1. Physics 2
Chapter 12
Sections 2-4

2. First Law
When a system absorbs an amount of heat Q and performs an amount of work W, the internal energy of the system changes by an amount ΔU where
ΔU = Q – W
Expression of conservation of energy
If heat flows out of a system or a system does work it loses energy

3. First Law
ΔU = Q – W
If heat flows into a system or work is done on a system it gains energy
By convention, work done by a system is + and heat absorbed by a system is +

4. Thermal Processes
4 common thermal processes that any system can undergo:
- isobaric, isochoric, isothermal, adiabatic
Assume that all are quasi-static (occurs slowly enough that uniform pressure and temp exist at all times)

5. Thermal Processes
Isobaric process – occurs at constant pressure

6. Thermal Processes Isochoric process – occurs at constant volume
- sometimes called isovolumetric or isometric

7. Thermal Processes
Isothermal process – occurs at constant temperature

8. Thermal Processes Isothermal process – occurs at constant temperature
If ideal gas W = nRT ln (Vf / Vi)
where n = number of moles
R = 8.31 J/K
T – temp in Kelvin

9. Thermal Processes
Adiabatic process – occurs without the transfer of heat

10. Thermal Processes
Adiabatic process – occurs without the transfer of heat
If monatomic ideal gas W = 3/2 nR (Ti – Tf)

11. Thermal Processes

12. Sample Problem
Heat is added to 1000 cm3 of water at atmospheric pressure. The water’s temperature increases by 50 ⁰C. What type of process is it? Find the work done by the water as it expands. Find the increase in the water’s internal energy.

13. Sample Problem
Find the work done by 2 moles of an ideal gas if its volume doubles at a constant temperature of 23 ⁰C. What type of process is this? Find the heat absorbed by the gas.

14. Sample Problem
Consider the PV diagram on your study guide for an ideal gas. Processes I, II, and III take place at constant volume, temperature, and pressure respectively. In process III the heat added to the gas is -61 cal and the work done by the gas is -24 cal. Find the change in internal energy for process III. Find Q, ΔU, and W for process I.

15. Second Law
Heat energy flows spontaneously from a substance at a high temperature to a substance at a low temperature and does not flow spontaneously in the reverse direction
Ex – cold drink in a warm room warms up
cold drink in a warm room won’t get
cooler

16. Entropy Measure of the disorder of a system
Defined by Clausius as heat divided by the temperature at which the heat flows
S = Q / T where S – entropy
Q – heat flow
T – temp in Kelvin
[S] = J / K
If you were to see a bunch of trees arranged in neat rows you would assume they were planted. Why? If left to its own devices, nature tends to produce a disorderly arrangement. Order is possible but not probable.
Toss 3 pennies into a cup. Possible outcomes are HHH, TTT, HHT, TTH, HTT, THH, HTH, and THT. Which outcomes are the most ordered? HHH and TTT Likelihood of ordered outcome? 2/8 Likelihood of a disordered outcome? 6/8

17. Entropy cont’d.
Increasing entropy decreases the energy available to do work
- if all gas particles
are headed toward
piston the energy
could be used to
do work on piston
- highly efficient
-if gas particles were moving randomly only some particles would collide with piston to do work

18. Second Law of Thermo
Heat flows spontaneously from a substance at high temperature to a substance at low temperature OR
Total entropy of the universe increases when an irreversible process occurs
Consider an ice cube in the experimental fluid at Chalky’s
Ice melts because heat flows into the system
This heat breaks down the ordered crystalline arrangement of molecules and makes them more disordered
---heat and entropy are related

19. Sample Problem
1200 J of heat energy flows spontaneously from a hot reservoir at 650 K to a cold reservoir at 350 K. Determine the amount by which this irreversible process changes the entropy of the universe.
1.58 J/K
Note: individual parts can have a decrease in S but a greater increase in S will take place somewhere else

20. Sample Problem
Suppose that by stirring 100 g of water you are able to increase its temperature from 19 C to 21 C. Find the increase in the water’s entropy.
2.9 J/K

21. Heat Death of the Universe
Since entropy of the universe is always increasing, less energy is available to do work
At some point there will be no energy available to do any work

22. Third Law
It is not possible to lower the temperature of any system to absolute zero in a finite number of steps
Can’t reach absolute zero