Answer :
The magnetic field is given by B = μ_0(i1/2πr) ln(c/b).
What is magnetic field?
The magnetic influence on moving charges, electric currents, as well as magnetic materials is described by a magnetic field, which is a vector field. A force perpendicular to the charge's own velocity and the magnetic field acts on it when the charge is moving through a magnetic field. The magnetic field of a permanent magnet pulls on ferromagnetic substances like iron and attracts and repels other magnets. Para magnetism, diamagnetism, as well as antiferromagnetic are three additional magnetic effects that a nonuniform magnetic field can have on "nonmagnetic" materials, though these forces are typically so minute that they can only be discovered by laboratory equipment. Electric currents, like those used in electromagnets, as well as electric fields that change in time produce magnetic fields that surround magnetised materials.
Where μ_0 is the permeability of free space, i1 is the current in the central conductor, r is the radius of the central conductor, b is the inner radius of the conducting tube, and c is the outer radius of the conducting tube.
How to derive magnitude?
B = (4π*10-7 N/A2)*30 A/(2π*0.01 m)
= 1.2*10-5/*0.02 = N/(As) = 6*10-4 T.
Near the center of the solenoid B = μ0nI
= 4π10-7(1000/0.4)*I T, with I in units of Ampere.
With B = 10-4 T, we need I = 10-4A/(4π*10-7 2500) A = 31.8 mA.
For comparison, near Knoxville, TN,
the strength of the Earth magnetic field is ~ 53 microT = 5.3*10-5 T.
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