Which one of the following statements about a thermodynamic engine is not true, i.e., which one of the statements is false?
1)The internal energy is always the same at the beginning and at the end of one cycle.
2)The absorbed and the exhausted heat have the same magnitude.
3)No engine can be more efficient than a Carnot engine.
4)The pressure is always the same at the beginning and at the end of one cycle.
5)The path in a pV-diagram is closed.
To determine which one of the statements about a thermodynamic engine is false, we need to evaluate each statement individually.
1) The statement "The internal energy is always the same at the beginning and at the end of one cycle" is true. In a thermodynamic engine, the internal energy of the working substance returns to its initial value after completing one cycle.
2) The statement "The absorbed and the exhausted heat have the same magnitude" is true. In a thermodynamic engine, the heat absorbed from the high-temperature reservoir is equal to the heat exhausted to the low-temperature reservoir. This is a consequence of the conservation of energy.
3) The statement "No engine can be more efficient than a Carnot engine" is true. The Carnot engine is the most efficient heat engine operating between two given temperature levels. It achieves the maximum possible efficiency, and no real engine can surpass it.
4) The statement "The pressure is always the same at the beginning and at the end of one cycle" is false. In a thermodynamic engine, the pressure of the working substance can change throughout the cycle. This is especially true for engines that operate on the Brayton or Otto cycles, where the pressure can vary significantly.
5) The statement "The path in a pV-diagram is closed" is true. In a pV-diagram representing the thermodynamic cycle of an engine, the path should form a closed loop. This indicates that the working substance returns to its initial state after completing the cycle.
Therefore, the false statement is number 4) "The pressure is always the same at the beginning and at the end of one cycle."