A phonograph record of radius R, which carries a uniformly distributed charge Q, is rotating with constant angular speed ω figure. Find the magnetic field at the center of the disk.
μ0Qω2πR
Let a ring of radius x and of thickness dx
Then charge on that ring; dq=2Qx dxR2
Magnetic field at centre due to this ring; dB=μ0i2x
dB=μ0Qω2πR2dx
Magnetic field due to whole disc =μQω2πR2∫R0dx = μ0Qω2πR