Consider the following statements about a solenoid: 1. The magnetic field strength in a solenoid depends upon the number of turns per unit length in the solenoid 2. The magnetic field strength in a solenoid depends upon the current flowing in the wire of the solenoid 3. The magnetic field strength in a solenoid depends upon the diameter of the solenoid Which of the statements given above are correct?

The correct answer is D, 1 and 2 only. The magnetic field inside a long solenoid is B = μ₀nI, where n is the number of turns per unit length and I is the current, so the field depends on these two and not on the diameter. A solenoid is a long coil of insulated wire wound like a spring. When current flows, the field inside is strong, nearly uniform and parallel to the axis, and the coil behaves like a bar magnet with north and south ends. Placing a soft iron core inside makes the field far stronger, which is how an electromagnet is made. Statement 1 is correct because more turns per unit length give a stronger field. Statement 2 is correct because a larger current gives a stronger field. Statement 3 is wrong because the diameter does not appear in the formula for a long solenoid. Option A is wrong because it includes the false statement 3. Option B is wrong because it includes statement 3 and leaves out statement 2. Option C is wrong because it includes statement 3 and leaves out statement 1. Exam tip: solenoid field B = μ₀nI; an iron core makes it an electromagnet.