magnetic and electric fields

[nemzy]

i have no idea how to do this problem:

a velocity selector consists of electric and magnetic fields described by the expressions E=E(k hat) and B=B(j hat), with B=14.5 mT. Find the vale of E such that a 828-eV electron moving along the positive x axis is undeflected.



hmm..since it is undeflected we can say that qE=qvB, right? and v=E/B...

but how could u solve for E? and how does the 828 eV electron fit into this problem?

[Andrew Mason]

i have no idea how to do this problem:

a velocity selector consists of electric and magnetic fields described by the expressions E=E(k hat) and B=B(j hat), with B=14.5 mT. Find the vale of E such that a 828-eV electron moving along the positive x axis is undeflected.



hmm..since it is undeflected we can say that qE=qvB, right? and v=E/B...

but how could u solve for E? and how does the 828 eV electron fit into this problem?
You have correctly stated the Lorentz force law:
\vec{F} = q \vec{E} + q \vec{v} \times \vec{B}

Determine electron speed from the electron kinetic energy of 828 eV. Since the net force = 0,
\vec{v} = \frac{-\vec{E}}{\vec{B}}

AM

[mborn]

1eV=1.602 \times 10^{-19} J
m_e=9.11 \times 10^{-31} Kg

M B