# 1st Law, Open Systems, Steady-State

In this chapter we will assume that no electrical or boundary work interactions occur. Therefore:
Summary of Characteristics of Steady-State Processes in Open Systems
1. All properties and process varaibles, such as flow rate, remain constant with respect to time at all positions within the system.

2. Properties and process variables within the system do not have the same value at all positions within the system.

3. All heat and work interactions remain constant with respect to time as well.

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### Ch 5, Lesson C, Page 3 - 1st Law, Open Systems, Steady-State

• The steady state form of the 1st Law for open systems looks an awful lot like the general form.
• The only difference is that the left-hand side is ZERO !
• In this chapter, and in most thermodynamics problems involving open systems, there is no electrical or boundary work.
• In fact an open system cannot have a boundary work interaction at steady state !
• When we eliminate the boundary and electrical work terms from Wsys, we are left with only shaft work, W sub s.
• The equation in the yellow box is the form of the 1st Law for steady-state processes on open systems that we will use.
• Before we move on to applications, I’d like you to review exactly what steady-state does and does not mean !
• Nothing changes with TIME.
• It does not mean ANYTHING about position.
• This seems simple, but it is often causes confusion.
• OK, now let’s see how the steady-state assumption affects the mass balance equation !