1. The problem statement, all variables and given/known data Capacitor plates are length L with radii a,b a<b are co-axial and intially covering eachother with line charge density of the inner plate Q/L. determine the force between plates as the inner plate is partially withdrawn along the axis. i) if cylinders charged to voltage V and then discnnected ii) if cylinders remain connected and maintained at voltage V 2. Relevant equations u=(1/2)cv^2=(1/2)qv F=-du/dL voltage between plates = Qln(b/a)/2Lpi*e0 capacitance between plates = 2Lpi*e0/ln(b/a) 3. The attempt at a solution using partial derivitives. for i) hold q constant differentiate for V du=(1/2)qdv -du/dl=(-1/2)Q(d/dL)(Qln(b/a)/2Lpi*e0) = (1/4)(Q^2)ln(b/a)/2(L^2)pi*e0) and is directed same direction to one being removed.. using partial derivitives. for i) hold v constant differentiate for q du=(1/2)(v^2)dc -du/dl=(-1/2)(v^2)(d/dL)(2Lpi*e0/ln(b/a)) = (-1/2)(v^2)(2pi*e0/ln(b/a)) and is directed in opposite direction to one being removed Are this working OK? Many thanks!