Photoelectric Effect

Introduction

This project demonstrates the Photoelectric Effect, a concept that won Albert Einstein his Nobel Prize. This project answers the question of whether or not UV light will excite an electrically charged zinc plate enough for it to actually lose it charge.

Materials

♦ Electroscope or other electrometer

Procedures

Electrically connect the zinc plate to the electroscope or electrometer via the alligator clips. Make sure the zinc plate is insulated from all other objects so it can maintain its charge.
Rub the plastic rod with the wool fabric thoroughly until a charge is produced (you should feel the static electricity).
Slowly move the rod over the zinc plate until the electroscope starts moving clockwise.
Slowly move the rod away from the zinc plate as the electroscope begins to balance itself and settles in a position. This should let you know the zinc plate is charged.
When the position of the electroscope has settled, turn on the UV light and shine it directly over the charged zinc plate. After a certain amount of time, the electroscope will slowly recede to its original position.
For the second part of the experiment, place your hand on top of the electroscope to reset it to its original position, and redo steps 2-4
Instead of shining the UV light on the plate, shine the incandescent light on it. The plate should remain electrically charged.

Safety Measures

Because this project deals with UV light, BE SURE to NEVER shine the light in your own or someone else's eyes as it can lead to blindness. Be certain where the beam is pointing prior to turning it on, and turn it off immediately after using it. Also be sure to be careful with the incandescent bulb, as it is glass.

♦ Zinc plate

♦ Plastic rod and piece of wool fabric

♦ UV lamp

♦ Incandescent lamp

♦ Alligator clips, 4-mm

Scientific Principle

The initial part of the experiment involved charging the zinc plate with electrons. Because the electrons have a negative charge, the electroscope was able to move freely simply by allowing that flow of electrons to move down the instrument. After the zinc has been charged and the UV light has been shined on it, the electroscope returns to its original position. This is where the actual science of the experiment comes in. The frequency and wavelength of ultraviolet light gives enough energy to the electrons to be excited, thereby liberating them from their energy levels. The reason no light is perceivable is that electrons that demonstrate this property also follow an "all-or-nothing" principle that essentially states that if the minimum energy for liberation is received, it will leave the orbital. If not, it will stay in its position. The reason the incandescent light does not excite the electrons and cause the electroscope to move is because visible light does not carry enough energy in its wavelength to excite the electrons to the point of liberation.

Page updated

Google Sites

Report abuse