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16.2. The first law of thermodynamics
A subsection of Physics, 9702, through 16. Thermodynamics
Listing 10 of 58 questions
State what is meant by the internal energy of a system. An ideal gas undergoes a cycle of changes as shown in . 3.00 2.80 2.60 2.40 2.20 2.00 pressure / 105 Pa volume / cm3 P R Q 97.0 J 280 K 332 K 372 K At point P, the gas has volume 950 cm3, pressure 2.10 × 105 Pa and temperature 280 K. The gas is heated at constant volume and 97.0 J of thermal energy is transferred to the gas. Its pressure and temperature change so that the gas is at point Q on . The gas then undergoes the change from point Q to point R and then from point R back to point P, as shown on . Some energy changes that take place during the cycle PQRP are shown in . change P → Q change Q → R change R → P thermal energy transferred to gas / J work done on gas / J increase in internal energy of gas / J +97.0 –42.5 +37.0 State the total change in internal energy of the gas during the complete cycle PQRP. Explain your answer. On , complete the energy changes for the gas during 1. the change P → Q, 2. the change Q → R, 3. the change R → P.
9702_w18_qp_41
THEORY
2018
Paper 4, Variant 1
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State what is meant by the internal energy of a system. An ideal gas undergoes a cycle of changes as shown in . 3.00 2.80 2.60 2.40 2.20 2.00 pressure / 105 Pa volume / cm3 P R Q 97.0 J 280 K 332 K 372 K At point P, the gas has volume 950 cm3, pressure 2.10 × 105 Pa and temperature 280 K. The gas is heated at constant volume and 97.0 J of thermal energy is transferred to the gas. Its pressure and temperature change so that the gas is at point Q on . The gas then undergoes the change from point Q to point R and then from point R back to point P, as shown on . Some energy changes that take place during the cycle PQRP are shown in . change P → Q change Q → R change R → P thermal energy transferred to gas / J work done on gas / J increase in internal energy of gas / J +97.0 –42.5 +37.0 State the total change in internal energy of the gas during the complete cycle PQRP. Explain your answer. On , complete the energy changes for the gas during 1. the change P → Q, 2. the change Q → R, 3. the change R → P.
9702_w18_qp_43
THEORY
2018
Paper 4, Variant 3
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State what is meant by work done. A lift of weight 13.0 kN is connected by a cable to a motor, as shown in . motor cable v lift weight 13.0 kN The lift is pulled up a vertical shaft by the cable. A constant frictional force of 2.0 kN acts on the lift when it is moving. The variation with time t of the speed v of the lift is shown in . 1.0 2.0 3.0 v / m s–1 t / s Use to determine: 1. the acceleration of the lift between time t = 0 and t = 3.0 s acceleration = m s–2 2. the work done by the motor to raise the lift between time t = 3.0 s and t = 6.0 s. work done = J The motor has an efficiency of 67%. The tension in the cable is 1.6 × 104 N at time t = 2.5 s. Determine the input power to the motor at this time. input power = W State and explain whether the increase in gravitational potential energy of the lift from time t = 0 to t = 7.0 s is less than, the same as, or greater than the work done by the motor. A calculation is not required.
9702_w19_qp_23
THEORY
2019
Paper 2, Variant 3
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Smoke particles are suspended in still air. Brownian motion of the smoke particles is seen through a microscope. Describe: what is seen through the microscope how Brownian motion provides evidence for the nature of the movement of gas molecules. A fixed mass of an ideal gas has volume 2.40 × 103 cm3 at pressure 3.51 × 105 Pa and temperature 290 K. The gas is heated at constant volume until the temperature is 310 K at pressure 3.75 × 105 Pa, as illustrated in . 2.40 × 103 cm3 3.51 × 105 Pa 290 K 2.40 × 103 cm3 3.75 × 105 Pa 310 K The quantity of thermal energy required to raise the temperature of 1.00 mol of the gas by 1.00 K at constant volume is 12.5 J. Calculate, to three significant figures: the amount, in mol, of the gas amount = mol the thermal energy transfer during the change. energy transfer = J For the change in the gas in , state: the quantity of external work done on the gas work done = J the change in internal energy, with the direction of this change. change = J direction
9702_w19_qp_42
THEORY
2019
Paper 4, Variant 2
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The first law of thermodynamics may be expressed as ΔU = (+q) + (+w) where ΔU is the increase in internal energy of the system. State the meaning of: +q +w. The variation with pressure p of the volume V of a fixed mass of an ideal gas is shown in . 2.2 2.0 2.4 2.8 3.2 3.6 V / 10–3 m3 p / 105 Pa 4.0 2.6 3.0 3.4 3.8 4.2 4.6 5.0 5.4 A C B The gas undergoes a cycle of changes A to B to C to A. During the change A to B, the volume of the gas increases from 2.3 × 10–3 m3 to 3.8 × 10–3 m3. Show that the magnitude of the work done during the change A to B is 390 J. State and explain the total change, if any, in the internal energy of the gas during one complete cycle. During the change A to B, 1370 J of thermal energy is transferred to the gas. During the change B to C, no thermal energy enters or leaves the gas. The work done on the gas during this change is 550 J. Use these data and the information in to complete Table 2.1. Table 2.1 change q / J w / J ΔU / J A to B B to C C to A
9702_w20_qp_41
THEORY
2020
Paper 4, Variant 1
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State what is meant by the internal energy of a system. The atoms of an ideal gas occupy a container of volume 2.30 × 10–3 m3 at pressure 2.60 × 105 Pa and temperature 180 K, as illustrated in . 2.30 × 10–3 m3 2.60 × 105 Pa 180 K 3.80 × 10–3 m3 2.60 × 105 Pa T 980 J The gas is heated at constant pressure so that its volume becomes 3.80 × 10–3 m3 at a temperature T. For the fixed mass of gas, calculate: the amount of substance, in mol amount = mol the temperature T, in K. T = K During the change in , the thermal energy supplied to the gas is 980 J. Determine the work done on the gas during this change. Explain your working. work done = J Determine the change ΔU in internal energy of the gas. ΔU = J
9702_w20_qp_42
THEORY
2020
Paper 4, Variant 2
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The first law of thermodynamics may be expressed as ΔU = (+q) + (+w) where ΔU is the increase in internal energy of the system. State the meaning of: +q +w. The variation with pressure p of the volume V of a fixed mass of an ideal gas is shown in . 2.2 2.0 2.4 2.8 3.2 3.6 V / 10–3 m3 p / 105 Pa 4.0 2.6 3.0 3.4 3.8 4.2 4.6 5.0 5.4 A C B The gas undergoes a cycle of changes A to B to C to A. During the change A to B, the volume of the gas increases from 2.3 × 10–3 m3 to 3.8 × 10–3 m3. Show that the magnitude of the work done during the change A to B is 390 J. State and explain the total change, if any, in the internal energy of the gas during one complete cycle. During the change A to B, 1370 J of thermal energy is transferred to the gas. During the change B to C, no thermal energy enters or leaves the gas. The work done on the gas during this change is 550 J. Use these data and the information in to complete Table 2.1. Table 2.1 change q / J w / J ΔU / J A to B B to C C to A
9702_w20_qp_43
THEORY
2020
Paper 4, Variant 3
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Questions Discovered
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