# Definitions

Objectives
 1. Definition of terms which are necessary before we move into the discussion of the first law 2. A reference for later sections Note: Other terms will be defined as they are needed.

Equilibrium (plural = equilibria)
No change or tendency to change[1]
No change with respect to time
There may however, be change with respect to position
System
Whatever we are interested in
For example: an engine, a reactor, or a living cell
Surroundings
Everything else outside of the system
Closed System
A system with no mass entering or leaving
Open System
A system with mass entering or leaving
Isolated System
A system with no mass or energy entering or leaving
State
A set of conditions which completely define a system
A state is given by its volume, temperature, pressure, and amount (moles) or mass
Extensive Property
A property dependent on the amount of material
For example, total volume: if we add two 250 mL cups of water together we get 500 mL of water
Intensive Property
A property not dependent on the amount of material
For example, temperature: if we add two 250 mL cups of 30 ° C water together the result is 30 ° C water, not 60 ° water
Pressure
Force divided by the surface area it is applied to
SI[2]unit is the Pascal (Pa) = 1 N/m2
Standard State Pressure
1 bar
1 bar = 105 Pa
previously 1 atmosphere (atm) was used -- 1 atm = 1.1013 bar
Temperature
see the next section

## Notes

1. Some people have learned the definition of equilibrium as "equal and opposite change". That definition is correct for some fields of study; however, in thermodynamics we are only interested in system as a whole -- not what is going on inside the system.
For example, look at a membrane with equal concentrations on both sides. There is movement across the membrane with equal amounts going each direction. However, if we only look at the system from the outside, nothing is changing.
2. Système Internationale d'Unités, commonly know as the metric system. In science it is the normally accepted unit system