Current Electricity and Electrical InstrumentHard
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?
Options
A.X stays the same, Y decreases
B.X increases, Y decreases
C.X increases, Y decreases
D.X decreases, Y increases
Solution
P = 
Initially, I = 
Power across PX = PY =
Finally, I =
, Power PX = 
Py = Pz = 
Hence Px increases, Py decreases.
Alternative method :
Brightness ∝i2 R when S is clo9sed current drawn from battery increases because Req decreases, i.e. current in X increases. So brightness of X increases and current in Y decreases. So brightness of Y decreases.
Initially, I = Power across PX = PY =
Finally, I =
, Power PX = Py = Pz = Hence Px increases, Py decreases.
Alternative method :
Brightness ∝i2 R when S is clo9sed current drawn from battery increases because Req decreases, i.e. current in X increases. So brightness of X increases and current in Y decreases. So brightness of Y decreases.
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