1. The problem statement, all variables and given/known data The expression for electric charge on the capacitor in the series RLC circuit is as follows: q(t)=A*exp(-Rt/2L)*cos(omega*t+phi) where omega=square_root(1/LC-R^2/4L^2) What is the phase phi, if the initial conditions are: q(t=0)=Q I(t=0)=0 2. Relevant equations The damped oscillator equation is: second_derivative(q)+R/L*first_derivative(q)+q/LC=0 3. The attempt at a solution I calculated the current (as a derivative of charge): I(t)=A/square_root(L*C)*exp(-Rt/2L)*cos(omega*t+phi-psi) where tg(psi)=2*L*omega/R When I set the initial condtitions, I get: A*cos(phi)=Q A/square_root(L*C)*cos(phi-psi)=0 and then I get: tg(phi)=-1/tg(psi)=-R/(2*L*omega) which seems really strange to me: shouldn't phi=0? If I start with some charge on capacitor and no current, the charge can only go down, at no moment the charge will be larger, so I think that q(t)=A*exp(-Rt/2L)*cos(omega*t) Or am I not right?