Recent content by spideyjj

I am studying Classical Mechanics in this semester, I want to know if there are any suggestions on some problem sets that will help me to master the skills needed.

In a free expansion, I know that we cannot use the equation dS=dQ/T...(1). Instead we use dS>dQ/T...(2). The question is that why we can use △S=ncᵥln(T_f/T_i)+nRln(V_f/V_i) , which is derived from the equation(1), to calculate the entropy change? Shouldn’t it be a inequality too?

The pocket size, like the one in mobile phones

What are the major obstacles in the development of battery encountered currently? Or there are different difficulties in different aspects? I have tried to search it online but it's like searching for a needle in a haystack.