Why am I getting two different results in emu and SI unit?

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Discussion Overview

The discussion revolves around the discrepancies in calculated force between two magnetic poles when using electromagnetic units (emu) and SI units. Participants explore the implications of unit conversions and the application of formulas in different unit systems.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant computes the force in emu and finds it to be 1 dyne, while in SI units, the computed force is significantly different, leading to confusion about the results.
  • Another participant points out that the units in the SI formula may not match correctly, suggesting a potential error in the application of the constant kA.
  • Several participants emphasize the importance of working with units to identify errors in calculations.
  • There is a suggestion that the pole strength (p) needs clarification regarding whether it is in abC or statC, which could affect the validity of the equations used.

Areas of Agreement / Disagreement

Participants express differing views on the correctness of the formulas and unit applications, indicating that the discussion remains unresolved with multiple competing perspectives on the issue.

Contextual Notes

There are unresolved questions regarding the definitions and conversions of units, particularly concerning the pole strength and the constants used in the calculations.

faheemahmed6000
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I am computing force between two magnetic poles each of one unit pole (in emu) and situated one centimeter apart.

In electromagnetic units:
##F_{dyne}=\dfrac{p^2}{r_{cm}^2}=\dfrac{1^2}{1^2}=1 dyne##
where ##p## is pole strength in emu

In SI units:
##F_{N}=k_A \dfrac{P^2}{r_m^2}=10^{-7} \dfrac{({1.25\times 10^{-7}})^2}{10^{-4}}=1.5625 \times 10^{-17} \neq 10^{-5}N=1dyne##
where ##P## is that same pole strength in SI units

with ##P=1.25\times10^{-7}p## (see here)

Now why am I getting two different results in emu and SI for the same configuration?
 
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The units in the formula "in SI units" don't match, at least not if ka is supposed to be ##\frac{\mu_0}{4 \pi}## as the numerical value used for it would suggest.
Working with units helps to spot errors.
 
mfb said:
The units in the formula "in SI units" don't match, at least not if ka is supposed to be ##\frac{\mu_0}{4 \pi}## as the numerical value used for it would suggest.
Working with units helps to spot errors.
Please can you explain a bit more elaborately?
 
Work with units, then the problem will become clear.
 
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faheemahmed6000 said:
In electromagnetic units:
##F_{dyne}=\dfrac{p^2}{r_{cm}^2}=\dfrac{1^2}{1^2}=1 dyne##
where ##p## is pole strength in emu
I agree with @mfb. You need to show the units in each of your intermediate steps. As it is, I can tell that F is in dyne and r is in cm, but I cannot tell if p is in abC or statC. If it is abC then your equation is wrong, and if it is in statC then you are using electrostatic units not electromagnetic units.
 

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