Question 19M.2.HL.TZ1.7b.ii
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.2.HL.TZ1.7b.ii |
| Level | HL | Paper | 2 | Time zone | TZ1 |
| Command term | Calculate | Question number | b.ii | Adapted from | N/A |
The work done during the isothermal expansion A → B is 540 J. Calculate the thermal energy that leaves the gas during one cycle.
[2]
heat into gas «is along AB» and equals
Qin «= ΔU + W = 0 + 540» = 540 «J» ✔
heat out is (1−e) Qin = (1−0.45) × 540 = 297 «J» ≈ 3.0 × 102 «J» ✔
Award [2] for bald correct answer.
The efficiency of the Carnot cycle was well calculated by most of the candidates, but the thermal energy that leaves the gas was well calculated only by the best candidates. Many candidates were able to establish that the heat added to the gas was 540 J - but struggled to then link this with the efficiency to determine the thermal energy leaving the gas. In iii) the better candidates used the formula given at i) to appropriately calculate the ratio but many were not able to manipulate the expressions to achieve the desired outcome. The reciprocal of 2.5 (0.4) was a common error in the final result and so was the incorrect use of the formula for isobaric expansion. The change in the entropy was well calculated by many candidates with ECF being very prominent from part biii). A complete and proper explanation in ii) was well formulated only by the better candidates. However, many were able to recognize that the Carnot engine was ideal and not real. Many answers referred to heat being lost to the environment but not to “additional losses” that make the engine less than its ideal capacity.
