IIT-JEE 2007 · Paper 1 · Physics Q1

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A resistance of 2 Ω is connected across one gap of a metre-bridge (the length of the wire is 100 cm) and an unknown resistance, greater than 2 Ω, is connected across the other gap. When these resistances are interchanged, the balance point shifts by 20 cm. Neglecting any corrections, the unknown resistance is

3 Ω

4 Ω

5 Ω

6 Ω

Answer and worked solution

Official answer: A

Balance before and after swapping. With the 2 Ω in the left gap, balance at $l$ : $\dfrac2X=\dfrac l{100-l}$ . Swapping moves the balance to $100-l$ , a shift of $|100-2l|=20$ cm. Since $X>2$ , $l<50$ , so $l=40$ .

Unknown. $X=2\times\dfrac{60}{40}=3\ \Omega$ .

(A)

Topics:Wheatstone bridge and metre bridge

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IIT-JEE 2007 · Paper 1, Physics question 1