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For JEE problems on magnetic effects:

Choose the right law: Biot-Savart for arbitrary geometries (loops, arcs, finite wires); Ampere's law for high-symmetry distributions (infinite wires, solenoids, toroids, cylindrical conductors).

Field calculation checklist: (1) Identify the geometry. (2) Apply the appropriate formula. (3) Use the right-hand rule for direction. (4) For composite shapes, decompose and superpose.

Force/torque problems: For straight wires in uniform fields: $F = BIL\sin\theta$. For parallel wires: $F/l = \mu_0 I_1 I_2/(2\pi d)$ — remember parallel attracts, antiparallel repels. For loops: $\tau = NIAB\sin\theta$.

Common traps to avoid: (1) Straight segments through the point give zero field — don't add phantom contributions. (2) Use $n = N/L$ (not $N$) in solenoid formula. (3) Toroid has zero field in the central hole AND outside. (4) At the end of a solenoid, $B = \mu_0 nI/2$. (5) Infinite wire field is twice the semi-infinite wire field. (6) The magnetic force on a single charge does no work, but the force on a current-carrying wire can (battery provides energy).

Numerical tips: $\mu_0 = 4\pi \times 10^{-7}$ T·m/A. Many calculations simplify when $\pi$ cancels. Always check units and powers of 10.