NTSE Stage 2 SAT 2018 Exam Question Paper

Question : 40 of 100

Marks: +1, -0

An electrical circuit, shown below, consists of a battery, an ammeter, three resistors and two keys.

Consider two cases:
(i) The key

K_1

is closed and the key

K_2

is open.
(ii) The key

K_2

is closed and the key

K_1

is open. The ratio of respective currents in those two cases will be:

Solution:

(i) When

K_1

is closed and

K_2

is open

{R}_{{eq}}=\;\;{R_{1}R_{2}}/{R_{1}+R_{2}}=\;\;{3×12}/{3+12}=\;\;{12}/{5}Ω

∴{I}_{1}=\;\;{V}/{R_{eq}}=\;\;{\;\;12/{{12}/{5}}=5{A}

(ii) When

K_2

is closed and

K_1

is open

{R}_{{eq}}=\;\;{4×12}/{16}=3Ω

∴

{I}_{2}=\;\;{V}/{R_{eq}}=\;\;{12}/{3}=4{A}

So,

\;\;{I_{1}}/{I_{2}}=\;\;{5}/{4}

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