heh heh, my physics teachers in high school hated when I used this phrase. But it's useful, as well as being good for a quick laugh, if you're around the right sort of people. I guess the only way to explain its use would be to give an example. Say you had a uniform distribution of charge covering a flat wall. Say you wanted to find the force on an electron 1 cm from that wall. The exact (but clearly wrong) way to do this is to integrate the electric field across the entire wall. The less exact, but much, much easier method (and therefore the Right Thing to do) is to say, hey, the wall is approximately infinite, so we can use the formula for E of an infinite sheet. Ain't that cool?

And to Kung: Approximately infinite just sounds better than effectively infinite, wouldn't you agree?

no no no!

Its not approximately infinite its effectively infinite.

You could say that the field produced by the charge on the wall may be approximated by considering the wall as an infinite sheet - but thats different.

and infinitely approximate sounds even better but it still doesnt make sense

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