16.2. The first law of thermodynamics
A subsection of Physics, 9702, through 16. Thermodynamics
Listing 10 of 58 questions
The first law of thermodynamics can be represented by the expression ΔU = q + w. State what is meant by the symbols in the expression. +DU +q +w A fixed mass of an ideal gas undergoes a cycle ABCA of changes, as shown in . 1.0 2.0 3.0 4.0 5.0 6.0 2.0 4.0 6.0 volume / 10–4 m3 pressure / 105 Pa C B A During the change from A to B, the energy supplied to the gas by heating is 442 J. Use the first law of thermodynamics to show that the internal energy of the gas increases by 265 J. During the change from B to C, the internal energy of the gas decreases by 313 J. By considering molecular energy, state and explain qualitatively the change, if any, in the temperature of the gas. For the change from C to A, use the data in and to calculate the change in internal energy. change in internal energy = J The temperature of the gas at point A is 227 °C. Calculate the number of molecules in the fixed mass of the gas. number =
9702_m17_qp_42
THEORY
2017
Paper 4, Variant 2
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A cylinder contains 5.12 mol of an ideal gas at pressure of 5.60 × 105 Pa and volume 3.80 × 104 cm3. Determine the temperature of the gas. temperature = K The average kinetic energy EK of a molecule of the gas is given by the expression EK = 3 2 kT where k is the Boltzmann constant and T is the thermodynamic temperature. The gas is heated at constant pressure so that its temperature rises by 125 K. Use your answer in to determine the new volume of the gas. volume = cm3 Calculate the increase in internal energy of the gas. Explain your working. increase in internal energy = J Use your answer in to determine the external work done during the expansion of the gas. work done = J Calculate the total thermal energy required to heat the gas in . energy = J
9702_m18_qp_42
THEORY
2018
Paper 4, Variant 2
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A gas is contained in a cylinder by a movable piston. 20.0 cm 30.0 cm 35.0 cm final position of piston gas cylinder initial position of piston The cylinder has a circular cross-section of diameter 20.0 cm. The pressure of the gas is 102 Pa and the piston is initially 30.0 cm from the base of the cylinder. The gas is heated causing the piston to move up so that it is 35.0 cm from the base. The pressure of the gas remains constant. How much work does the gas do in moving the piston?
9702_m21_qp_12
MCQ
2021
Paper 1, Variant 2
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A fixed mass of an ideal gas has a volume V and a pressure p. The gas undergoes a cycle of changes, X to Y to Z to X, as shown in . V Y Z X p Table 2.1 shows data for p, V and temperature T for the gas at points X, Y and Z. Table 2.1 p / 105 Pa V / 10–3 m3 T / K X 1.5 4.2 Y Z 5.1 State the change in internal energy ΔU for one complete cycle, XYZX. ΔU = J Calculate the amount n of gas. n = mol Complete Table 2.1. Use the space below for any working. The first law of thermodynamics for a system may be represented by the equation ΔU = q + W. State, with reference to the system, what is meant by: ΔU : q : W : Explain how the first law of thermodynamics applies to the change Z to X.
9702_m22_qp_42
THEORY
2022
Paper 4, Variant 2
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By referring to both kinetic energy and potential energy, explain what is meant by the internal energy of an ideal gas. A fixed mass of an ideal gas at a temperature of 20 °C is sealed in a cylinder by a piston, as shown in . cylinder gas piston The initial volume of the gas is 1.24 × 10–4 m3. Thermal energy is supplied to the gas and its volume increases by 5.20 × 10–5 m3. The piston is freely moving so that the gas is always at atmospheric pressure. Atmospheric pressure is 1.01 × 105 Pa. Calculate the work done by the gas. work done by gas = J Calculate the final thermodynamic temperature T of the gas. T = K The mass of the gas is 16 g. For this expansion, there is a net transfer of 960 J of thermal energy to the gas. Calculate the specific heat capacity c of the gas at this pressure. c = J kg–1 K–1 The gas in is allowed to return to its starting temperature. The piston is now fixed in position. Thermal energy is supplied to increase the temperature to the same final temperature as in . Use the first law of thermodynamics to suggest and explain how the specific heat capacity of the gas for this situation compares with the value in .
9702_m24_qp_42
THEORY
2024
Paper 4, Variant 2
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Explain what is meant by work done. A car is travelling along a road that has a uniform downhill gradient, as shown in . 25 m s–1 7.5° The car has a total mass of 850 kg. The angle of the road to the horizontal is 7.5°. Calculate the component of the weight of the car down the slope. component of weight = N The car in is travelling at a constant speed of 25 m s–1. The driver then applies the brakes to stop the car. The constant force resisting the motion of the car is 4600 N. Show that the deceleration of the car with the brakes applied is 4.1 m s–2. Calculate the distance the car travels from when the brakes are applied until the car comes to rest. distance = m Calculate 1. the loss of kinetic energy of the car, loss of kinetic energy = J 2. the work done by the resisting force of 4600 N. work done = J The quantities in part 1 and in part 2 are not equal. Explain why these two quantities are not equal.
9702_s11_qp_21
THEORY
2011
Paper 2, Variant 1
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Explain what is meant by work done. A boy on a board B slides down a slope, as shown in . 30° horizontal boy on board B The angle of the slope to the horizontal is 30°. The total resistive force F acting on B is constant. State a word equation that links the work done by the force F on B to the changes in potential and kinetic energy. The boy on the board B moves with velocity v down the slope. The variation with time t of v is shown in . 2.0 1.0 2.0 t / s v / m s–1 3.0 4.0 6.0 8.0
9702_s14_qp_23
THEORY
2014
Paper 2, Variant 3
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Questions Discovered
58