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A closely wound solenoid 80 cm long has 5 layers of 400 turns each and carries a current of 8.0 A. The magnitude of B near its centre is approximately:
A2.5 × 10⁻³ T
B2.5 × 10⁻² T
C5.0 × 10⁻² T
D1.3 × 10⁻² T
Answer & Solution
Correct answer: B. 2.5 × 10⁻² T
1. Total turns: $N = 5\times400 = 2000$.
2. Turns per unit length: $n = \dfrac{2000}{0.80} = 2500$ m⁻¹.
3. Inside field: $B = \mu_0 n I = 4\pi\times10^{-7}\times2500\times8.0$.
4. $B = 4\pi\times10^{-7}\times2\times10^{4} = 8\pi\times10^{-3}$.
5. $B \approx 2.5\times10^{-2}$ T.
_Source: NCERT Class 12 Physics Ch 4 "Moving Charges and Magnetism", p.158_
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