Eleven 15Ω and four 7Ω resistors are connected in series. If a new resistance of 21Ω is added in series, the change in the effective resistance would be:
21
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Solution
The correct option is A 21 Given: 11 resistors each of resistance 15Ω and 4 resistors each of resistance 7Ω are connected in series. ⟹ Effective resistance in series, Reff=11×15Ω+4×7Ω=193Ω A new resistor of resistance 21Ω is added to the series. ⟹ New effective resistance, R′eff=Reff+21Ω=193+21=214Ω ∴ Change in effective resiatance =R′eff−Reff=214−193=21Ω.