# 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 Ã— R