A plane loop is shaped in the form as shown in Fig. with radii a = 20 cm and b = 10 cm and is placed in a uniform time varying magnetic field B = B 0 sin ω t where B 0 = 10 mT and ω = 100 rad/s. Find the amplitude for the current induced in the loop if its resistance per unit length is equal to 50 × 10 –3 Ω /m. The inductance of the loop is negligible.

Text Solution
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Sol. Instantaneous flux through loop is
φ = π a 2 B cos 0º + π b 2 B cos 180º
= π (a 2 – b 2 ) B
= π (a 2 – b 2 ) B 0 sin ω t
Differentiating w.r.t. time,
= π (a 2 – b 2 ) B 0 ω cos ω t
i =
(only the modulus)
i max =
, ρ = resistance per unit length
=
(a – b) B 0 ω
=
(20 – 10) × 10 2 × 10 × 10 –3 × 
i max = 1 A
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