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. Gas molecules expand to 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.
Orange and white ping-pong ball demo
What are the chances that after shaking the balls will all be lined up again?

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

5. 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
350.0 °F

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 Metal bar at T1 Metal bar at T2 > T1
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. Thermal expansion for Copper* 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 room for expansion result 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

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
metal B

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 switchON
OFF
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  Average Kinetic 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. Heat  what is it ? ? ? System B System A at temp TB at temp TA
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- it is 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 A
at temp TA
cold
System B
at temp TB >TA
hot
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. But what about refrigerators?
A refrigerator is a device that moves heat from something cold (inside) to something hot (the room).
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