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Example of thermal and chemical effect of current Example : twenty one electric bulb are connected in series with the mains of a 220 V supply. After one bulb is fused, the remaining 20 bulbs are again connected in series across the same main. By what percentage will the illumination of (i) a bulb change (ii) all the bulbs change? Solution : Let R be the resistance of each bulb. Total resistance of all the 21 bulbs in series = 21 R. Therefore, current       I = 220/(21R) Illuminating power of each bulb, P = I 2 R=(220/21R) 2 ×R = (220/21) 2 × 1/R Illuminating power of all the bulbs, P’ =(220/21R) 2 × 21 R = (220) 2 /21R When one bulb is fused, then current, I’ =220/20R Now illuminating power of each bulb, P’ =(220/20R) 2 ×R = (220/20) 2 × 1/R Illuminating power of all the bulbs, P’ = (220/20R) 2 × 20R= (220) 2 /20R (i)       %   increases in illuminating power of one bulb = (p’ –p)/p   × 100 = ...