Physics · Magnetism And Matter · NEET
It is a VECTOR. Its size tells you how strong the magnet is, and its direction points from the South pole to the North pole of the magnet (that is, along the magnet's axis). Because it is a vector, when you combine two magnets you must add their moments like vectors (using the parallelogram rule), not just add the numbers. NEET loves this in bent-magnet problems.
By convention the magnetic dipole moment vector m points from S to N INSIDE the magnet (along its axis). This is the opposite of the field lines OUTSIDE the magnet, which go N to S. Do not mix them up: field lines outside go N to S, but the moment arrow and the field lines inside the magnet both go S to N.
The SI unit is ampere metre squared (A m squared). You can see why from the current-loop formula m = N I A: current (A) times area (m squared). It is the same unit whether you get the moment from a bar magnet (pole strength x length) or from a current loop (N I A).
Pole strength (q_m, unit A m) is the 'charge-like' strength of one pole. Magnetic moment (m, unit A m squared) is pole strength times the length between the two poles: m = q_m x 2l. So pole strength is a property of a single pole, while moment describes the whole dipole. When you cut a magnet in half the pole strength stays the same but the length halves, so the moment halves.
A loop carrying current behaves exactly like a tiny bar magnet. Its magnetic moment is m = N I A, where N is the number of turns, I the current, and A the area of the loop. The direction is given by the right-hand rule: curl your fingers along the current, your thumb points along m. This is the bridge between electromagnetism (Chapter 4) and magnetism (Chapter 5) that NEET tests often.
An iron bar of length L has magnetic moment M. It is bent at the middle of its length such that the two arms make an angle 60 degrees with each other. The magnetic moment of this new magnet is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
m = q_m x 2l, where q_m is the pole strength and 2l is the distance between the two poles. Its direction is from South pole to North pole and its unit is A m squared.
m = N I A, where N is the number of turns, I is the current, and A is the area of the loop. This shows a current loop acts like a magnetic dipole, which is why a bar magnet is equivalent to a solenoid.
In a uniform field B the torque is tau = m x B, with magnitude tau = m B sin(theta). So m controls how strongly the field tries to align the magnet. A larger moment means a larger turning effect.
Yes. Cutting it in half along its length keeps the pole strength but halves the length, so each piece has moment m/2. You never get a single pole (monopole); each piece is a full dipole with its own N and S.
It points from the South pole to the North pole along the magnet's axis (inside the magnet). For a current loop, use the right-hand rule: curl fingers along the current and the thumb gives the moment direction.
