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

This question is really asking 4 things. i. Mechanical energy can be transformed into heat with 100 percent efficiency. If an object slides across a rough horizontal table and comes to a stops, all of its mechanical energy is transformed into heat. ii. Mechanical energy can also be transformed into internal energy with 100 percent efficiency. If a mass is placed on a frictionless, massless piston that presses vertically against a gas in an insulated container, all of the gravitational potential energy becomes internal energy of the gas. iii. In an isothermal process, the change in internal energy is zero, and Q=W. In an isothermal expansion, 100 J of heat is converted into 100 J of mechanical work. It is tempting to say that heat can never be converted completely to mechanical energy because the second law of thermodynamics says it is impossible to make a 100 percent efficient heat engine. But the question does not say that we must consider a complete cycle; It only asks if it is “ever” possible, and it is. In fact, the previous question gives an example. iv. Internal energy, in an ideal process, can be perfectly transformed into mechanical energy. The first law of thermodynamics tells us that if Q=0, then a decrease in internal energy results in the same amount of work. $$\Delta U = Q-W$$ A gas that expands adiabatically is an example of a system that transforms internal energy perfectly into mechanical energy.