Question
(a) State what is meant by the internal energy of a system.
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(b) An ideal gas undergoes a cycle of changes as shown in Fig. 2.1.
At point P, the gas has volume 950cm3, pressure 2.10 × 105Pa and temperature 280K.
The gas is heated at constant volume and 97.0J of thermal energy is transferred to the gas.
Its pressure and temperature change so that the gas is at point Q on Fig. 2.1.
The gas then undergoes the change from point Q to point R and then from point R back to
point P, as shown on Fig. 2.1.
Some energy changes that take place during the cycle PQRP are shown in Fig. 2.2.
(i) State the total change in internal energy of the gas during the complete cycle PQRP.
Explain your answer.
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(ii) On Fig. 2.2, complete the energy changes for the gas during
1. the change P → Q,
2. the change Q → R,
3. the change R → P.
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Answer/Explanation
(a) sum of potential and kinetic energies (of molecules/atoms/particles)
(energy of) molecules/atoms/particles in random motion
(b)(i) final temperature = initial temperature
no change in internal energy
(b)(ii)
1. work done on gas \((P→Q): 0 \)
increase in internal energy \( (P→Q): (+)97.0J\)
2. increase in internal energy \((Q→R): –42.5J \)
3. increase in internal energy \((R→P): –54.5J \)
Question
(a) State what is meant by internal energy.
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(b) The variation with volume V of the pressure p of an ideal gas as it undergoes a cycle ABCA of changes is shown in Fig. 2.1.
Determine
(i) the amount, in mol, of gas,
amount = ………………………………………….. mol [2]
(ii) the temperature of the gas at C.
temperature = …………………………………………….. K [2]
(c) Explain why the change from C to A involves external work and a change in internal energy.
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Answer/Explanation
Ans: