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A 480 turn circular-loop coil 7.25 cm in radius is initially aligned so that its axis is parallel to the Earth's magnetic field. In 2.77 ms, the coil is flipped so that its axis is perpendicular to the Earth's magnetic field. If an average voltage of 0.170 V is thereby induced in the coil, what is the value of the Earth's magnetic field (in μT) at that location? 

(1) 59 

(2) 48 

(3) 65 

(4) 39 

(5) 43 

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Correct option: (1) 59


The emf for a circular coil which is rotated in this way is E = Nπr2 B/Δt, where N = 480, r =0.0725, E = 0.170 and Δt = 0.00277. This yields B = 59 μT.

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