Answer to Question #109234 in Electricity and Magnetism for Mukelo Madolo

Question

27.3 The interior of a three-way incandescent lightbulb, which provides three levels of light intensity. The socket of the lamp is equipped with a four-position switch for selecting different light intensities, with the positions described as follows;(1) off (switches S1 and S2 closed both open),(2) switch S1 closed,(3) switch S2 closed, and (4) switches S1 and S2 both closed. The lightbulb contains two filaments. When the lamp is connected to a 120-V source, one filament receives 100 W of power and the other receives 75 W. What is the total power input to the light bulb when (a) only switch S1 is closed, (b) only swith S2 is closed, and (c) both switches are closed? (d) What if? suppose the 75-W filament breaks and no longer is able to carry a current. How many switch positions will result in light leaving the bulb and what will be the power input to the bulb in those positions?

Answer 1

Look at the figure:

The total power input to the light bulb when

(a) only switch S1 is closed:

75 W because the current cannot flow in 100-W filament.

(b) only switch S2 is closed:

100 W because the current cannot flow in 75-W filament.

(c) both switches are closed:

175W because the current flows in both filaments.

(d) If the 75-W filament breaks and no longer is able to carry a current, the switch will have only 2 positions (on and off) and the power will be 100 W.