L 17 - Thermodynamics [2]

1. L 17 - Thermodynamics [2] • Science that studies the relationship between heat (random molecular motion) and work (organized motion) and the conversion of one into the other • it applies to all living and non-living things • it predicts the direction in which natural (spontaneous) processes go • ice melts (rather than getting colder!) • gases expand to fill entire volumes

2. Orange and white ping-pong ball demo • What are the chances that after shaking the balls will all be lined up again? Gas molecules expandto fill a volume This always happens spontaneously This never happens Under normal conditions there are several times 1023 molecules in a box. The chance that all of these are moving to the left at the same time is zero.

3. Some processes are possible, but not probable • The second law of thermodynamics is essentially a statement of the overwhelming likelihood of what happens in systems that contain very large numbers of particles

4. 350 C two different metals AMPS But first, some practical matters Thermocouples

5. 350.0 °F Thermocouples- measure Temp. • The temperature sensor you stick into your turkey is a thermocouple • it is composed of 2 wires composed of different metals welded together - when it gets hot it makes electrical current flow

6. Thermocouples protect you! • a thermocouple is used in gas heaters and dryers to protect against explosions • a thermocouple is placed in the pilot light • as long as the pilot light is on, the thermocouple is hot and current flows • a circuit senses the current and allows the main gas valve to open • if the pilot light is out, the circuit prevents the main gas valve from opening

7. Thermal Expansion L1 Metal bar at T1 Metal bar at T2 > T1 L2 • Expansion occurs in all dimensions • Expansion must be taken into account when designing roads and bridges in climates that vary significantly from winter to summer – all materials expand, steel, concrete, asphalt . . .

8. for Copper* Thermal expansion change in Temp = 100 – 22 = 78 degrees C * for concrete  = 10 x 106 per degree C

9. winter/summer expansion gaps

10. expansion gaps on bridges

11. Thermal Expansion problems Forgot to leave roomfor expansionresult buckling

12. Areas and volumes expand too! cold hot cold hot

13. Does the whole get bigger or smaller when heated? cold hot Both the inner and outer diameters increase when the ring is heated

15. H2O is Weird Stuff ! E X P A N S I O N

17. Fire sprinklers use thermal expansion

18. Bi-Metal strips • thermal expansion of metals is put to good use in a bi-metallic strip. • this is two strips of different metals bonded together metal A metalB

19. Heating a Bi-metal strip • when heat is applied to the bi-metallic strip, both metals expand, but by different amounts! • result: this can be used as a switch – in your coffee maker!

20. Bi-Metal strip thermal switch OFF ON when a preset temperature is reached, the bi-metallic strip opens up

21. Internal energy, Temperature and Heat • in a gas the molecules have energy because they are moving. • the sum of all the energies of all the molecules is the system’s internal energy • the temperature of the system is a measure of the average kinetic energy of the atoms, • TemperatureAverageKinetic Energy

22. Temperature and Heat • Temperature measures the average kinetic energy (KE) of the molecules • Heat (heat content) is the average KE multiplied by the number of molecules 100 C 100 C 100 C

23. System B at temp TB System A at temp TA Heat  what is it ? ? ? • Heat is the energy that flows from one system to another because of their temperature difference. • Heat stops flowing when the two systems come to the same temperature. • Heat was first thought to be an actual fluid (caloric), but it is not a fluid- itis energy!

24. Heat Flow and the laws of thermodynamics • System A has a certain amount of internal energy and so does system B • If energy is transferred and the internal energy of B decreases by some amount then internal energy of A must increase by the same amount. [the first law] • If the temperature of A is less than the temperature of B then heat flows from B to A (hot to cold).  [the second law]

25. 1st and 2nd Laws of Thermodynamics System B at temp TB >TA hot System A at temp TA cold • First Law of Thermodynamics: Energy is conserved. For two isolated systems A and B that exchange heat energy, the heat lost by one system = the heat gained by the other system. • Second Law of Thermodynamics: In a spontaneous process involving two systems in contact, heat flows from the hot system to the cold system. In the top case heat flows from B to A

26. Reverse heat flow- why ice melts Notice that the 1st law does not say that ice must melt. The first law does not prohibit that the ice from getting colder and the water warmer! The 2nd law specifies the direction of heat flow in natural (spontaneous)processes.

27. A refrigerator is a device that moves heat from something cold (inside) to something hot (the room). But what about refrigerators? needs to be plugged in to work! [energy in]

28. Refrigerators and the 2nd Law • Does this violate the 2nd law? NO, because it is not a spontaneous process • Refrigerators require energy input (work) (electricity) to operate. • Heat does not flow spontaneously from cold to hot, but it can be made to flow backwards if there is an input of WORK. • It uses electrical energy to pump heat from cold to hot.

29. How does Heat flow ? preview • HEAT  the energy that flows from one system to another because of temperature differences. • There are 3 processes by which heat flows • convection • conduction • radiation