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## Homework Statement

The Earth's magnetic field is essentially that of a magnetic dipole. If the field near the North Pole is about 10^-4 T, what will it be (approximately) 13,000 km above the surface at the North Pole?

## Homework Equations

[tex]B = \frac{\mu _0}{2\pi}\frac{\mu}{(R^2+x^2)^{3/2}}[/tex]

for x >> R

[tex]B = \frac{\mu _0}{2\pi}\frac{\mu}{x^3}[/tex]

## The Attempt at a Solution

I know how to find the magnetic field of a dipole on its axis for a circle. My textbook uses the second formula to solve this problem. But I cannot understand what "x" and "R" are for the earth, and how we assumed x>>R.