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The magnetic field generated along the axis of a solenoid is proportional to:

1. its length
2. square of current flowing in it
3. number of turns per unit length in it
4. reciprocal of its radius

Correct answer: number of turns per unit length in it

Solution

For a long solenoid, the magnetic field along its axis is given by B = μ₀ n I, where n is the number of turns per unit length. So the field is directly proportional to the turn density, not the length, current squared, or radius.

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