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Question

# The magnetic field inside a toroid of radius R is B. If the current through it is doubled and the radius increased four times keeping the number of turns per unit length same, then the magnetic field produced by it will be

A
B
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B
2B
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C
4B
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D
B2
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Solution

## The correct option is B 2BMagnetic field produced by toroid inside the core of toroid is given by B=μ0Ni2πr=μ0ni Where, n=N2πr= number of tunrs per unit length ∴B∝i (for same n) So, B2B1=i2i1 From question, B1=B; i2=2i1 ∴B2=2B ∴ New magnetic field, B1=2B Hence, option (b) is correct. Why this question ? Caution: In this question n is constant but N is not. Here change in the radius is given to create confusion in the student's mind.

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