In an \(R C\) series circuit consisting of a \(20.0-\Omega\) resistor, a \(300-\mu \mathrm{F}\) capacitor, and an AC source, the source frequency is \(150 \mathrm{~s}^{-1}\). If the current has its maximum value at \(t=0\), at what later instant is the source emf at its maximum value?