E/m

e/m lab manual

Goal
Our goal is to determine the fundamental charge "e" of an electron by comparing electric force to gravitational force on a tiny drop of mineral oil with some positive or negative charge.

Theory
In this experiment, we spray mineral oil from an atomizer into a viewing chamber between two plates with high voltage. Many of the oil droplets carry a charge (positive or negative) (how? why do they have a charge?), which we want to show are integral multiples of some fundamental constant. Not only do the oil droplets have different charges, they will also have different masses, which we want to show is related to the number of fundamental particles present. A grid on the back of the ocular is lit so that we can track individual oil droplets as they fall. We will measure the fall time of an oil droplet for a set distance (between every major grid line is 1 mm) once the droplet appears to have reached terminal velocity to determine the mass of the particle (the gravitational force on the particle mg= the frictional force of air on the particle kv... m=kv/g). Then, we turn on the voltage between the electric plates and calculate the electrical force on the particle by F=Eq, where E is the field strength (Voltage divided by distance between plates) and q is the charge on the particle. We then measure the rise time of the droplet in the field over the same distance, and we find the charge on the electron by equating mg+kv=Eq, or q=d(mg+kv)/V.