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Q.17.A circuit has a line of 5A. how many lamps ofrating 40W; 220 V can simultaneously run on thisline safely? [Board Term I, Set-57289R, 2014]

Answer»

Power rating of bulb is 40watts and 220vresistance of bulb,R=V²/p⇒220*220/40⇒1210ohmsmaximum voltage=220vmaximum current= 5 amp.Maximum resistance, when 'n' no of resistances are connected in parallel.R₁=v/I⇒220/5⇒44 ohmsNet resistance=R₁=r/n⇒44=1210/nn=27.5so 27 lamps can be lighted.



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