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MHS Physics Department AP Unit II C 2 Laws of Thermodynamics Ref: Chapter 12.

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Presentation on theme: "MHS Physics Department AP Unit II C 2 Laws of Thermodynamics Ref: Chapter 12."— Presentation transcript:

1 MHS Physics Department AP Unit II C 2 Laws of Thermodynamics Ref: Chapter 12

2 MHS Physics Department a) Students should know how to apply the first law of thermodynamics, so they can: (1) Relate the heat absorbed by a gas, the work performed by the gas, and the internal energy change of the gas for any of the processes above. (2) Relate the work performed by a gas in a cyclic process to the area enclosed by a curve on the PV diagram.

3 MHS Physics Department b) Students should understand the second law of thermodynamics, the concept of entropy and heat engines and the Carnot cycle, so they can: (1) Determine whether entropy will increase, decrease, or remain the same during a particular situation. (2) Compute the maximum possible efficiency of a heat engine operating between two given temperatures. (3) Compute the actual efficiency of a heat engine. (4) Relate the heats exchanged at each thermal reservoir in a Carnot cycle to the temperatures of the reservoirs.

4 MHS Physics Department 1st Law of thermodynamics. This is a statement of the conservation of energy. An insulated container filled with an ideal gas rests on a heat reservoir. The container is fitted with a snug but frictionless weighted piston that can be raised or lowered. The confined gas is the system and the piston and heat reservoir are the surroundings

5 MHS Physics Department Heat Exchange

6 MHS Physics Department Work is done on the system when the gas is compressed. W = - F Δs. Since F = PA and A Δs = V we have W = -P ΔV and vice versa. W is negative when the system does work against its surroundings, and positive when the surroundings do work on the system isochoric - constant volume, isobaric - constant pressure, isothermal - constant temperature, adiabatic - no heat exchanged

7 MHS Physics Department First Law Thermodynamics The system’s internal energy ΔU = Q (heat energy) + W (work done on the gas) ΔU = Q + W A 0.5 mol of an ideal gas (C V = 12.5J/mol K, C p = 20.8 J/mol K) is brought from state a to state b along the path shown in the following P-V diagram

8 MHS Physics Department 1.5 x 10 5 Pa P V (x10 -3 m 3 10 30 ab What are the values of each of the following (a) Temperature at a and b b) Work done by the gas during ab c) Heat added to the gas during ab d) change in internal energy of gas

9 MHS Physics Department 1.R in Physics is 8.31 j/mol K. In chemistry they use J/liters K. If 1 mol of gas is 22.4 liters, what will be R in chemistry? 2. A gas expand from 4 m 3 to 85 m 3 at constant pressure of 1 atmosphere. How much work does it do. 3.A gas increases in pressure from 1 atmosphere to 3 atmospheres at a constant volume of 4 m 3. What is the work done?

10 MHS Physics Department 2 nd Law Thermodynamics Heat Engine (E.g. Internal combustion engine) are Cyclic Engines ΔU = 0. They have a hot reservoir (Q H ) and a cold reservoir (Q C ). Q net = -W = Q H – Q C. Efficiency = work out/work in = (Q H – Q C )/Q H = 1- Q C /Q H Efficiency is always less than one!!!!!!!

11 MHS Physics Department Carnot Engine Theoretically most efficient engine Since Q is proportional to absolute temperature Efficiency = work out/work in = (T H – T C )/T H = 1- (T C /T H )

12 MHS Physics Department Diagram of pressure and volume graph for Carnot Engine

13 MHS Physics Department Entropy The change in entropy of a system (ΔS) is equal to the heat flowing into or out of the system (ΔQ) divided by the absolute temperature (T). Entropy is a measure of the disorder of the universe. Reactions tend to go in the direction of increasing Entropy. ΔS = ΔQ/T Therefore the second law of thermodynamics can be stated in two ways: 1. Heat will not flow spontaneously from a cold object to a hot object. 2. No heat engine operating in a cycle can absorb thermal energy from a reservoir and perform an equal amount of work

14 MHS Physics Department 1. If the heat flowing into a system is 200 Joules at 373K, what is the entropy of the system? 2. If the Carnot internal combustion engine operates at 800°C and exhausts to 25 °C, what is the theoretical efficiency of the engine? 3. If 330 ml of steam at 1 x 10 5 Pa is cooled from 100 °C to 0 °C, at constant volume, what is the change in pressure?

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