A long straight wire carries 400 A. How far from the wire will the magnetic field generated equal that of earth (i. e. 3X 10-4 T)?

Can someone explain to me how to do this problem ?

(a) assume the equation x = at^3 + bt describes the motion of a particular object, with x having the dimension of length and t having the dimension of time. determine the dimensions of the constants a and b. (use the following as necessary: l and t, where l is the unit of length and t is the unit of time.) (b) determine the dimensions of the derivative dx/dt = 3at^2 + b. (use the following as necessary: l and t, where l is the unit of length and t is the unit of time.)

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