Laws of Thermodynamics

1. Laws of Thermodynamics • First law—is basically stating the law of conservation of energy. • It states that the change in the energy of a system is the amount of energy added to the system minus the energy spent doing work. • ∆U = q – w

2. Laws of Thermodynamics • ∆U = q – w • ∆U is the change in internal energy of the system. • q is the energy transferred into the system. If heat flows out of the system, then q is negative. • w is the work done by the system. If the surroundings do work on the system, w is negative.

3. Quick check • When work is done on a system, what happens to its energy and temperature? • When heat is added to a system, what happens to its ability to do work? • A can filled with liquid is shaken for a length of time. What happens to its temperature? Why?

4. Laws of Thermodynamics • Copy the illustration on white board and use to explain the behavior of the piston, the mass, and the internal energy of the piston if we change the following: • Increase gas temperature? • Increase size of mass? • Decrease size of mass? • Decrease gas temperature? • Put more gas in the piston?

5. Thermodynamic processes • Isochoric processes occur with no change in volume. • Suppose we have a sealed can on a stove. • Can any work be done by it? • Then w is 0 and the equation becomes ∆U = q.

6. Thermodynamic processes • Adiabatic processes • Suppose we keep the system well insulated or the process occurs very quickly so that no heat can enter or leave the system. Then q is 0 and the equation becomes ∆U = -w.

7. Thermodynamic processes Adiabatic processes occur in nature and result in events known as rain shadows.

8. Thermodynamic processes • Isobaric processes occur with no pressure change and apply to the piston diagram looked at earlier. You get ∆U = q – w and the work is calculated as P∙∆V at constant pressure. • ∆U = q – P∙∆V

9. Laws of Thermodynamics • Work, in this case, is calculated as pressure times volume and is called PV work. • Prove that pressure multiplied by volume is equal to work using their appropriate units.

10. Thermodynamic processes • Isothermal processes occur at very slow speeds and have constant temperature. An example is melting ice or boiling water. • The equation remains ∆U = q – w

11. Laws of Thermodynamics • The second law can be stated in about 100 different ways depending on the field of science using it. • 3 most common are: • Heat flows from a hot body to a cold one. • Heat cannot be completely converted into work; there are always losses. • Every system becomes disordered over time.

12. Laws of Thermodynamics • The third law of thermodynamics is also stated in many forms but generally means that you will never be able to reach absolute zero and the entropy will never reach zero but will reach a very low constant value. • This is referred to as heat death of the universe.

13. Laws of Thermodynamics • The zeroth law of thermodynamics states that if two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. • Says if A = B and B = C, then A = C.

14. Law of entropy • Entropy is a measure of the degree of disorder of a system. • The law of entropy states that everything in the universe tends to become more disordered over time unless energy is used to prevent this.

15. Examples of entropy • A deck of playing cards thrown up in the air come down disordered. It doesn’t work in the opposite way. • Ice melts. • Desks become messy. • Weather reduces rocks to sand and gravel. • Tearing paper increases entropy. • Can you reverse these? Does it take energy?

16. Entropy in nature • Water naturally evaporates due to heat from Sun. • Disordered water molecules in clouds lose heat, form droplets and fall as rain. • Photosynthesis combines CO2 and H2O to form sugars and allows plants to grow. • Dead plants naturally decay back to CO2 and H2O. • Which steps require energy?

17. Entropy in nature • Water naturally evaporates due to heat from Sun. • Disordered water molecules in clouds lose heat, form droplets and fall as rain. • Photosynthesis combines CO2 and H2O to form sugars and allows plants to grow. • Dead plants naturally decay back to CO2 and H2O. • Which steps require energy?

18. Heat engines • Heat engines are devices that turn internal energy into mechanical work. • These include steam, internal combustion, and jet engines. • All of these have a hot reservoir and a cold reservoir. Heat flowing between them creates work.

19. Heat engines • The maximum efficiency of a heat engine is calculated using the Carnot equation: • (Thot– Tcold)/ Thot • Be sure temperature is in Kelvin. • Calculate the efficiency of a heat engine with a hot reservoir temperature of 1500 0C and a cold reservoir temperature of 350 0C

20. Heat engines • Automobiles use 4 stroke or 4 cycle engines: • Intake, Compression, Power, Exhaust. • Refrigeration uses a compressor and two different tubing sizes to achieve cooling.

21. Heat engines Intake and exhaust are isochoric. Why? Power is adiabatic with work done by the system. Why? Compression is adiabatic with work done on the system. Why?

22. Heat engines http://www.samarins.com/glossary/ohv_engine.gif