# Why is a heat sink required? [duplicate]

In 'Thermodynamics' , the section under "heat engines" mentions, that, it is practically impossible to create an engine which can operate with just a heat source and no heat sink present. Why isnt that possible? Cant a ship just take heat from the ocean to drive itself, without burning fuel? Also, why is it not possible to convert all(100%) heat intake into work, while the reverse of it is possible? (Tell me if I am wrong somewhere, with the statement on heat engines)

• Source? But it is obvious to us who have studied it that what you are confused about is the physics understanding of entropy. There is no short answer. You need to study this properly from a good textbook for 3 months to fully understand why. Jan 4 at 7:24
• The link answers your question. As for your second question: $efficiency=\frac{W}{Q_1}$ which implies $efficiency=1-\frac{T_1}{T_2}$(I hope you understand the steps that I skipped). Now since $T_1$ can't be zero, efficiency can't be 1, i.e. 100%. I'll further add that work is high grade energy and heat is low grade energy; so the former can be converted fully into the latter 100% but not vice versa. Jan 4 at 8:17
• The engine has to operate in a cycle, and to get back to the same initial state to start the cycle over, heat has to be removed from the working fluid. Jan 4 at 11:57