Internal Energy and the First Law of Thermodynamics Prof. Marlon Flores Sacedon Department of Mathematics and Physics College of Arts and Sciences Visayas State University, Visca Baybay City, Leyte, Phiippines

Internal Energy (U) What is Internal Energy? 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 = 150J – 100J = +50 J System Surroundings (environment) Q = -150J W = -100J Where: = change in internal energy (J) W = work done (J) Q = heat quantity (J) = Q-W = -150J – (-100J) = -50 J System Surroundings (environment) Q = 150J W = 150J = Q-W = 0

Human daily cyclic thermodynamic process

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? (Ans. a) -1.15x105 b) 2.55x105J, out of gas, c) no

EXAMPLE: The pV-diagram of Fig. shows a series of thermodynamic processes. In process ab, 150 J of heat is added to the system; in process bd, 600 J of heat is added. Find (a) the internal energy change in process ab; (b) the internal energy change in process abd (shown in light blue); and (c) the total heat added in process acd (shown in dark blue).

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