PV work w = - Pex dV = - PexV QUESTION: A system expands, resulting in the transfer of 5.0 J between system and surroundings. Which of the following is/are true? The system did 5.0 J of work w = - 5.0 J Both Neither When a system expands (against a nonzero external pressure), it is said to be doing work. A system doing work is losing energy. In most modern Chemistry and Physics textbooks, the first law of thermodynamics is: U = q+w Convention: w < 0 if done by the system Old convention: U = q-w  w > 0 if done by the system A system expands, resulting in the transfer of 5.0 joules of energy between system and surroundings. Which of the following is or are true? The system did 5.0 joules of work w = negative 5.0 joules Both statements A and B are true, or Neither statement is true PAUSE When a system expands, HIGHLIGHT “expands” it is said to be doing work. HIGHLIGHT “doing work” Therefore, the statement in choice A is True. CALLOUT “TRUE” pointing to choice A. The system did work, HIGHLIGHT “did” in choice A and the amount of work done is the amount of energy transferred HIGHLIGHT “5.0 J” in question and in choice A What about choice B? Whether it’s true or not depends on the textbook you are using. CALLOUT “True or False depending on textbook” pointing to choice B CLICK In most modern Chemistry and Physics textbooks, the first law of thermodynamics is written as Delta U equals q plus w HIGHLIGHT U = q+w CALLOUT “E in some textbooks” pointing to “U” In this case, w is negative if the system is doing the work. HIGHLIGHT “w<0 if done by the system” A negative value for U is associated with a drop in energy. If a system is doing work, it’s losing energy, ….so, the w term in the expression must be negative. However, in older textbooks, the first law of thermodynamics is written as Delta U equals q MINUS w HIGHLIGHT U = q-w If the system is doing the work, the w term in the expression must be positive. CLICK Work due to expansion or compression of an object is called “pressure volume work”,… or PV work Using the modern sign convention, PV work is defined as the NEGATIVE of the integral of the external pressure HIGHLIGHT Pex in formula with respect to volume. When a system expands, it typically encounters a resistance from the surroundings. That resistance is called the external pressure. If the external pressure is constant while the system is expanding, then the formula simplifies to… negative of external pressure, times change in volume HIGHLIGHT –Pex V Note that expansion means an increase in volume,… and a positive value for deltaV, …. And a negative value for w. On the other hand, if a system is compressed due to an external pressure, … deltaV would be negative, …. And the value for w would be positive. Finally, if the system were to expand into a vacuum, it would not encounter a resistance, and we would say that the external pressure is zero. In this case, we say that the no work is involved…. w equals zero. CLICK PAUSE END RECORDING

Video ID: 8-6-1 © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Assisted by Johuan Gasery In partial fulfillment of the requirements for CHEM 481, Fall 2008.