My PBL Project

Introduction

In this experiment, a zinc plate is given a negative charge and then returned to a neutral charge by shining a UV light over it. But what causes the negatively charged plate to lose charge as the UV light is shined over it?

Scientific Principle

The charge initially created by the wool was conducted by the zinc plate. This charge was negative, meaning that it excited the electrons in the zinc plate. That is what the electroscope was measuring. The science behind the UV light is intriguing. The UV rays emitted by the light are absorbed by the excited electrons and are used to liberate the electrons from the zinc plate, returning the charge back to neutral. The reason visible light does not do this is because the electrons follow a sort of "all or nothing" principle, in which the electrons absorb the energy from the light photons and escape or the energy is remitted. Visible light does not have enough energy to liberate the electrons from the zinc plate and so the energy is remitted in the forms of light and thermal energy.