# Warm-up Complete the Free response you picked up at the door.

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Warm-up Complete the Free response you picked up at the door.

Do Now (11/4/13): What are the Laws of Thermodynamics?

Objectives Describe thermodynamic processes on P-V diagrams.
State and apply second law principles. Apply the laws of thermodynamics to the Otto cycle.

Today’s Plan Finish efficiency discussion. Discuss Thermodynamics.
Discuss the Otto cycle. Homework problems due Thursday! Quiz Friday.

Specific Heat How much heat is required to raise the temp of an empty 20-kg vat made of iron (c=450J/kg°C) from 10°C to 90°C

Calorimetry If 200cm3 of tea (c=4186J/kg°C) at 95°C is poured into a 150-g glass cup (c=840J/kg°C) initially at 25°C, what will be the final temperature T of the mixture when equilibrium is reached, assuming no heat flows to the surroundings?

First Law U=Won + Qinto
Where U=internal energy, Q=net heat added to system, W=net work done on the system. Conventionally, heat added is +, lost is negative Work done on system is +, done by system is negative Statement of energy conservation

Work Energy transfer between system and surroundings due to organized motion in the surroundings. (rubbing a block of wood vigorously, stir a glass of water, allow a gas to expand against an external pressure.)

Heat Energy transfer between system and surroundings as a result of random motion in the surroundings. Flows spontaneously from high temp to low temp. Work can be used to make heat flow opposite natural flow direction.

Efficiency Ratio of work done by the gas to the heat that flows into the system. e=Wby / Qin Develop a procedure to determine the efficiency of a microwave and a hot plate. Which is more efficient?

Pressure-Volume Work P= F/A F=P*A Wby = F*d = PV
Graphical representation. Area under curve = work done by gas

Example: Gas in a cylinder is at a pressure of 8000 Pa and the piston has an area of 0.10 m2. As heat is slowly added to the gas, the piston is pushed up a distance of 4 cm. Calculate the work done on the surroundings by the expanding gas. 32 J

PV Diagrams In general, the work done in an expansion from some initial state to final state is the area under the curve on a PV diagram

Cyclical Direction matters

Example: Find the work done for each process
What is the total work done?

Example: To find the work done by the gas, find the area under each segment, remembering the sign convention.

Example: For a constant volume process like B -> C, no work is done by the gas. The total work done for the entire cycle is the area enclosed within the graph. In this example, the sum of the work is Which is the area of the enclosed triangle

Heat engines such as automobile engines operate in a cyclic manner, adding energy in the form of heat in one part of the cycle and using that energy to do useful work in another part of the cycle.

Thermodynamic Systems
Isothermal—work done by the gas equals the heat added to the gas. U=0 Adiabatic—no heat is allowed to flow into or out of the system. Q=0 therefore, U=Won. Isobaric—pressure is constant Isochoric (isovolumetric) —volume is constant

Isothermal Constant temperature PV=constant
Graphical representation and isotherms As heat is added slowly, gas expands at constant temperature. Work is done by the gas. U=0, so Wby = Qin.

Adiabatic No heat is exchanged between the system and surroundings. Q = 0. U = Won Internal energy decreases as gas expands. (U=3/2 NkT, so temperature will decrease.)

Isobaric Pressure of system remains constant. W= PV

Isochoric Volume remains constant W = 0.

Second Law Heat flows naturally from a hot object to a cold object; heat will not flow spontaneously from a cold object to a hot object.

Heat Engines Heat Engines Carnot Otto Cycle Rankine 2 stroke engine
Refrigeration cycle

Heat Engines Mechanical energy is obtained from thermal energy when heat is allowed to flow from high to low temperature regions. QH=W+QL e=W/QH = 1-(QL/QH) Theoretical efficiency in Carnot (ideal) cycle: eideal=1-TL/TH

Heat Engine

Carnot Engine Theoretical heat engine where all processes are considered reversible. (very slow processes) Actual cycles have turbulence and friction. Ideally, QH and QL are proportional to TH and TL. Theoretical efficiency : eideal=1-TL/TH

Practice: Complete problem #5 in your textbook

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