First slide

Electrical power

Question

If X, Y and Z in figure are identical lamps, which of the following changes to the brightness of the lamps occur when switch S is closed?

Moderate

A

X stays the same, Y decreases
B

X increases, Y decreases
C

X increases, Y stays the same
D

X decreases, Y increases

Solution

$P = \frac{V^{2}}{R}; Initially, I = \frac{V}{2 R}$
Power across $P_{X} = P_{Y} = \frac{V^{2}}{4 R}$
Finally, $I = \frac{2 V}{3 R}, Power P_{X} = \frac{V^{2}}{9 R}, P_{Y} = P_{Z} = \frac{V^{2}}{9 R}$
Hence P_X increases, P_Y decreases.

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