Torque on Current Loop — Motor principle
Magnetic Effects of Current
13
JEE Qs
8%
Hard
60
min
Always correctly identify the direction of the magnetic moment (M) and the magnetic field (B) to determine the angle for scalar calculations or use the vector cross product (M x B) for direction.
🧮 Key Formulas
✅ Key Points for JEE
- 1The magnetic dipole moment (M) vector's direction is determined by the right-hand thumb rule: curl fingers in the direction of current, thumb points in direction of M (perpendicular to the loop's area).
- 2The angle θ in τ = NIABsinθ is the angle between the magnetic dipole moment vector (M) and the external magnetic field vector (B). It is NOT the angle between the plane of the loop and B.
- 3For a current loop placed in a uniform magnetic field, the net force acting on the loop is always zero, but a net torque generally acts on it.
- 4The torque tends to align the magnetic dipole moment (M) of the loop with the direction of the external magnetic field (B), leading to stable equilibrium when M || B and unstable equilibrium when M || -B.
⚠️ Common Mistakes
- ✕Incorrectly identifying the angle θ; often confusing it with the angle between the plane of the coil and the magnetic field.
- ✕Misapplying the right-hand rule to determine the direction of the magnetic dipole moment (M) or the direction of the torque (τ).
- ✕Forgetting to include the number of turns (N) when calculating magnetic moment or torque for a multi-turn coil.
📝 Practice Questions
See allQ43.Match List - I with List - II. List - I List - II (A) Magnetic induction (I) Ampere meter 2 (B) Magnetic intensity (II) Weber (C) Magnetic flux (III) Gauss (D) Magnetic moment (IV) Ampere/meter (1) (A)-(I), (B)-(II), (C)-(III), (D)-(IV) (2) (A)-(III), (B)-(IV), (C)-(I), (D)-(II) (3) (A)-(III), (B)-(II), (C)-(I), (D)-(IV) (4) (A)-(III), (B)-(IV), (C)-(II), (D)-(I)
Q46.The magnetic field inside a 200 turns solenoid of radius 10 cm is 2.9 × 10−4Tesla. If the solenoid carries a current of 0.29 A , then the length of the solenoid is ________ πcm .
Q47.A parallel plate capacitor consisting of two circular plates of radius 10 cm is being charged by a constant current of 0.15 A . If the rate of change of potential difference between the plates is 7 × 108 V/s then the integer value of the distance between the parallel plates is ( Take, ϵ0 = 9 × 10−12 mF , π = 227 ) ________ μm . 2025 (29 Jan Shift 2) JEE Main Previous Year Paper
Q37.If B is magnetic field and μ0 is permeability of free space, then the dimensions of (B/μ0) is (1) ML2 T−2 A−1 (2) MT−2 A−1 (3) L−1 A (4) LT−2 A−1
Q46.A proton is moving undeflected in a region of crossed electric and magnetic fields at a constant speed of 2 × 105 ms−1 . When the electric field is switched off, the proton moves along a circular path of radius 2 cm . The magnitude of electric field is x × 104 N/C. The value of x is _______ Take the mass of the proton = 1.6 × 10−27 kg.
Q50.Two long parallel wires X and Y , separated by a distance of 6 cm , carry currents of 5A and 4A, respectively, in opposite directions as shown in the figure. Magnitude of the resultant magnetic field at point P at a distance 2025 (22 Jan Shift 2) JEE Main Previous Year Paper of 4 cm from wire Y is x × 10−5 T. The value of x is__________ . Take permeability of free space as μ0 = 4π × 10−7SI units.
NCERT Chapters
- Class 12 Physics Ch 4: Moving Charges and Magnetism