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Energy Balance on a Closed System

In this lesson, we will incorporate the effects of mass flow across the system boundaries on the

In chapter 4, we applied the

to closed systems.

Change in U, EK,EP
and other forms of energy

Net energy transfer by heat and work

In chapter 4, we discussed how heat and work interactions fit into the

for a closed system.

In this chapter, we need to discover how mass flow across the system boundary can be incorporated in the 1st Law or energy balance equation.

Closed
System

Lesson 5B: page 1 of 12

In chapter 4 we applied the 1st Law to closed systems.
The result was an energy balance equation of the form shown here.
In a closed system, there are only two ways a system can exchange energy with its surroundings: heat and work.
In an open system, the mass that crosses the system boundary carries energy with it.
And, as we’ll se later, fluid pushing in and out of the system also represents a new form of work interaction.
So, for these two reasons, energy balances on open systems are a bit more complicated.
In this lesson, we will derive a general form of 1st Law for open systems.
Then, in later lessons, we will look at how this big, messy, general equation can be simplified for many common processes.
OK, let’s get started with our open system.