Systems and Surroundings: The First Law of Thermodynamics

The term system is used to describe the particular part of the universe we are focusing our attention on: a beaker, a cell, Earth and its atmosphere, et cetera; everything outside the system is considered the surroundings or environment (i.e., system + surroundings = universe). A system may be classified as:

• Isolated—when it cannot exchange energy or matter with the surroundings, as with an insulated bomb reactor or a well-insulated Thermos flask
• Closed—when it can exchange energy but not matter with the surroundings, as with a steam radiator or a stoppered test tube out of (and into) which heat can flow
• Open—when it can exchange both matter and energy with the surroundings, as with a pot of boiling water: Water molecules are escaping into the gas phase, bringing energy with them.

Basic Concept

Three Types of Systems

• Isolated
• Closed
• Open

You may have learned that energy, though interconvertible among all its different forms (kinetic, potential, et cetera), is conserved: The total amount of energy has to be constant. This is true of a particular system only if it is isolated. Since energy can neither go in nor go out, it has to be conserved. If the system is closed or open, the amount of energy in the system can certainly change. A system can exchange energy with its surroundings in two general ways: as heat or as work. The first law of thermodynamics states that the change in the internal energy of a system is equal to the heat added to the system, q, minus the work that a system does, w:

∆E (or ∆U) = q − w

If work is done on a system, w is negative. Note, however, that sometimes w is defined as the work done on, rather than by, the system, in which case the equation is written as ∆E = q + w, and work done by the system is considered negative. Regardless of which convention is used, if work is done on a system, its energy will increase; if work is done by the system, its energy will decrease. Work is generally associated with movement against some force. For ideal gas systems, for example, expansion against some external pressure means that work is done by the system, while compression implies work being done on the system.