Q27 of 126 Page 198

A bulb is made using two filaments. A switch selects whether the filaments are used individually or in parallel. When used with a 15 V battery, the bulb can be operated at 5W, 10W or 15W. What should be the resistances of the filaments?

and

Given, Voltage of the battery,


Power operated P=5W, 10W or 15W


We know,


Now,





The parallel combination of resistances is always less than individual resistances. Therefore, has to be parallel combination while and are the resistances.


More from this chapter

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25

Three bulbs, each having a resistance of 180 Ω, are connected in parallel to an ideal battery of emf 60 V. Find the current delivered by the battery when

(a) all the bulbs are switched on,


(b) two of the bulbs are switched on and


(c) only one bulb is switched on.


 26

Suppose you have three resistors of 20Ω, 50Ω and 100Ω. What minimum and maximum resistances can you obtain from these resistors?

 28

Figure shows a part of a circuit. If a current of 12 mA exists in the 5 kΩ resistor, find the currents in the other three resistors. What is the potential difference between the points A and B?


 29

An ideal battery sends a current of 5A in a resistor. When another resistor of value 10 Ω is connected in parallel, the current through the battery is increased to 6A. Find the resistance of the first resistor.