Steam engines use thermodynamic cycles for their operation. In such a cycle the

Question

Steam engines use thermodynamic cycles for their operation. In such a cycle the change in internal energy of the gas (water vapor) is zero. None-the-less usable work is extracted. Why? The work is not internal energy, but external energy According to the first law of thermodynamics, if Q=W we can still have non-zero work, zero change in internal energy. Steam engines don't do much work so the first law of thermodynamics is only slightly violated Nobody knows how steam engines can do work The energy comes from the chemical combustion of the steam

Explanation / Answer

In general an engine converts energy to mechanical work. Heat engines distinguish themselves from other types of engines by the fact that their efficiency is fundamentally limited by Carnot's theorem. Although this efficiency limitation can be a drawback, an advantage of heat engines is that most forms of energy can be easily converted to heat by processes like exothermic reactions (such as combustion), absorption of light or energetic particles, friction, dissipation and resistance. Since the heat source that supplies thermal energy to the engine can thus be powered by virtually any kind of energy, heat engines are very versatile and have a wide range of applicability.

So option 1 is correct :

The work is not internal energy but external energy.

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