Question

An electron travels with speed 2.0 × 107 m/s between the two parallel charged plates shown in the figure. The plates are separated by a 100 V battery.

(a)What is the direction of the magnetic field to allow the electron to pass between the plates without being deflected? Explain how you determine the direction.

(b) What magnetic field strength will allow the electron to pass between the plates without being deflected? SHOW FULL WORK

Answer 1

Answer 2

a)

the direction of the magnetic field = into the page so that on negative charge magnetic force can ac downward to balance an upward electric force

see figure

A Fe -e (V B E V FB

E = V/d =200/ 0.01 = 2*10⁴ V/m

for no deflection

F_net = 0   

Fb = Fe  

qVB = qE

B = E/V = 2*10^4/( 2*10^7) = 0.001 T  

magnetic field strength   = 0.001 T