Answer
See answers.
Work Step by Step
The electric force must balance the magnetic force, since the electrons are undeflected.
$$qE=qvB$$
$$v=\frac{E}{B}=\frac{7700V/m}{0.0075T}=1.0\times10^6m/s$$
If the electric field is turned off, the magnetic force causes circular motion. Use the relationship from Example 20-6, page 567, to find the radius.
$$r=\frac{mv}{qB}=\frac{(9.11\times10^{-31}kg)(1.027\times10^6m/s)}{(1.6\times10^{-19}C)(0.0075T)}=7.8\times10^{-4}m $$
