[SOLVED] Help with understanding the uncertainty principle I may have posted this in the wrong forum. If so I am sure a moderator could move it to the correct one. Problem statement: I have an assignment in which I need to calculate the lowest possible mean of an electron's KE , <KE>, based on the uncertainty of the electron's moment. Given data: The radius in which the electron is allowed to move, d=2.81*10^-10 Attempt at solution: The uncertainty of the electrons position corresponds to d, deltax=d Using the uncertainty principle I got, deltap = hbar/(2*deltax), where deltap is the lowest possible deviation. I got deltaKE from, deltaKE = deltap^2/(2m), (m is the electron mass) A fellow student explained to me that because my problem involves a stationary system (an electron bound to an atom), <KE> should be zero, which gives me: (deltaKE)^2 = <KE^2> - <KE>^2 => (deltaKE)^2 = <KE^2> But how do I proceed from here?