| Date | May 2014 | Marks available | 4 | Reference code | 14M.2.HL.TZ1.7 |
| Level | Higher level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | Calculate, Explain, Show, and State | Question number | 7 | Adapted from | N/A |
Question
Part 2 A heat engine
The piston of an engine contains a fixed mass of an ideal gas. During one cycle of the engine, the gas undergoes the thermodynamic processes shown below.
(i) State what is meant by an isothermal process.
(ii) Show that process AB is isothermal.
State the nature of process BC.
During the cycle ABCD, the net work done by the gas is 550J. Calculate the net thermal energy absorbed by the gas.
Explain why it is not possible for this engine, operating in this cycle, to be 100% efficient.
Markscheme
(i) a process in which temperature remains constant;
(ii) calculation to show that pV=16(×102) at any point from A to B;
calculation to show that pV=16(×102) at any other point from A to B;
pV is constant;
isochoric / isovolumetric / occurs at constant volume;
\(\Delta U = 0\);
Q=W=550(J);
the gas/system must return to original conditions (P, V, and T);
(to do that) some of the thermal energy absorbed by gas must be given off to the surroundings;
hence not all thermal energy absorbed by gas can be converted to work;
it is the second law of thermodynamics;
Examiners report
ei) Most recognized the meaning of isothermal.
eii) The calculations were successfully done but some candidates missed a concluding statement.
This was well answered.
A significant number attempted a calculation based on the area under the graph on the previous page.
Many were able to relate this to the second law of thermodynamics and recognized that some of the thermal energy was given off to the surroundings.
