We always read the "Third Law" of magnetic attraction and repulsion as "the force varies inversely as the square of the distance separating the magnetic poles." I had to puzzle through the English on this one a few times. What happens is pretty simple and stated, I think, more clearly it's this: "the force varies such that it is the inverse of the new distance (between the poles) squared." Thus if poles are moved a further 2 feet apart from an original distance X, the force is the inverse of 2 squared, i.e., 1/4 of the original.

Just a thought to make some of the language clearer for newcomers. Any newcomers agree, or is it just me?

Tony

 

Originally posted by AWSariti@Jan 5 2005, 08:52 AM
We always read the "Third Law" of magnetic attraction and repulsion as "the force varies inversely as the square of the distance separating the magnetic poles." I had to puzzle through the English on this one a few times. What happens is pretty simple and stated, I think, more clearly it's this: "the force varies such that it is the inverse of the new distance (between the poles) squared." Thus if poles are moved a further 2 feet apart from an original distance X, the force is the inverse of 2 squared, i.e., 1/4 of the original.

Just a thought to make some of the language clearer for newcomers. Any newcomers agree, or is it just me?

Tony

[post=4425]Quoted post[/post]​

I agree, except its written like that for the math. Like RMS. The root of the mean of the square of the signal. Confusing but it has a purpose. In the beginning it gets stupid, but when you start doing multiple forces at different directions and have to integrate over time and/or distance, there is a method to the madness.

 

That inverse square law thing applies to radio wave/propagation as well. At the source the Intensity is I. For any distance (D) away from the source the intensity is divided by the square of the distance.

At the source you have intensity I.

Moving away from the source; I (new distance) = I (source) / (D-squared)

 

"Thus if poles are moved a further 2 feet apart from an original distance X, the force is the inverse of 2 squared, i.e., 1/4 of the original"

True only if X=2 ft.!

If you multiply the distance by two, you quadruple the force.
If you add two distance units, this does not tell you anything unless you know the original distance.

Taking your example, consider that original distance was one foot. You added two feet, making the new distance three feet. You have multiplied the distance by three, and your new force is one ninth the original.