1)

The supply voltage in a room is 120 V. The resistance of the lead wires is 6 Ω. A 60 W  bulb is already switched on. What is the decrease of voltage across the bulb, when a 240 W heater is switched on in parallel to the bulb?


A) zero

B) 2.9 V

C) 13.3 V

D) 10.04 V

Answer:

Option D

Explanation:

P=V2R

3032021560_P6.JPG

R=120×12060=240  Ω

  Req = 240+6=246 Ω

    i1=VReq=120246

303202129_p7.JPG

 

v1=240246×120=117.073V

     i2=12048+6

3032021825_p8.JPG

  v2=4854×120=106.66V

   v1-v2= 10.04 V