Question

The potential difference $V$ and the current $I$ flowing through an instrument in an ac circuit of frequency f are given by V = $5$ cos $\omega t$ volts and $I=$2$sin\omega t$ amperes (where $\omega =2\pi f$). The power dissipated in the instrument is :-

• A. Zero
• B. $10W$
• C. $5W$
• D. $2.5W$

Answer 1

The correct option is A Zero

Given,

$I=2sin\omega t$

$V=5cos\omega t=5sin(\omega t+\frac{\pi }{2})$

Power $=V_{rms}\times I_{rms}\times cos\varphi =0$

$($ Since $\varphi =\frac{\pi }{2}$, therefore, $cos\varphi =cos\frac{\pi }{2}=0$ $)$