In a certain circuit supplied from 50 Hz mains, the potential difference has a maximum value of 500 V and the current has a maximum value of 10 A. At the instant t=0, the instantaneous values of the p. d. and the current are 400 V and 4 A respectively, both increasing positively. Assuming sinusoidal variation, state trigonometrical expressions for the instantaneous values of the p. d. and the current at time t. Calculate the instantaneous values at the instant t = 0.015 s and find the angle of phase difference between the p. d. and the current.​