## Physics: Principles with Applications (7th Edition)

One example is a system that undergoes a change of state (melting/freezing, boiling/condensing) because heat is transferred but the temperature remains constant. Another example may be found from the first law of thermodynamics. $$\Delta U = Q-W$$ The change in internal energy can be zero, even though Q is nonzero, if Q=W. The amount of work done on or done by the system equals the heat expelled or added. An example is the isothermal expansion of an ideal gas.