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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 is
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$
