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$\newcommand{\tl}[1]{\tag{#1}\label{#1}}$

Magnetic Field of a current is: $$B = \mu_0 I / (2 \pi r)\tl{01}$$ Magnetic Field of a Moving Charge is: $$B = \mu_0 qv\sin\theta / (4\pi r^2)\tl{02}$$

So I have $$\mu_0 I / (2\pi r) = \mu_0 qv\sin\theta / (4\pi r^2)\implies I = qv\sin\theta / (2r)\tl{03}$$ Since $I = q/t$, $$q/t = qv\sin\theta / (2r)\implies vt\sin\theta = 2r\tl{04}$$ Since $l = vt$, $$l\sin\theta = 2r\implies\frac{l}{r}\sin\theta = 2\tl{05}$$ Since $l / r =\cos\theta$, $$\cos\theta\sin\theta = 2\tl{06}$$

This equation has no solution. It doesn't make sense. Can someone please point out where I'm wrong at?

What I write is just what I think in my head, if you don't understand something, please ask me because you may not find it on any website.

Dodo
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T H
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1 Answers1

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Your first formula is for an (infinite ) straight line of moving charges. The second is for only one of the charges in that line.

R.W. Bird
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  • More specifically the first equation will fail for a single point charge because it assumes some contributions from left & right of observation point will cancel each other out. – Señor O Dec 28 '21 at 16:19
  • With a cross product, extra contributions from segments of current do not cancel. – R.W. Bird Dec 28 '21 at 17:30