Laws of Thermodynamics

Size: px

Start display at page:

Download "Laws of Thermodynamics"

Laurence Moody
6 years ago
Views:

1 Laws of Thermodynamics The Three Laws of Thermodynamics - The first lawof thermodynamics, also called conservation of energy. We can use this knowledge to determine the amount of energy in a system, the amount lost as waste heat, and the efficiency of the system. -The second lawof thermodynamics states that the disorder in the universe always increases. As the disorder in the universe increases, the energy is transformed into less usable forms. Thus, the efficiency of any process will always be less than 100%. - The third law of thermodynamics tells us that all molecular movement stops at a temperature we call absolute zero, or 0 Kelvin (-273 o C). Since temperature is a measure of molecular movement, there can be no temperature lower than absolute zero. At this temperature, a perfect crystal has no disorder.

2 Entropy a measure of the disorder of a system.the concept of entropy addresses the transformation of energy, and tells us how much energy can be converted from one form into another, and in particular, how much energy is available for doing useful work. ENTROPY It is impossible to construct a heat engine that performs an amount of work equal to the heat absorbed by the system! The entropy of the universe increases in all natural processes.

3 ENTROPY The change in entropy of a system, ΔS, is equal to the heat, ΔQ, flowing into the system as it changes from one state to another divided by absolute temperature.

4 ENTROPY The second law of thermodynamics enables us to classify all the processes under two main categories: reversible (or ideal processes) and irreversible (or natural processes). REVERSIBLE PROCESS - The process in which the system and surroundings can be restored to the initial state from the final state without producing any changes in the thermodynamics properties. IRREVERSIBLE PROCESS the initial state of the system and surroundings cannot be restored from the final state the various states of the system on the path of change from initial state to final state are not in equilibrium with each other. the entropy of the system increases decisively and it cannot be reduced back to its initial value.

5 ENTROPY In a reversible process, ΔS remains constant In a irreversible process, ΔS increases due to loss of energy (through friction, leakage, damping, etc.) ΔS = ΔQ/T ΔS - positive when disorder increases or order decreases ΔQ - heat exchange between system and surroundings (=0 in adiabatic process)

6 The Gasoline Engine A gas engine means an engine running on a gas (any type of gas: bio fuel, petroleum, natural gas, etc.).

Otto Cycle - Gasoline Engine The process for a four-stroke gasoline engine is modeled using the Otto cycle. (1) Air drawn into system (O to A). Volume increases from V 2 to V 1.

7 Otto Cycle - Gasoline Engine The process for a four-stroke gasoline engine is modeled using the Otto cycle. (1) Air drawn into system (O to A). Volume increases from V 2 to V 1. (2) From A to B: (compression stroke) Air-fuel mixture is compressed from V 1 to V 2 adiabatically (w/o heat being transferred to the surroundings). Temp increases from T 1 to T 2. Work done on system. (3) From B to C: (Q h, heat flows in due to combustion). Both temp and pressure rise rapidly but volume remains constant. No work done. (4) From C to D: (power stroke) Gas expands adiabatically. Volume increases from V 2 to V 1, causing temp to fall. Work is done by system. (5) From D to A: (Q c, heat extracted from system; hot gas replaced by cool gas) Pressure falls at constant volume. No work done. (6) Exhaust stroke A to O. Residual gases expelled. Volume decreases from V 1 to V 2. Cycle complete.

8 Engine Efficiency Modern gasoline engines have max. thermal efficiencies of 25-30% when used to power a car. Even when the engine is operating at its point of maximum thermal efficiency about 70-75% is rejected as heat without being turned into useful work. At idle, the thermal efficiency is zero since no usable work is being drawn from the engine. Engines using the Diesel cycle are usually more efficient, although the Diesel cycle itself is less efficient at equal compression ratios. Diesel engines use much higher compression ratios because the heat of compression is used to ignite the slow-burning diesel fuel. The most efficient type, direct injection Diesels, are able to reach an efficiency of about 40% in the engine speed range of idle to about 1,800 rpm. At high speeds, efficiency in both types of engine is reduced by pumping and mechanical frictional losses, and the shorter time period within which combustion has to take place.

9 Carnot Engine The Carnot cycle is a theoretical thermodynamic cycle. It is the most efficient cycle for converting a given amount of thermal energy into work, or conversely, creating a temperature difference (e.g. refrigeration) by doing a given amount of work. A Carnot cycle acting as a heat engine. The cycle takes place between a hot reservoir at temperature T H and a cold reservoir at temperature T C. It is not a practical engine cycle because the heat transfer into the engine in the isothermal process is too slow to be of practical value. As Schroeder puts it "So don't bother installing a Carnot engine in your car; while it would increase your gas mileage, you would be passed on the highway by pedestrians."

10 Carnot Cycle 1 to 2: Reversible isothermal expansion of the gas at the "hot" temperature, T H (isothermal heat addition or absorption). During this step (A to B on T-S diagram, 1 to 2 P-V diagram) the expanding gas makes the piston work on the surroundings. The gas expansion is propelled by absorption of quantity Q 1 of heat from the high temperature reservoir.

11 Carnot Cycle 2 to 3: Reversible adiabaticexpansion of the gas.for this step (B to C on T-S diagram, 2 to 3 on P-V diagram) the piston and cylinder are assumed to be thermally insulated, thus they neither gain nor lose heat. The gas continues to expand, working on the surroundings. The gas expansion causes it to cool to the "cold" temperature, T C.

12 Carnot Cycle 3 to 4: Reversible isothermal compression of the gas at the "cold" temperature, T C. (isothermal heat rejection) (C to D on T-S diagram, 3 to 4 on P-V diagram). Now the surroundings do work on the gas, causing quantity Q 2 of heat to flow out of the gas to the low temperature reservoir.

Carnot Cycle 4 to 1: Reversible adiabaticcompression of the.(d to A on T-S diagram, 4 to 1 on P-V diagram) Once again the piston and cylinder are assumed to be thermally insulated.

13 Carnot Cycle 4 to 1: Reversible adiabaticcompression of the.(d to A on T-S diagram, 4 to 1 on P-V diagram) Once again the piston and cylinder are assumed to be thermally insulated. During this step, the surroundings do work on the gas, compressing it and causing the temperature to rise to T H. At this point the gas is in the same state as at the start of step 1.

14 Carnot Cycle Establishes upper limit on efficiency of all engines Gives largest amount of work possible for a given amount of heat supplied to system All processes in the cycle are either adiabatic or isothermal and operate in a reversible cycle (cycle where system & surroundings return to initial state) Adiabatic complete combustion of fuel; no lingering ΔQ input in other parts of cycle (no exchange of heat with surroundings) Isothermal no heat loss to walls; no turbulence of fluids; no friction; heat transfer is done VERY slowly in order to retain isothermal quality Carnot Efficiency (Temp in Kelvins!) Eff c = 1 T c /T h

15 Example In each cycle, a heat engine absorbs 375 J of heat and performs 25 J of work. (a) Find the efficiency of the engine

16 Example In each cycle, a heat engine absorbs 375 J of heat and performs 25 J of work. (b) Find the heat expelled in each cycle

17 Example The exhaust temperature of a Carnot heat engine is 300 C. What is the intake temperature if the efficiency of the engine is 30%?

18 Example An ice tray contains 500g of water at 0 C. Calculate the entropy change of the water as it freezes completely and slowly at 0 C.

Chapter 20. Heat Engines, Entropy and the Second Law of Thermodynamics. Dr. Armen Kocharian

Chapter 20. Heat Engines, Entropy and the Second Law of Thermodynamics. Dr. Armen Kocharian Chapter 20 Heat Engines, Entropy and the Second Law of Thermodynamics Dr. Armen Kocharian First Law of Thermodynamics Review Review: The first law states that a change in internal energy in a system can