cycle when the working fluid is an ideal monatomic gas. Indicate in which part of each cycle heat is transferred to and rejected by the fluid.
b) The maximum temperature reached in an Otto cycle is 1000 K, and the minimum temperature is 350 K. The heat transfer into the engine is 4 J per cycle and 4 J is rejected. Calculate the two intermediate temperatures reached within this ideal cycle for 0.001 moles of gas.
c) Describe how heat is transferred to and rejected by the working fluid in an internal combustion engine. What practically limits the efficiency of this type of engine?
The Lennard-Jones interaction potential can be written as:
V(r) = 49 e / 、 12 - / 、 6a .
(d) Discuss briefly what this function describes, and explain the physical origin of the two terms in the square brackets. (e) With the aid of a diagram(s), explain the microscopic origin for the thermal expansion of simple solids.
(f)(i) A cylindrical aluminium bar has length 2 m and radius 0.2 m, and is at equilibrium at 300 K. What is the length of the bar at 350 K? The linear expansion coefficient of aluminium is 2.3 × 10−5 K− 1 .
(ii) If the aluminium bar is held so that it cannot expand, what pressure is exerted on the ends of the bar when the temperature is increased from 300 to 350 K? The Young’s modulus for aluminium is 6.9 × 1010 Pa.
3. (a) Explain two cases where defects can be beneficial to the functional properties of a material, and two cases where defects can be detrimental to the functional properties of a material.
(b) In a diffraction experiment on a cubic crystal of unit cell length 4.3 A˚ , at what 2o values would you find the 111 and the 220 reflections, using X-rays of wavelength 0.154 nm?
(c) A cubic ionic crystal has a chemical composition AB. If ion B has a radius of 0.15 nm, calculate the smallest radius of ion A for which AB has a crystal structure with eight nearest neighbours and the A ions touch the B ions.
(d) By sketching isotherms of the van der Waals equation of state at temperatures above and below the critical point, explain Maxwell’s equal area rule and its physical implications with reference to experimental observations.
(e) The vapour pressure of carbon dioxide increases from pvap to 1.5pvap when the temperature increases by 20 degrees Kelvin. The enthalpy of vaporisation of carbon dioxide is 15.3 kJ mol− 1 . Calculate the value of the temperature before it was increased.
2022-04-22