Question 19M.2.HL.TZ1.c.ii
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.2.HL.TZ1.c.ii |
| Level | HL | Paper | 2 | Time zone | TZ1 |
| Command term | Explain | Question number | c.ii | Adapted from | N/A |
Explain, by reference to the second law of thermodynamics, why a real engine operating between the temperatures of 620 K and 340 K cannot have an efficiency greater than the answer to (b)(i).
[2]
the Carnot cycle has the maximum efficiency «for heat engines operating between two given temperatures »✔
real engine can not work at Carnot cycle/ideal cycle ✔
the second law of thermodynamics says that it is impossible to convert all the input heat into mechanical work ✔
a real engine would have additional losses due to friction etc ✔
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.
