循环过程和孤立系统

An isolated system is a thermodynamic system that does not exchange heat with its surroundings and performs no work on them. For isolated systems, the change in internal energy is always zero because no energy enters or leaves the system. This principle is fundamental to understanding energy conservation in thermodynamic processes.

In a cyclic process, a system returns to its initial state after undergoing a series of changes. Since internal energy is a state function depending only on initial and final states, returning to the initial state means internal energy returns to its original value, making the net change zero. Therefore, the net work done equals the net heat transfer in the cycle.

On a pV diagram, a cyclic process is represented by a closed path. The net work done equals the area enclosed within this closed path. If the cycle follows a clockwise direction, the net work is positive; if counter-clockwise, the net work is negative. This graphical representation helps visualize energy transformations during the complete cycle.

In a cyclic process, since internal energy change is zero, the net work done by the system equals the net heat transfer into the system. If the net heat transfer is positive, the system does positive work on surroundings. If negative, work is done on the system. This relationship directly follows from the first law of thermodynamics applied to cyclic processes.

To find net work in a multi-step cyclic process, first calculate the total heat transfer by summing heat values for each step. Since internal energy change is zero in a cyclic process, the net work done equals this total heat transfer. For example, if individual heat transfers are 40 J, −80 J, −20 J, and 100 J, the net work is their sum: 40 J.

Positive net work in a cyclic process means the system does more work on its surroundings than work done on the system. This occurs when the cycle follows a clockwise path on a pV diagram. The heat energy supplied to the system is converted into external work, making the process useful for energy conversion applications like heat engines.

Both isolated systems and cyclic processes maintain constant internal energy. An isolated system has zero internal energy change because it exchanges no heat or work. A cyclic process in a non-isolated system also has zero internal energy change because it returns to its initial state. Both demonstrate that internal energy depends only on the system's state, not the path taken.