Magnetic Field from Current Strip

In the x y xy plane, a thin metal strip of width ( w = 1 m ) (w = \text{1 m}) is oriented with its left side at ( x = 0 m ) (x = \text{0 m}) and its right side at ( x = 1 m ) (x = \text{1 m}) . The strip has infinite length, and its length is perpendicular to the x y xy plane.

The strip carries a constant current of 1 A 1 A , uniformly distributed over its width. The current is directed into the page.

There is a test point on the x x -axis at a distance ( d = 2 m ) (d = \text{2 m}) from the origin.

What is the magnitude of the magnetic flux density ( B ) (B) at the test point?

Details and assumptions:
- The surrounding medium is vacuum. μ 0 = 4 π × 1 0 7 H / m \mu_0 = 4 \pi \times 10^{-7} H/m
- Give your answer in nano-Teslas, to 3 decimal places.


The answer is 138.629.

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1 solution

Let l = x i ^ + z k ^ \vec{l}=x\hat{i}+z\hat{k} be the parametrization of the metal strip, where z z goes from -\infty to \infty and x x goes from 0 0 to 1 1 . Let's find the magnetic field in a point P = d i ^ \vec{P}=d\hat{i} . By Biot-Savart Law we have: B = μ 0 I 4 π 0 1 d l × ( P l ) P l 3 d x = μ 0 I 4 π 0 1 ( d x ) j ^ ( d x ) i ^ z k ^ 3 d z d x = μ 0 I 4 π 0 1 ( d x ) j ^ ( ( d x ) 2 + z 2 ) 3 / 2 d z d x = μ 0 I 4 π 0 1 1 d x π / 2 π / 2 cos θ j ^ d θ d x = μ 0 I 2 π 0 1 1 d x d x = μ 0 I 2 π ( ln ( d ) ln ( d 1 ) ) \begin{aligned} \vec{B} &= \dfrac{\mu_0 I}{4\pi} \int_0^1 \int_{-\infty}^{\infty} \dfrac{\mathrm{d}\vec{l} \times \left(\vec{P}-\vec{l}\right)}{\left\lVert \vec{P}-\vec{l}\right\rVert^3} \mathrm{d}x\\ &= \dfrac{\mu_0 I}{4\pi} \int_0^1 \int_{-\infty}^{\infty} \dfrac{(d-x)\hat{j}}{\left\lVert (d-x)\hat{i}-z\hat{k} \right\rVert^3} \mathrm{d}z\mathrm{d}x\\ &= \dfrac{\mu_0 I}{4\pi} \int_0^1 \int_{-\infty}^{\infty} \dfrac{(d-x)\hat{j}}{((d-x)^2+z^2)^{3/2}}\mathrm{d}z\mathrm{d}x\\ &= \dfrac{\mu_0 I}{4\pi} \int_0^1 \dfrac{1}{d-x} \int_{-\pi/2}^{\pi/2} \cos \theta \hat{j}\mathrm{d}\theta\mathrm{d}x\\ &= \dfrac{\mu_0 I}{2\pi} \int_0^1 \dfrac{1}{d-x} \mathrm{d}x\\ &= \dfrac{\mu_0 I}{2\pi} (\ln(d)-\ln(d-1)) \end{aligned} Finally, substituting d = 2 m d=2\text{ m} and I = 1 A I=-1\text{ A} we have B = 2 × 1 0 7 ln ( 2 ) j ^ T \vec{B} = -2 \times 10^{-7} \ln(2)\hat{j}\text{ T} , and B 138.629 nT \left\lVert \vec{B} \right\rVert \approx \boxed{138.629}\text{ nT} .

Interesting approach. You can also make it a 1D problem by resolving the strip into infinitesimally thin, infinitely long wires.

Steven Chase - 3 years, 10 months ago

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