Ch 4 covers the magnetic field due to current-carrying conductors (Biot-Savart, Ampere's law), force on moving charges and conductors in magnetic fields, and devices like the cyclotron and galvanometer.
Biot-Savart law: dB = (μ₀/4π)(Idl × r̂/r²). B at centre of circular loop: μ₀I/2R. B of long straight wire: μ₀I/2πr. Ampere's circuital law: ∮B·dl = μ₀I_enclosed. Solenoid: B = μ₀nI (inside, nearly uniform). Toroid: B = μ₀NI/2πr (inside).
Lorentz force: F = qv × B + qE. Magnetic force on current-carrying conductor: F = IL × B. Two parallel currents: same direction attract, opposite repel (F/L = μ₀I₁I₂/2πd). Cyclotron: charged particles spiral in B field; r = mv/qB; frequency = qB/2πm (independent of speed). Moving coil galvanometer: measures current using torque on coil in B field.
Download: https://ncert.nic.in/textbook/pdf/leph104.pdf | Part I: https://ncert.nic.in/textbook/pdf/leph1ps.zip
The magnetic force F = qv×B is always perpendicular to the velocity. A constant perpendicular force changes the direction of velocity without changing its magnitude (speed). This is exactly the condition for circular motion. The radius is r = mv/qB. Faster or heavier particles make larger circles.
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