Q. What will happen when a 40 W, 220 V lamp and 100 W, 220 V lamp are connected in series across 40 V supply?
CMC MedicalCMC Medical 2013
Solution:
When current $ i $ flows, across potential V, then power $ {{v}_{i}}\,, $ and $ V=i\,E $ (Ohms law)
The currents required by the two lamps for their normal brightness are $ {{i}_{1}}=\frac{{{p}_{1}}}{{{V}_{1}}}=\frac{40}{220} $ $ =\frac{2}{11}=0.18\,A $ $ i=\frac{V}{{{R}_{1}}+{{R}_{2}}} $ $ =\frac{40}{1694}=0.236 $ $ {{i}_{2}}=\frac{{{p}_{2}}}{{{V}_{2}}}=\frac{100}{200}=0.45A $ The resistance of the filaments are $ {{R}_{1}}-\frac{V}{{{i}_{1}}}=\frac{220\times 11}{2}=121\,\Omega $
