# First Law of Thermodynamics

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First Law of Thermodynamics
Created by: Marlon Flores Sacedon Physics section, DMPS June 2010

The First Law of Thermodynamics
Thermodynamic system is a system that can interact (and exchange energy) with its surroundings, or environment, in at least two ways, one of which is heat transfer. Thermodynamic process is a process in which there are changes in the state thermodynamic system. Work Done during volume changes

Work Done by the system

Work Done by the system F A dS Where: W = work done by the system [J]
p = pressure [pa] dV = differential volume [m3] V1 & V2 = initial and final volume [m3]

Work Done by the system Where: W = work done by the system
p = pressure dV = differential volume V1 & V2 = initial and final volume p V V1 V2 pV-diagram

Work Done by the system Signs of work done

Work Done by the system If the pressure is constant during thermodynamic process

Problem

Work Done by the system Paths Between Thermodynamics States

Internal Energy (U) Internal Energy of a system is the sum of kinetic energies of all of its constituent particles, plus the sum of all the potential energies of interaction among these particles. Where: = change in internal energy U1 = initial internal energy U2 = final internal energy

The First Law of Thermodynamics
System Surroundings (environment) Q = 150J W = 100J = Q-W = +50 J System Surroundings (environment) Q = -150J W = -100J = Q-W = -50 J Where: = change in internal energy (J) W = work done (J) Q = heat quantity (J) System Surroundings (environment) Q = 150J W = 150J = Q-W = 0

The First Law of Thermodynamics

The First Law of Thermodynamics
Ex. A gas in a cylinder is held at a constant pressure of 2.30x105 Pa and is cooled and compressed from 1.70 m3 to 1.20 m3. The internal energy of the gas decreases by 1.40x105 J. a) Find the work done by the gas. b) Find the absolute value of the heat flow into or out of the gas, and state the direction of heat flow. c) Does it matter whether or not the gas is ideal? J, b) 2.55x105J, out of gas, c) no (Ans. a) -1.15x105 Ex. A gas in a cylinder is held at a constant pressure of 2.30 x 105 Pa and is cooled and compressed from 1.70 m3 to 1.20 m3. The internal energy of the gas decreases by 1.40 x 105 J. a) Find the work done by the gas, b) Find the absolute value |Q| of the heat flow into or out of the gas, and state the direction of heat flow, c) Does it matter whether or not the gas if ideal? Why or who not?

Kinds of Thermodynamic Process
1. Adiabatic Process (pronounced “ay-dee-ah-bat-ic”) is defined as one with no heat transfer into or out of a system: Q = 0. (adiabatic process) 2. Isochoric Process (pronounced “eye-so-kor-ic”) is a constant-volume process. When the volume of thermodynamic system is constant W=0. (isochoric process) 3. Isobaric Process (pronounced “eye-so-bear-ic”) is a constant –pressure process. (Isobaric process) 4. Isothermal Process (pronounced “eye-so-bear-ic”) is a constant –temperature process. (Isothermal process)

Kinds of Thermodynamic Process

Internal Energy of an Ideal Gas
Property of Ideal Gas: The internal energy of an ideal gas depends only on its temperature, and not on its pressure and volume.

Heat Capacity of an Ideal Gas
Molar heat capacity at constant volume (CV) Molar heat capacity at constant pressure (Cp) or (First Law) At constant volume At constant pressure (from First Law) ( from pV=nRT ) (because dQ=dU) Where: Cp = molar specific at constant pressure (J/mol.K) CV = molar specific at constant volume (J/mol.K) R = ideal gas constant initial and final volume (Molar heat capacities of an ideal gas) (ratio of heat capacities)

Molar Heat Capacities of Gases
Type of Gas Gas CV (J/mol.K) Cp (J/mol.K) Cp-CV (J/mol.K) (J/mol.K) Monatomic He 12.47 20.78 8.31 1.67 Ar Diatomic H2 20.42 28.74 8.32 1.41 N2 20.76 29.07 1.40 O2 20.85 29.17 CO 29.16 Polyatomic CO2 28.46 36.94 8.48 1.30 SO2 31.39 40.37 8.98 1.29 H2S 25.95 34.60 8.65 1.33

Heat Capacity of an Ideal Gas
Molar heat capacities for Monatomic ideal gas Molar heat capacities for Diatomic ideal gas Molar heat capacities for Polyatomic ideal gas

Example. In an experiment to simulate conditions within an automobile engine, 645J of heat is transferred to mol of air-conditioned within a cylinder of volume 40.0cm3. Initially the nitrogen is at a pressure of 3.00x106 Pa and a temperature of 780K. a) If the volume of the cylinder is held fixed, what is the final temperature of the air? Assume that the air is essentially nitrogen gas, use the Table. Draw a pV-diagram for this process. b) Find the final temperature of the air if the pressure remains constant. Draw a pV-diagram for this process

Adiabatic Process for an Ideal Gas
No heat transfer, Q = 0

Adiabatic Process for an Ideal Gas