# Magnetic and electric fields

1. Nov 25, 2004

### 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?

2. Nov 25, 2004

### Andrew Mason

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

3. Nov 25, 2004

### mborn

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

M B