In an AC circuit with only a capacitor this diagram represents the relation between the current and the voltage in it (the current leads the voltage by 90 degrees). and because: (I= dQ/dt) and ( Q=C*V) where: Q is the amount of charge, C is the capacitance and V is the potential difference between the plates of the capacitor. Then: I=C (dV/dt). In the diagram above my textbook refers to the slope of the tangent of the voltage curve to be (dV/dt), and I don't understand it, could you please explain this for me? Regarding that I don't actually know how to determine the slope of a tangent of a curve.