Question about the 2nd law of thermodynamics (heat loss)

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In a closed system with an ideal gas and a movable piston, the first law of thermodynamics states that the heat added (dQ) equals the change in internal energy (dU) plus the work done (dW). The second law of thermodynamics indicates that while mechanical energy can be fully converted to heat, the reverse is not entirely possible. When heat is added to the cylinder at constant temperature, some energy is lost due to inefficiencies in the conversion process. Although heat energy can be converted to work, this cannot be achieved in a cyclic process without some energy loss. Thus, maintaining constant temperature while adding heat results in energy dissipation, highlighting the limitations of energy conversion in thermodynamic systems.
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Assume that a closed system of cylindar filled with ideal gas consists of a movable piston.We know from the 1st law dQ=dU +dW.
According to the 2nd law mechanical energy can be totally converted into heat energy but heat energy cannot be converted completely into mechanical energy.The question is"If we give heat to the cylindar and keep the temperature constant how will some of the heat energy be lost ?(here dU=0,and we know dQ=dU+dW)
 
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You can completely convert heat energy to work. You just can't do it by operating the system in a cycle.
 
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