Question

A series R-L circuit is connected to 110V ac source .If voltage across resistor is 85V find power dissipated in circuit

Answer 1

Since the voltages across the inductor and the resistor are $90^0$out of phase, we obtain the voltage across the inductor in the phase domain as follows: If the voltage across the resistor is$V_1$, the voltage across the inductor will be$jV_2$.

Assume the voltage of the

source is

$V_3$

Applying KVL law

$$V_1+jV_2=V_1cos(wt)+V_2sin(wt)=V_3cos(wt-\theta)=85+V_2sin(wt)=110cos(wt-\theta)$$

$110=\sqrt{85^2+V_2^2}$

$V_2=69.82V$

For example

$P_{ave}=\frac{1}{2}\frac{V^2}{R}$

Where V=85V

Assume $R=20 \Omega$

Put value

$P_{av}=\frac{1}{2}\frac{85^2}{20}=180.6 W$