Download Section 11

Section 11­2.notebook
February 28, 2012
Conservation of Energy thus far does not deal with friction. If friction is included then we are no longer dealing with the system but the environment. Example is the roller coaster on page 409.
It deals with the KE, PE and U (sum of internal energy) In this situation the increase of internal energy is equal to the decrease of mechanical energy. The internal energy is changed to energy dissipated into the air as heat and sound.
First Law of Thermodynamics ­ the principle of energy conservation that takes into account a system's internal energy as well as work and heat.
The First Law of Thermodynamics
ΔU = Q ­ W
Change in system's internal energy = energy transferred to or from system as heat ­ energy transferred to or from system as work.
1
Section 11­2.notebook
February 28, 2012
The First Law of Thermodynamics for special processes.
First Law of Conditions
Process
Thermodynamics
Isovolumetric ­ no work done
ΔV = 0 so PΔV = 0
and W = 0 so ΔU = Q
Isothermal Adiabatic
Isolated
System
no change in temperature
and internal energy
no energy transferred as heat
no heat or work
interaction with surroundings
ΔT = 0 so ΔU = 0
ΔU = Q ­ W = 0
Q = W
Q = 0 ΔU = ­W
Q = W = 0
ΔU = 0 Ui = Uf
Interpretation
Energy added Q > 0
Energy removed
Q < 0
Energy added to system as heat is removed as work W < 0 increases internal energy no change in the system's internal energy
2
Section 11­2.notebook
February 28, 2012
A total of 135 J of work is done on a gaseous refrigerant as it undergoes compression. If the internal energy of the gas increases by 114 J during the process, what is the total amount of energy transferred as heat? Has energy been added to or removed from the refrigerant as heat?
ΔU = Q ­ W
3
Section 11­2.notebook
February 28, 2012
Cyclic Process ­ a thermodynamic process in which a system returns to the same conditions under which it started
A refrigerator performs work to create a temperature difference between its closed interior and its environment. It can be represented by a low temperature Tcold, Tc and a high temperature Thot, Th. The refrigerator uses work performed by an electric motor to compress the refrigerant. The process occurs with four basic steps. Found on page 414.
1. (A) liquid refrigerant at a low temperature and pressure (colder than the air in the refrigerator) the refrigerant absorbs the warm air. The refrigerant absorbs energy until it begins to boil and eventually vaporize.
2. (B) Once in vapor phase the refrigerant is passed through a compressor. The compressor does work on the gas by decreasing its volume without transferring energy as heat. The adiabatic process increases the pressure and internal energy of the gaseous refrigerant.
4
Section 11­2.notebook
February 28, 2012
3. (C) The refrigerant is moved to outer parts where thermal contact with the outside air (which is at a lower temperature). The gaseous refrigerant at high pressure then condenses at a constant temperature to a liquid.
4 (D) The refrigerant is brought back in by passing an expansion valve and expands without absorbing energy as heat. The liquid then does work at it moves from a high pressure to a low pressure and its volume increases . By doing work against the spring in the valve the refrigerant reduces its internal energy 5
Section 11­2.notebook
February 28, 2012
Step A
Q
+
W
0
ΔU
+
B 0 ­
+
C
­
0
­
D
0
+
­
6
Section 11­2.notebook
February 28, 2012
The change in internal energy of a system is zero in a cyclic process
The initial internal energy is the same as the final internal energy
ΔUnet = 0 and Qnet = Wnet
The net work done during a cycle must equal the net amount of energy transferred as heat. Energy is transferred as heat from the cold interior of the refrigerator Qcold, Energy is transferred as heat from the hot condensing refrigerant to the outside cooler air Qhot. Therefore the difference between Qh and Qc equals the net change of work
Wnet = Qh ­ Qc where Qh > Qc
7
Section 11­2.notebook
February 28, 2012
A heat engine uses heat to do work.
An example is a water wheel. Where water falls from one level to another and using the potential energy increases the kinetic energy so the water does work on the wheel. A heat engine does work by transferring energy from a high temperature to a lower temperature substance. Wnet = Qh ­ Qc
The internal­combustion engine is another type of heat engine.
In the engine some of the matter in the system is not isolated and is removed. There are still the four steps to the process. The steps are found on page 418
8
Section 11­2.notebook
February 28, 2012
Practice 11B ­ #1, 2, 4
Section Review ­ #1, 2, 5, 7
9