decrease such that there is no net change in energy for both the water and
ice cube. This simple example shows that in applying the law it is neces-
sary to think carefully about which objects must be considered in applying
energy conservation. Therefore, first we will look at some definitions.
Objects are considered to be either part of a systemor the surroundings.
The system is the set of objects under consideration and the surroundings
are everything else. In some cases, the surroundings may be considered
as being much larger than the system and so not changing as a result of
an energy exchange with the system. For example, the surroundings may
be represented as a large water bath that remains at constant temperature
regardless of energy flow into or out of the system.
The energy of the system is defined as the capacity of the system to
perform work. Heat and work are forms of energy that can transfer into
a system, or out of it into the surroundings. Energy can be exchanged
between the system and surroundings by either performing work or heat-
ing. A system does work when the energy transfer is coupled with a motion
against an opposing force. For example, gas molecules in a chamber do
work when they push against a piston and change its position. Heating
occurs when the energy transfer is associated with a temperature differ-
ence between the system and surroundings.
The flow of matter and energy in and out of systems can be divided
into three categories: open systems, closed systems, and isolated systems
(Figure 2.1). Each of these systems is defined in terms of the allowed flow
of matter and energy into and out of the system.
- Open systems: a system is described as open when both energy and
matter of the system can be exchanged with the surroundings. An
example of this is a cup of hot water in which matter can leave in the
form of water vapor and water can also heat the surroundings.
24 PARTI THERMODYNAMICS AND KINETICS
Open Closed Isolated
Figure 2.1A system is described as open if it can exchange both energy
and matter with its surroundings, closed if it can exchange only energy,
and isolated if neither type of exchange is possible.
