Physics · Moving Charges And Magnetism · NEET
For a single loop (one turn) m = IA. When the same wire is wound into N turns, each turn adds its own IA and they all point the same way, so m = NIA. In NEET numericals, always check the number of turns N. If a coil has 100 turns, m is 100 times larger than a single loop with the same current and area.
Use the right-hand curl rule. Curl the fingers of your right hand along the direction of current flow around the loop; your thumb points along the magnetic moment m. This is the same axis as the loop's magnetic field at the centre (the North-pole side). m is perpendicular to the plane of the loop, never in the plane.
The unit of m = NIA is ampere times metre squared, written A·m². You can check it: current (A) multiplied by area (m²). An equivalent unit is joule per tesla (J/T), which is useful because energy U = -m·B and torque tau = m×B.
No. m = NIA depends only on the loop itself (turns, current, area), not on any external field B. What changes in a stronger field is the torque (tau = mB sin theta) and the potential energy (U = -mB cos theta), not m. Students often confuse m with torque because both appear together in tau = m×B.
First find how many turns N the wire makes: N = total length / perimeter of one turn. Then find the area A of one turn, and use m = NIA. Fewer, larger turns can give more or less m than many small turns, so you must compute N and A carefully - this is a very common NEET trap.
A 100-turn closely wound circular coil of radius 5 cm has a magnetic field of 3.14 × 10⁻³ T at its centre. The current flowing through the coil, and the magnitude of the magnetic moment of this coil are, respectively: (Take μ₀ = 4π × 10⁻⁷ T m/A)
A 2 A current is flowing through two different small circular copper coils having radii ratio 1 : 2. The ratio of their respective magnetic moments will be:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
m = NIA, where N is the number of turns, I is the current, and A is the area enclosed by one turn. Its direction is along the loop axis by the right-hand curl rule, and its unit is A·m² (also J/T).
A current loop produces a magnetic field with a North and South pole just like a small bar magnet. Its field pattern at large distances matches that of a magnetic dipole, so we describe it by a single vector m = NIA pointing from the South to the North face.
It is a vector. Its magnitude is NIA and its direction is perpendicular to the plane of the loop, along the axis given by the right-hand curl rule. This direction matters in tau = m×B and U = -m·B.
When the loop is placed in a magnetic field B, the field exerts a torque tau = m×B, magnitude tau = mB sin theta = NIAB sin theta. The moment m is a property of the loop; the torque only appears when an external field is present.
Only through the area A. For the same current and turns, a loop enclosing more area has a larger moment. When a fixed length of wire is bent into different shapes, both N and A change, so you must recompute m = NIA for each shape.
