LH vs Gaussian units in Biot-Savart Law

AI Thread Summary
The discussion centers on the differences in the Biot-Savart formula between Gaussian and Lorentz-Heaviside (LH) units, specifically the presence of a factor of 1/4π in LH units that is absent in Gaussian units. The transformation for the magnetic field B between these units is given by B_LH = B_G/√4π, raising questions about the origin of the additional √4π factor. Participants argue that when considering the current I, defined as I = qv, the charge transformation q_LH = √4π q_G leads to an increase in the numerator rather than a decrease in the denominator. This prompts confusion about how the factorization aligns with the expected transformations. The conversation emphasizes the importance of careful unit conversion and mathematical consistency in electromagnetic theory.
avikarto
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I have a question regarding the relationship between the Biot-Savart formula in Gaussian and Lorentz-Heaviside units. In Gaussian, we have a ##\frac{1}{c}## outside the integral, but in LH units we have a ##\frac{1}{4\pi}\frac{1}{c}##. This does not make sense, considering the transformation between Gaussian and LH for B is that ##B_{LH}=\frac{B_G}{\sqrt{4\pi}}##. Where does the extra ##\sqrt{4\pi}## come from? Thanks.

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avikarto said:
the transformation between Gaussian and LH for B is that ##B_{LH}=\frac{B_G}{\sqrt{4\pi}}##. Where does the extra ##\sqrt{4\pi}## come from? Thanks.

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it comes from I.
 
If we take ##I=q v##, then using that ##q_{LH}=\sqrt{4\pi} q_G##, we would have that ##I_{LH}=\sqrt{4\pi} I_G##. This seems to brings an extra ##\sqrt{4\pi}## into the numerator, not the denominator, and would therefore remove the factor all together. I don't understand how this resolves the issue. Besides, the transformation on B should be the transformation on B, without also having to transform its subcomponents additionally. Could you please elaborate?
 
avikarto said:
This seems to brings an extra 4π−−√4π\sqrt{4\pi} into the numerator, not the denominator, and would therefore remove the factor all together.
Check your math. Write the equation in one set of units, and then substitute. The factor is correct.
 
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