Physics · Moving Charges And Magnetism · NEET
n is turns per unit length, n = N/L, measured in turns per metre. It is NOT the total turns N. Example: 100 turns over 0.50 m gives n = 100/0.50 = 200 turns/m, not 100. Forgetting to divide by length is the number one mistake in NEET solenoid questions.
For a long solenoid (length much greater than radius) the field inside is uniform and depends only on n and I. Ampere's law gives B = mu0 n I with no radius term. So if a question gives you the radius, it is usually a distractor. In NEET 2022, the '1 mm radius' was there only to confuse you.
1 m = 1000 mm, so multiply by 1000. n = 100 turns/mm x 1000 mm/m = 100000 = 10^5 turns/m. Similarly turns/cm is multiplied by 100. Always end up in turns/m before using SI units.
Use B = mu0 n I for a long solenoid (a long tube of many turns), where n = N/L. Use B = mu0 N I / (2R) for a flat circular coil of N turns and radius R, at its centre. Read the question: 'solenoid of length L' means the first formula; 'coil of radius R' means the second.
Not directly. The field depends on n = N/L. If you make the solenoid longer while adding turns to keep n the same, B stays the same. But if you stretch a fixed number of turns over a longer length, n drops and B falls. So length matters only through n.
A long solenoid of 50 cm length having 100 turns carries a current 2.5 A. The magnetic field at the centre of the solenoid is (mu0 = 4 pi x 10^-7 T m A^-1)
A long solenoid of radius 1 mm has 100 turns per mm. If 1 A current flows in the solenoid, the magnetic field strength at the centre of the solenoid is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
B = mu0 n I, where mu0 = 4 pi x 10^-7 T m/A, n = N/L is turns per metre, and I is the current. The field is uniform and parallel to the axis inside a long solenoid.
n is the number of turns per unit length, n = N/L, in turns per metre. It is the total turns N divided by the total length L of the winding.
At the open end of a long solenoid the field is half the interior value, B_end = (1/2) mu0 n I. NEET questions usually ask for the centre, where B = mu0 n I.
No. Reversing the current only flips the direction of B (found by the right-hand rule); the magnitude B = mu0 n I is unchanged because it depends on the size of I, not its sign.
A solenoid is a straight tube with field B = mu0 n I inside and almost zero outside. A toroid is a solenoid bent into a ring; its field B = mu0 N I / (2 pi r) stays inside the ring and depends on the radial distance r. See the toroid page next.
