Question

What uniform magnetic field must be set up in space to permit a proton of speed 1.0x10^7 m/s to move in a circle the size of the earth’s equator? (R = 6400 km) 

Answer 1

We know that

$R=\frac{mv}{qB}$

$B=\frac{mv}{qR}$

$$B=\frac{1.6\times10^{-27}\times10^7}{1.6\times10^{-19}\times64\times10^5}=1.63\times10^{-8}T$$