Size of an object not connected to safety physical earth, to be effective earth?!

[GrimJosef]

Originally Posted by Neutrino

... Even if you used other base units the relationships would be the same except perhaps for numerical factors ...

In a sense this is true. But the critical thing is that the numerical factors are not necessarily dimensionless.

You are right in saying that in SI electric charge has the dimensions s A

In cgs electric charge has the dimensions g^0.5 cm^1.5 s^-1

It really does https://en.wikipedia.org/wiki/Gaussi...Unit_of_charge. They are not the same.

Cheers,

GJ

[Hartley118]

Originally Posted by Neutrino

Capacitance expressed in terms of SI base units is: m^−2 kg^−1 s^4 A^2
(Sorry, I do not see how to do superscript text here.)

The symbols for the SI base units are: m, kg, s, A, K, mol and cd. Even if you used other base units the relationships would be the same except perhaps for numerical factors..........

David

Agreed. Dimensions are independent of units actually used - only the constants change. The dimensions of charge connect to the fundamentals length, mass and time via the force exerted between two charges and the force between conductors carrying currents (rate of flow of charge).

I won't attempt equations - as you indicate, they don't come out well on the Forum. It would also lead us (almost but not quite) off-topic.

Martin

[GrimJosef]

Originally Posted by Hartley118

... Dimensions are independent of units actually used - only the constants change ...

How can this be true if the constants are not dimensionless ? In SI units the force between charges involves the constant 'epsilon' (the permittivity). In cgs units the force between the charges does not involve epsilon. Epsilon is a constant. But it is not dimensionless.

Cheers,

GJ

[G8HQP Dave]

Having just read the Wikipedia article on cgs as applied to electromagnetism I am glad that I never had to learn it or use it. What a mess!

PS Perhaps I should have said 'them' instead of "it" in the above sentence because it appears there are three or four cgs versions for electromagnetism. They differ in where the factors of c appear.

[julie_m]

Turning moment is measured in Newtons-metres (= kg.m2.s-2); which look a lot like Joules, dimensionally, but aren't. (Although, if the magnitude of the force remains constant, the turning moment is equal to the work done per radian of angle turned through.)

Otherwise, if anything else has dimensions kg.m2.s-2, it's probably energy.

[GrimJosef]

Originally Posted by G8HQP Dave

Having just read the Wikipedia article on cgs as applied to electromagnetism I am glad that I never had to learn it or use it. What a mess!

Me too, to be honest. I think if you work in a relatively small area then it can make that small area simpler and tidier. But when it comes to interfacing reliably with the rest of physics keeping track of the conventions becomes very hard work. Had I had to use it in anger then sooner or later I would have dropped the ball over something important.

It was a bit annoying though because when I was studying this the most authoritative book on the whole subject https://en.wikipedia.org/wiki/Classi...dynamics_(book) was written using cgs units. Fortunately I only needed it occasionally. And now the latest edition at least starts in SI. Phew !

Cheers,

GJ

[G8HQP Dave]

My copy of Jackson is quite recent. I originally learnt EM from Landau and Lifschitz but I can't remember what units they used. I had enough trouble with particle physics, where c=h=1 is assumed but then the result of a calculation is expressed in MeV or millibarns!

[Al (astral highway)]

Thank you , folks. There is much to digest here, very interesting stuff indeed. I am minded to do some basic experiments at some point in the (far from immediate!) future...