What is the electric field strength between the disks?

Question

Two 4.0cm-diameter disks face each other, 1.5mm apart. They are charged to +/- 11nC.

a) What is the electric field strength between the disks?

b) A proton is shot from the negative disk toward the positive disk. What launch speed must the proton have to just barely reach the positive disk?

Answer 1

a) E=Q/(ε₀*A), where A=(πD^2)/4 ==>

E=4Q/ (πε₀D^2)=4*11e-9/(π*8.854e-12*0.04^2)

E=988652 V/m

b) e=1.6e-19 proton charge, m=1.6726e-27 proton mass, d=0.0015 m

Ekin(neg disc)=Epot(pos disc)

0.5mv^2=eU, where U=dE ==>

v=√(2edE/m)=

√(2*1.6e-19*0.0015*988652/1.6726e-27)=

532.655 km/s