then answer the question based on what you see there. A. 5. (This problem is fro

Question

then answer the question based on what you see there. A. 5. (This problem is from the 2nd mid-term of Winter 2011, and it was probably the hardest problem on that exam.) A simple light circuit is shown at right. There are four identical light bulbs, each of which normally acts as a 1 resistor. When a light bulb fails, it acts like a switch opening and current cannot flow through it (in effect, its resistance becomes infinite). The brightness of a light bulb is given by the power dissipated in- the bulb: the more power, the brighter it is. If no power is dissipated in a

Explanation / Answer

Power = VI

Since the bulbs are identical, the same current flows through all of them and the potential drop across each bulb is also the same (V=IR) hence all four bulbs will glow with equal brightness.

Before bulb A fails, the equivalent resistance of the circuit was 1/2 +1/2 = 1 ohm. After bulb A fails, the resistance becomes 1 +1/2 = 1.5 ohm . Since the resistance of the circuit increases, the current flowing through it decreases and hence the bulbs 3) gets fainter but doesn't go out. (Even though the path through A is open, the current can still flow through B).

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