An electric current passes through a long straight copper wire. At a distance 5cm from the straight wire, the magnetic field is B. The magnetic field at 20cm from the straight wire would be,

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answer is B.

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Detailed Solution

Due to infinte wire, B=μoi2πr ⇒B ∝ 1r ⇒B1B2=r2r1 ⇒BB2=205 ⇒B2=B4

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