Work (6.1.4) Flashcards
• By convention, energy, heat, or work entering a system has a positive sign, while
energy, heat, or work coming out of a system has a negative sign.
• By convention, energy, heat, or work entering a system has a positive sign, while
energy, heat, or work coming out of a system has a negative sign.
• PV work is work performed by a gas expanding or contracting.
• PV work is work performed by a gas expanding or contracting.
If energy flows from the surroundings to the
system, the change in internal energy (∆E) is
positive. If energy flows from the system to the
surroundings, ∆E is negative.
If heat flows from the surroundings to the system,
heat (q) is positive. If heat flows from the system to
the surroundings, q is negative.
If work (w) is done on the system by the
surroundings, w is positive. If work is done on the
surroundings by the system, w is negative.
PV work is work performed by a gas expanding or
contracting.
In general, work (w) equals force (F) multiplied by
distance (d). For PV work, the force is equal to the
external pressure (Pex) times the area (A), and the
distance is equal to the change in height (∆h). Area
multiplied by change in height equals change in
volume (∆V). Therefore, the work done is equal to
Pex multiplied by ∆V. Since PV work is done on the
system by the surroundings, w = –Pex∆V.
Problem: Calculate the work done in joules when
a gas expands from 1.00 L to 2.00 L against a
constant external pressure of 1.00 atm. Recall:
1 L•atm = 101.325 J.
Plugging the given quantities into the equation
w = –Pex∆V yields the work (–101 J).
If energy flows from the surroundings to the
system, the change in internal energy (∆E) is
positive. If energy flows from the system to the
surroundings, ∆E is negative.
If heat flows from the surroundings to the system,
heat (q) is positive. If heat flows from the system to
the surroundings, q is negative.
If work (w) is done on the system by the
surroundings, w is positive. If work is done on the
surroundings by the system, w is negative.
PV work is work performed by a gas expanding or
contracting.
In general, work (w) equals force (F) multiplied by
distance (d). For PV work, the force is equal to the
external pressure (Pex) times the area (A), and the
distance is equal to the change in height (∆h). Area
multiplied by change in height equals change in
volume (∆V). Therefore, the work done is equal to
Pex multiplied by ∆V. Since PV work is done on the
system by the surroundings, w = –Pex∆V.
Problem: Calculate the work done in joules when
a gas expands from 1.00 L to 2.00 L against a
constant external pressure of 1.00 atm. Recall:
1 L•atm = 101.325 J.
Plugging the given quantities into the equation
w = –Pex∆V yields the work (–101 J).
