Question

what is the resistance of a bulb of 40W connected in a line of 220W

Answer 1

case 1 : power of bulb , P = 40 watt

voltage supplied across the bulb , V = 220V

we know, power supplied by bulb , P = Vi

or, 40 = 220 × i

or, i = 40/220 = 2/11 = 0.18 Amp

again, use formula, P = V²/R to find resistance of bulb

or, P = V²/R

or, R = V²/P

or, R = (220)²/40 = 48400/40 = 1210 ohm

case 2 : if given bulb is replaced by a bulb operating 25 watt , 220 volt.

so, current, I = P'/V = 25/220 = 0.1136 Amp

resistance, R' = V²/P' = (220)²/25 = 1936 ohm.

here it is clearly shown that when we change bulb , current and resistance will be change.

change in current = 0.18 - 0.1136

= 0.0664 Amp [ current decreases when we use 25 watt bulb in place of 40 watt bulb ]

change in resistance = 1936 - 1210= 726 ohm [ resistance increase when we use 25 watt bulb in place of 40 watt ]