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Find the magnetic induction intensity (college physics)
Because I am>& gtr, I can be regarded as an infinite solenoid.

The electric quantity per unit length along the axial direction is σ/(2πR), and the angular velocity of uniform rotation is ω, so the rotation period is 2π/ω, so the current per unit length along the axial direction is σω/(4π? r)

Then internal B=μ0σω/(4π? R) direction is the horizontal direction on the right in the figure.