There must be reasons that real macroscopic processes cannot be reversible. We can imagine them going in reverse. For example, heat transfer occurs spontaneously from hot to cold and never spontaneously the reverse. Yet it would not violate the first law of thermodynamics for this to happen. In fact, all spontaneous processes, such as bubbles bursting, never go in reverse. There is a second thermodynamic law that forbids them from going in reverse. When we study this law, we will learn something about nature and also find that such a law limits the efficiency of heat engines. We will find that heat engines with the greatest possible theoretical efficiency would have to use reversible processes, and even they cannot convert all heat transfer into doing work.
[link] summarizes the simpler thermodynamic processes and their definitions.
Summary of simple thermodynamic processes
Isobaric
Constant pressure
Isochoric
Constant volume
Isothermal
Constant temperature
Adiabatic
No heat transfer
Phet explorations: states of matter
Watch different types of molecules form a solid, liquid, or gas. Add or remove heat and watch the phase change. Change the temperature or volume of a container and see a pressure-temperature diagram respond in real time. Relate the interaction potential to the forces between molecules.
A thermodynamic process begins at 1.2 × 10
6 N/m
2 and 5 L. The state then changes to 1.2 × 10
6 N/m
2 and 2 L. Next it becomes 2.2 × 10
6 N/m
2 and 2 L. The next change is 2.2 × 10
6 N/m
2 and 5 L. Finally, the system ends at 1.0 × 10
6 N/m
2 and 5 L.
The first step of a thermodynamic cycle is an isobaric process with increasing volume. The second is an isochoric process, with decreasing pressure. The last step may be either an isothermal or adiabatic process, ending at the starting point of the isobaric process. Sketch a graph of these two possibilities, and comment on which will have greater net work per cycle.
Look at
[link] , and assign values to the three pressures and two volumes given in the graph. Then calculate the net work for the cycle ABCFEDCFA using those values. How does this work compare to the heat output or input of the system? Which value(s) would you change to maximize the net work per cycle?
One of the important implications of the first law of thermodynamics is that machines can be harnessed to do work that humans previously did by hand or by external energy supplies such as running water or the heat of the Sun. A machine that uses heat transfer to do work is known as a heat engine.
There are several simple processes, used by heat engines, that flow from the first law of thermodynamics. Among them are the isobaric, isochoric, isothermal and adiabatic processes.
These processes differ from one another based on how they affect pressure, volume, temperature, and heat transfer.
If the work done is performed on the outside environment, work (
) will be a positive value. If the work done is done to the heat engine system, work (
) will be a negative value.
Some thermodynamic processes, including isothermal and adiabatic processes, are reversible in theory; that is, both the thermodynamic system and the environment can be returned to their initial states. However, because of loss of energy owing to the second law of thermodynamics, complete reversibility does not work in practice.
Questions & Answers
if three forces F1.f2 .f3 act at a point on a Cartesian plane in the daigram .....so if the question says write down the x and y components ..... I really don't understand
a fixed gas of a mass is held at standard pressure temperature of 15 degrees Celsius .Calculate the temperature of the gas in Celsius if the pressure is changed to 2×10 to the power 4
