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Some Standard Cases for Series and Parallel Combinations

  • If n identical bulbs are first connected in series so PS = P/n and then connected in parallel. So PP = nP. Hence, PP/PS = n2.
  • To operate a bulb on voltage which is more than its rated voltage, a proper resistance is connected in series with it. For example, to glow a bulb of 30 W, 6 V with full intensity on 126 V required series resistance calculated as follows:
    Bulb will glow with its full intensity if applied voltage on it is 6 V, i.e., 120 V appears across the series resistance R. Current flows through bulb = current flows through resistance
Fig. 17
Hence, for resistance V = iR, i.e., 120 = 5 × R
 R = 24 Ω


  • If you want to learn short trick, then remember 76478.png
  • An electric kettle has two coils. When one coil is switched on, it takes time t1 to boil water and when the second coil is switched on, it takes time t2 to boil the same water.

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