<p>I believe I understand this equation for the dynamo effect of planets as far as iron-nickel cores go, where ρ is density in kilograms per cubic meter, Ω is rotation speed in radians per second, σ is electrical conductivity in the inverse of ohm-meters, and B is the magnetic field on the surface of the core in teslas (e.g. as described on <a href="https://en.wikipedia.org/wiki/Dynamo_theory" rel="nofollow noreferrer">wikipedia's page on dynamo theory</a>):</p>
<p><span class="math-container">$$B \propto \sqrt\frac{\rho\Omega}{\sigma}$$</span></p>
<p>However, what I do not understand is, assuming there's a fluid in the core, energy is provided by rotation, and convective motion occurs in the core, why can only conductors in the core form magnetic fields and not insulators?</p>
<p>From what I understand, conductors have a relatively high conductivity, which would mean the denominator is high, so the magnetic field would be comparatively weak. Insulators like sulfur have a low conductivity, so the denominator would be low and the magnetic field would be strong. Could someone tell me if there is another effect that I am unaware of in comparing a conducting core and an insulating core?</p>

CC BY-SA 4.0
2023-10-26T13:22:51.500Z
This is an excellent question. I replaced your image of the formula by the actual formula using MathJax. This helps e.g. screen readers - and makes talking about arbitrary formula easy IMHO.
planetmaker
CC BY-SA 4.0
2023-10-26T14:27:08.997Z
Low conductivity will allow for persistent separation of charges - which will appear as currents and hence generate the magnetic field. If an insulator does not have moving charges in the first place, the formula is not applicable.
AtmosphericPrisonEscape
CC BY-SA 4.0
2023-10-26T14:29:57.203Z
@AtmosphericPrisonEscape So the moving charges are only in conductors?
Pyrania
<magnetic-field><planetary-science><core>
Clarifications on Planetary Magnetic Fields
Pyrania
# Clarifications on Planetary Magnetic Fields

(Asked by: username_0 on 2023-10-26)

I believe I understand this equation for the dynamo effect of planets as far as iron-nickel cores go, where ρ is density in kilograms per cubic meter, Ω is rotation speed in radians per second, σ is electrical conductivity in the inverse of ohm-meters, and B is the magnetic field on the surface of the core in teslas (e.g. as described on [wikipedia's page on dynamo theory](https://en.wikipedia.org/wiki/Dynamo_theory)):

$$B \propto \sqrt\frac{\rho\Omega}{\sigma}$$

However, what I do not understand is, assuming there's a fluid in the core, energy is provided by rotation, and convective motion occurs in the core, why can only conductors in the core form magnetic fields and not insulators?

From what I understand, conductors have a relatively high conductivity, which would mean the denominator is high, so the magnetic field would be comparatively weak. Insulators like sulfur have a low conductivity, so the denominator would be low and the magnetic field would be strong. Could someone tell me if there is another effect that I am unaware of in comparing a conducting core and an insulating core?

**Tags:** magnetic-field, planetary-science, core

> **Comments:**
> **username_1**: This is an excellent question. I replaced your image of the formula by the actual formula using MathJax. This helps e.g. screen readers - and makes talking about arbitrary formula easy IMHO.  
> **username_2**: Low conductivity will allow for persistent separation of charges - which will appear as currents and hence generate the magnetic field. If an insulator does not have moving charges in the first place, the formula is not applicable.  
> **username_0**: @username_2 So the moving charges are only in conductors?  

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## 0 answers

