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A ring of radius (R) carries a linear charge density (\lambda). It is rotating with angular speed (\omega). What is the magnetic field at the centre?

Discussion:

Linear charge density $$\lambda=\frac{Q}{2\pi R}$$
When the ring is rotated about the axis, the motion of the electrons in a circular orbit is equivalent to a current carrying loop.
Current $$I=\frac{Q}{T}=\frac{Q\omega}{2\pi}$$
since Time period (T=2\pi/\omega).
Now, magnetic field around the centre of a current carrying loop is given by $$B=\mu_0I/2R$$
Putting the value of (I) in the above equation, we get
$$B=\frac{\mu_0\omega}{2}.\frac{Q}{2\pi R}$$$$\Rightarrow B=\frac{\mu_0\lambda\omega}{2}$$