### Introduction

Consider a thin insulated rod that carries a known negative charge Qrod that is uniformly distributed. It is possible to determine the electric field along a line perpendicular to the rod that passes through its center using the following equation – often derived in introductory texts: $\vec E_{{\text{rod}}}^{{\text{theory}}} = \frac{{\left( {k{Q_{{\text{rod}}}}} \right)}} {{r\sqrt {{r^2} + {{(L/2)}^2}} }}\hat i$

where L is the length of the charged part of the rod, r is the distance from the test charge to the center of the charged part of the rod, and Qrod is its total charge. The constant k is the well-known Coulomb constant.

### Objectives

In this activity, you will

• Examine a digital movie of a charged rod exerting a force on a hanging “test charge” along with a Logger Pro analysis to determine if the theoretical equation describes the relationship between r, L and the measured electric field, $\vec E_{{\text{rod}}}^{{\text{meas}}}$ at the location of the test charge.