then answer the question based on what you see there A 5 Thi

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

Solution

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).