# What is Thermodynamics second law: Definition and 14 Discussions

The second law of thermodynamics establishes the concept of entropy as a physical property of a thermodynamic system. Entropy predicts the direction of spontaneous processes, and determines whether they are irreversible or impossible, despite obeying the requirement of conservation of energy, which is established in the first law of thermodynamics. The second law may be formulated by the observation that the entropy of isolated systems left to spontaneous evolution cannot decrease, as they always arrive at a state of thermodynamic equilibrium, where the entropy is highest. If all processes in the system are reversible, the entropy is constant. An increase in entropy accounts for the irreversibility of natural processes, often referred to in the concept of the arrow of time.Historically, the second law was an empirical finding that was accepted as an axiom of thermodynamic theory. Statistical mechanics provides a microscopic explanation of the law in terms of probability distributions of the states of large assemblies of atoms or molecules. The second law has been expressed in many ways. Its first formulation, which preceded the proper definition of entropy and was based on caloric theory, is Carnot's theorem, credited to the French scientist Sadi Carnot, who in 1824 showed that the efficiency of conversion of heat to work in a heat engine has an upper limit. The first rigorous definition of the second law based on the concept of entropy came from German scientist Rudolph Clausius in the 1850s including his statement that heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.
The second law of thermodynamics can also be used to define the concept of thermodynamic temperature, but this is usually delegated to the zeroth law of thermodynamics.

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1. ### I State equations for a thermodynamic substance/system

Hi, as follow up to this thread I believe for any substance/thermodynamic system there exists actually a set of 3 state equations between the 5 variables ##(U,T,S,p,V)##. For example in the case of ideal gas which are the 3 equations ? Thanks.
2. ### I Thermodynamic temperature scale and Carnot cycle

Hi all, recently I started following the MIT course "Statistical Mechanics I: Statistical Mechanics Of Particles" by MIT (here). In the second lesson Prof. Kardar introduces the concept of thermodynamic temperature analyzing the behavior of two Carnot engines that share a thermal reservour at...
3. ### I Magnetic fields are exceptions to the second law of thermodynamics?

Einstein famously said “{Thermodynamics} is the only physical theory of universal content, which I am convinced, that within the framework of applicability of its basic concepts will never be overthrown.” I don't think any of us want to argue with Einstein, but it's worth noting the "within the...
4. ### What is the Relationship Between Entropy and Temperature?

Help! Hi, I need in the secodn law of thermodynamic, we have the ENTROPY "S". Well, I need help for this: We have dS ≈ dQ Then we have dS = λ *dQ where we have λ = λ (T, ... ) I have to demostrate that : λ = 1/T , where T = temperature. Thanks for the advices and help!
5. ### Nearly Perfect Efficiency - 2nd Law of Thermodynamics

Recently read this article on Physorg: https://phys.org/news/2018-01-efficiency.html My questions are: 1. If "converting" information into energy requires a system (the demon I suppose) having a photodiode, processor, laser (light trap) all expending energy in order to cause the conversion...
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### Correct statement of 2nd law of thermodynamics?

Thermodynamics is stated in different ways. E.g. In isolated systems entropy never decreases Heat never spontaneously pass from colder to warmer body Total energy quality decreases in all processes. Energy disperses But what is it exactly? What is the correct description of the 2nd law of...
7. ### How do I state 2nd law of thermodynamics for an open system

Ecosystems are open systems, they receive solar energy and other materials from outside the ecosystem and migration of animals is also witnessed. I read Physics uptil class XII and have done a bit of research on google books and have come across these three books: 1.Towards a Thermodynamic...
8. ### I What will be the flow direction?

I guess there would be no flow in case 2; case 3 seems easy to me applying fluid mechanics principle; flow will be 1 to 2. But I doubt case 1; it is not in thermal equilibrium ? Pls help
9. ### Determine the coefficient of performance of this cycle

Homework Statement In an ideal refrigeration cycle, the temperature of the condensing vapour is 40oC and the temperature during evaporation is -20oC. Determine the coefficient of performance of this cycle for the working fluids; R12 and ammonia. Homework Equations C.O.Pc = TL/(TH-TL) The...
10. ### Calculate Turbine gross and net output

Homework Statement A steam power plant consists of a boiler, a turbine, a condenser and a pump. The temperature of the inner walls of the boiler is 350oC and the temperature of the condenser cooling water is 20oC. During a certain interval of time, the heat added to the boiler is 2.9x106 kJ and...
11. ### Hidden Heat Reservoirs and Violations of the 2nd Law of Thermodynamics

I have a quick question concerning the 2nd law of thermodynamics. So my textbook uses an illustration to explain how the violation of the Clausius statement of the 2nd law implies a violation of the Kelvin-Plank. But while looking over the diagram I stated thinking about a certain situation. In...
12. ### Efficiency of Irreversible Engine

I have a small confusion that can an irreversible engine have greater efficiency than reversible engine if both have different source and sink temperatures?I know that under same conditions reversible engines are more efficient but what can be concluded under different conditions like both...
13. ### Is Hydroelectric power plant a perpetual motion machine?

Is Hydroelectric power plant a perpetual motion machine of second kind from this classification https://en.wikipedia.org/wiki/Perpetual_motion#Classification ? or not? If not which kind of perpetual motion machine is it? :)
14. ### Ideal gas temperature proportional to absolute temperature

Homework Statement I[/B] have some difficulties proving that the ideal gas temperature is directly proportional to absolute temperature defined by the second law of thermodynamics. Homework Equations The ideal gas temp. is defined by the ideal gas equation: pV=NkTi(T), where k is Boltzmann's...