Hello there. New here. I have read and tried to understand the Kelvin Plank statement of the second law. Essentially, it would imply, it is impossible to convert a given quantity of heat completely to work. Why is it so? I mean, what is the justification for this? I am ready to accept that it's practically impossible (to "devise" a heat engine) to convert heat completely to work, due to technological aspects like friction can't be zero, dissipation effects cannot be completely nullified, but theoretically in the most ideal case, why is it impossible? And I am not looking for explanations that include entropy, because entropy is a consequence of the second law. That'd be a circular argument. If I go by first law, change in internal energy = heat - work. Therefore, Work=IE + Heat. For work=heat, we need change in internal energy to be zero. Why is the change in internal energy not reversible? That is, I supply a heat (Say Q) to a body. Temperature (hence internal energy) increases. I want to take away the supplied heat Q in the form of work, so that internal energy reverts back to original. Why is that impossible?