I was wondering that why does the equivalent resistance actually increase in a series connection of resistors and why does it actually decrease in a parallel connection of resistors? I know that we can prove it by the formula $V/I = R$ but what's actually the reason behind it. The current remains...

I'm currently reading Griffiths' book about Quantum Mechanics but I cannot understand how he derives the formula for the time derivative of the expected value of position in 1 dimension. He writes: $$\frac{\mathrm d\langle x \rangle}{ \mathrm dt} = \int x\,\frac{\partial}{\partial t}\left(|\psi|^...