A velocity selector consists of electric and magnetic fields described by the expressions vector E = E k hat bold and vector B = B j hat bold, with B = 0.0140 T. Find the value of E such that a 670 -eV electron moving along the negative x axis is undeflected.

Question

A velocity selector consists of electric and magnetic fields described by the expressions vector E  = E k hat bold and vector B  = B j hat bold, with B = 0.0140 T. Find the value of E such that a 670 -eV electron moving along the negative x axis is undeflected.
  kV/m

Elena · Answer

B=0.014 T
KE=670 eV =1.6•10⁻¹⁹•670 =1.072•10⁻¹⁶ J
KE=mv²/2
v=sqrt{2•KE/m} = 2•1.072•10⁻¹⁶/9.1•10⁻³¹=1.5•10⁻⁷ m/s
F(el) = F(mag)
eE =evB
E=vB =1.5•10⁷•0.014=
=2.1•10⁵ V/m=210 kV/m