In the plane, a thin metal strip of width is oriented with its left side at and its right side at . The strip has infinite length, and its length is perpendicular to the plane.
The strip carries a constant current of , uniformly distributed over its width. The current is directed into the page.
There is a test point on the -axis at a distance from the origin.
What is the magnitude of the magnetic flux density at the test point?
Details and assumptions:
- The surrounding medium is vacuum.
- Give your answer in nano-Teslas, to 3 decimal places.
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Let l = x i ^ + z k ^ be the parametrization of the metal strip, where z goes from − ∞ to ∞ and x goes from 0 to 1 . Let's find the magnetic field in a point P = d i ^ . By Biot-Savart Law we have: B = 4 π μ 0 I ∫ 0 1 ∫ − ∞ ∞ ∥ ∥ ∥ P − l ∥ ∥ ∥ 3 d l × ( P − l ) d x = 4 π μ 0 I ∫ 0 1 ∫ − ∞ ∞ ∥ ∥ ∥ ( d − x ) i ^ − z k ^ ∥ ∥ ∥ 3 ( d − x ) j ^ d z d x = 4 π μ 0 I ∫ 0 1 ∫ − ∞ ∞ ( ( d − x ) 2 + z 2 ) 3 / 2 ( d − x ) j ^ d z d x = 4 π μ 0 I ∫ 0 1 d − x 1 ∫ − π / 2 π / 2 cos θ j ^ d θ d x = 2 π μ 0 I ∫ 0 1 d − x 1 d x = 2 π μ 0 I ( ln ( d ) − ln ( d − 1 ) ) Finally, substituting d = 2 m and I = − 1 A we have B = − 2 × 1 0 − 7 ln ( 2 ) j ^ T , and ∥ ∥ ∥ B ∥ ∥ ∥ ≈ 1 3 8 . 6 2 9 nT .