A piston compresses the air in the cylinder by doing 97.0 J of work on the air. As a result, the air gives off 191.0 J of heat to the surroundings. If the system is the air in the cylinder, what is the change in its internal energy?

Explanation:

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In this case, since the first law of thermodynamics allows us to understand how the energy behaves when heat and work are involved during a process, we can mathematically define it as:

Whereas Q is the heat added to the system, W the work done by the system and ΔU the change in the internal energy, we can notice that since 97.0 J of work are done on the system it would be negative and the given off heat is also heat due to the used convention; therefore the change in its internal energy turns out:

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