Answer
0.24 T
Work Step by Step
This is discussed in example 20-6. The magnetic force causes circular motion. The alpha particle moves in a clockwise circular path. The radius of the motion is calculated in the book’s example.
$$r=\frac{mv}{qB}$$
$$B=\frac{mv}{qr}$$
$$=\frac{(6.6\times10^{-27}kg)( 1.6\times10^6m/s)}{ 2(1.60\times10^{-19}C) (0.14m)}$$
$$=0.24T$$