Moving Charges and MagnetismNEET Physics · Class 12 · NCERT Chapter 4

You can expect 1 to 2 questions in NEET 2027. The chapter has high PYQ frequency. Lorentz force, motion in magnetic field (radius and period), Biot-Savart for straight wire and circular coil, solenoid, and torque on a current loop are favourites.

A charge q moving with velocity v in a magnetic field B feels a force F equals q v cross B. The magnitude is q v B sin theta, where theta is the angle between v and B. Direction is perpendicular to both v and B (right-hand rule). The force does no work because it is always perpendicular to v.

When v is perpendicular to B, the particle moves in a circle. Magnetic force provides the centripetal force: q v B equals m v squared over r, giving r equals m v over (q B). The time period T equals 2 pi m over (q B), independent of speed.

It gives the magnetic field at point P due to a small current element I dL: dB equals (mu_0 over 4 pi) times (I dL cross r unit vector) over r squared. Direction is perpendicular to both dL and the line from element to point. For a finite wire or loop, integrate over the geometry.

B equals mu_0 n I, where n is the number of turns per unit length and I is the current. The field is uniform along the axis and zero outside (ideal long solenoid). At the END of a finite solenoid, the field is half this value.

Two wires carrying currents I_1 and I_2 in the same direction attract each other. In opposite directions, they repel. Force per unit length is f equals (mu_0 I_1 I_2) over (2 pi d), where d is the separation. This is the definition of the ampere: 1 A is the current that produces 2 times 10 to the minus 7 N per metre between two wires 1 m apart.

A coil of N turns, area A carrying current I has a magnetic dipole moment m equals N I A (perpendicular to the coil). In a uniform field B, torque is tau equals m cross B, magnitude m B sin theta. This is the principle behind the moving coil galvanometer and electric motor.