In an a.c. circuit voltage v and current i are given by v = 100 sin 100 t volts, i = 100 sin {100 t + π/3} mA.  The power dissipated in the circuit is

2.5W 5W 7.5W 10W

2.5W

$P = V_{rms}I_{rms} cos\phi = \frac{100V}{\sqrt 2} \times \frac{100\times 10^{-3}}{\sqrt 2} \times cos\frac{\pi}{3} = 2.5W$