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  4. A torch bulb rated as 4.5 W , 1.5 V is connected as shown in the figure. The e.m.f. of the cell needed to make the bulb glow at full intensity is

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Q. A torch bulb rated as $4.5 \,W , 1.5 \,V$ is connected as shown in the figure. The e.m.f. of the cell needed to make the bulb glow at full intensity isPhysics Question Image

Current Electricity

Solution:

Current in the bulb $=\frac{P}{V}=\frac{4.5}{1.5}=3\, A$
Current in $1 \,\Omega$ resistance $=\frac{1.5}{1}=1.5 \,A$
Hence, total current from the cell, $i=3+1.5=4.5\, A$
By using $E=V+i r $
$\Rightarrow E=1.5+4.5 \times(2.67)=13.5 \,V$