two lamps one rated 100w at 220v and other 60w ĺat 220v are connected in paralleland series to electric mains supply. what current is drawn from the line if the supply voltage is 220v?
Answers
i take this question as two ways
First as two separate case like
A) connected in parallel & B) Connected in Series
P1 = 100W
P1 = V²/R1
100 = (220)²/R1
R1 = 484 Ω
P2= 60W
60 = 220²/R2
R2 = 2420/3 Ω
LAMP in Series = Resistance added up
RS = R1+R2 = 484 + 2420/3 = (1452+2420)/3= 3872/3 Ω
RS = Resistance in Series
I = V/R
IS = current in series
IS = V/RS = 220/(3872/3)=660/3872 = 0.17 A
current in Series = 0.17A
Resistance in Parallell
1/RP = 1/R1 +1/R2
1/RP = 1/484 + 1/(2420/3)
1/RP = 1/484 + 3/2420
2420/RP = 5+3
RP = 2420/8
RP = 302.5 Ω
Current in Parallel
IP = V/RP = 220/302.5
IP = 0.727 A
Current in parallel = 0.727A
Secondly Now considering Question as total 4 lamp
2,2 are connected in series and then connected parallely
RS1 = 484 Ω RS2 = 484 Ω (from last solution)
1/RP = 1/RS1 + 1/RS2
1/RP = 1/484 + 1/484
RP = 484/2
RP = 242 Ω
I = 220/242 = 0.91 A
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