1. Purdue University, Physics 220
Final Exam May 5th 7-9 pm STW183
Lecture 24
Purdue University, Physics 220

2. Purdue University, Physics 220
Lecture 24
Heat
Lecture 24
Purdue University, Physics 220

3. Purdue University, Physics 220
Internal Energy
Energy of all molecules including
Translational and rotational kinetic energy of molecules due to their individual random motions.
Vibrational energy (both kinetic and potential) of molecules and of atoms within molecules due to random vibrations about their equilibrium points.
Potential energy due to interactions between the atoms and molecules of the systems.
Chemical and nuclear energy (kinetic and potential energy associated with the binding of atoms to form molecules, the binding of electrons to nuclei to form atoms, and the binding of protons and neutrons to form nuclei).
DOES NOT INCLUDE
Macroscopic motion of object
Potential energy due to interactions with other objects
Lecture 24
Purdue University, Physics 220

4. Purdue University, Physics 220
Heat
Definition: Flow of energy between two objects due to difference in temperature
Note: similar to WORK
Object does not “have” heat (it has energy)
Heat flows from a system at higher temperature to one at lower temperature
Lecture 24
Purdue University, Physics 220

5. Purdue University, Physics 220
Heat
Units:
joule (J)
calorie
Amount of heat needed to raise 1g of water 1ºC
1 Calorie = 1 kcal = 1000 cal = 4186 Joules
Sign definition:
Added to the system: Q>0
Removed from the system: Q<0.
Lecture 24
Purdue University, Physics 220

6. Heat Capacity and Specific Heat
Heat capacity =Q/T
shows how much heat is required to change the T of object (system)
Specific heat c= Q/mT
Q = c m DT
Heat required to increase temperature depends on amount of material (m) and type of material
Heat adds energy to object/system
IF there is no dissipation then:
Heat increases internal energy: Q = DU
Heat increases temperature: Q = C DT
Lecture 24
Purdue University, Physics 220

7. Purdue University, Physics 220
Exercise
After a grueling work out, you drink a liter of cold water (0 C). How many Calories does it take for your body to raise the water up to body temperature of 36 C?
A) B) C) 3, D) 36,000
1 liter = 1,000 grams of H20
1000 g x 1 calorie/(gram degree) x (36 degree) = 36,000 calories
36,000 calories = 36 Calories!
Lecture 24
Purdue University, Physics 220

8. Purdue University, Physics 220
Question
Suppose you have equal masses of aluminum and copper at the same initial temperature. You add 1000 J of heat to each of them. Which one ends up at the higher final temperature
A) aluminum
B) copper
C) the same
Substance c in J/(kg-C)
aluminum 900
copper 387
iron 452
lead 128
human body
water
ice
T = Q/cm
Lecture 24
Purdue University, Physics 220

9. Purdue University, Physics 220
iClicker
Two insulated buckets contain the same amount of water at room temperature. Two blocks of metal of the same mass, both at the same temperature, are warmer than the water in the buckets. One block is made of aluminum (c=0.9) and one is made of copper. You put the aluminum block into one bucket of water, and the copper (c=0.385) block into the other. After waiting a while you measure the temperature of the water in both buckets. Which is warmer?
A) The water in the bucket containing the aluminum block
B) The water in the bucket containing the copper block
C) The water in both buckets will be at the same temperature
Since aluminum has a higher specific heat than copper, you are adding more heat to the water when you dump the aluminum in the bucket (Q=mcDT).
Lecture 24
Purdue University, Physics 220

10. Specific Heat for Ideal Gas
Monatomic Gas (single atom)
Translational kinetic energy only
At constant Volume work = 0
Q = DU = 3/2 nRDT
CV = 3/2 R = 12.5 J/(K mole)
Cv – specific heat at constant volume.
Diatomic Gas (two atoms)
Can also rotate
CV = 5/2 R = 20.8 J/(K mole)
Lecture 24
Purdue University, Physics 220

11. Purdue University, Physics 220
Phase Transitions
A phase transition occurs whenever a material is changed from one phase, such as the solid phase, to another phase, such as the liquid phase.
Phase transitions occur at constant temperature.
The latent heat of vaporization LV is the heat per unit mass that must flow to change the phase from liquid to gas or from gas to liquid.
Fusion occurs when a liquid turns into a solid.
Evaporation occurs when a liquid turns into a gas.
Sublimation occurs when a solid changes directly to a gas without going into a liquid form.
Lecture 24
Purdue University, Physics 220

12. Latent Heat
As you add heat to water, the temperature increases for a while, then it remains constant, despite the additional heat!
Latent Heat L [J/kg] is heat which must be added (or removed) for material to change phase (liquid-gas).
|Q| = m L T
Q added to water
water temp rises
water changes to steam (boils)
steam temp rises
100oC
Latent Heat
Substance Lf (J/kg) Lv (J/kg)
water x x 105
f=fusion
v=vaporization
Lecture 24
Purdue University, Physics 220

13. Purdue University, Physics 220
Phase Diagram
H2O
Lecture 24
Purdue University, Physics 220

14. Purdue University, Physics 220
Phase Diagram
CO2
Lecture 24
Purdue University, Physics 220

15. Purdue University, Physics 220
Boiling Point
Going from Lafayette to Denver the temperature at which water boils:
A) Increases B) Decreases C) Same
Lecture 24
Purdue University, Physics 220

16. iClicker
Which can absorb more energy from your soda, a “cooler” filled with water at 0 C, or a cooler filled with ice at 0 C.
A) Water B) About Same C) Ice
Latent Heat L [J/kg] is heat which must be added (or removed) for material to change phase (liquid-gas).
ice temp rises
ice changes to water (melts)
water temp rises
After ACT add information about latent heat of fusion and numbers. T
Q added to water
0oC
Substance Lf (J/kg) Lv (J/kg)
water x x 105
Latent Heat
Lecture 24
Purdue University, Physics 220

17. Purdue University, Physics 220
Exercise
During a tough work out, your body sweats (and evaporates) 1 liter of water to keep cool (37 C). How much cold water would you need to drink (at 2 C) to achieve the same thermal cooling? (recall CV = 4.2 J/g for water, Lv=2.2x103 J/g)
A) 0.15 liters B) 1.0 liters C) 15 liters D) 150 liters
Qevaporative = L m = 2.2x103 kJ/kg x 1kg
Qc = c m Dt = 4.2kJ/kgK x 35K x m
m = 2.2x103 / 147 = 15kg or 15 liters!
Lecture 24
Purdue University, Physics 220

18. Purdue University, Physics 220
Exercise
How much ice (at 0 C) do you need to add to 0.5 liters of a water at 25 C, to cool it down to 10 C?
(L = 80 cal/g, c = 1 cal/g C)
Cool Pool w/ ice cubes on transparency?
Lecture 24
Purdue University, Physics 220

19. Purdue University, Physics 220
Exercise
Ice cube trays are filled with 0.5 kg of water at 20 C and placed into the freezer. How much energy must be removed from the water to turn it into ice cubes at -5 C?
(L = 80 cal/g, cwater = 1 cal/g C, cice = 0.5 cal/g C)
Water going from 20 C to 0 C:
Water turning into ice at 0 C:
Cool Pool w/ ice cubes on transparency?
Ice going from 0 C to -5 C:
Lecture 24
Purdue University, Physics 220