First Law - Open Systems: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • At steady-state, the properties (T, P, H, mass flow rates, etc.) do not change with time at a given location.
  • An open system has mass flows in and/or out of the system.
  • When a gas flows into a tank, the gas does flow work on the tank and this increases the temperature of the tank contents.
  • The energy flowing into or out of a container is expressed as enthalpy in order to account for flow work.
  • The energy with a tank is expressed as internal energy.
From studying this module, you should now be able to:
  • Apply the first law for open systems in its differential equation form and its integrated form.
  • Apply the first law (energy balance) to steady-state flow systems, including throttles, turbines, compressors, and pumps.
  • Apply the unsteady-state first law to flow into or out of a tank.
  • Understand how gas flowing into a tank does work on the tank contents.
  • Explain the concept of flow work and why enthalpy is used for energy flowing into or out of a system.
Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder