PHOTOELECTRIC PHOTOELECTRIC DEVICESDEVICES

Prepared by CynthiaYoungblood

Jay High School, Santa Rosa County

A photoelectric effect is any effect in which light energy is converted to electricity.

When light strikes certainWhen light strikes certain light-sensitive materials light-sensitive materials, ,

It may cause them to give electrons.It may cause them to give electrons. It may change their ability to It may change their ability to

conduct electricity.conduct electricity. It may cause them to develop an It may cause them to develop an

electrical potential, or voltage across electrical potential, or voltage across two surfaces. ( These are two surfaces. ( These are called photoelectric devices.)called photoelectric devices.)

These devices that make use of the These devices that make use of the photoelectric effect are often calledphotoelectric effect are often called

A photo electric cellA photo electric cellA photo cell, orA photo cell, orAn electric eyeAn electric eye

KINDS OF PHOTOELECTRIC KINDS OF PHOTOELECTRIC CELLSCELLS

Photoemissive cellsPhotoemissive cellsPhotoconductive cellsPhotoconductive cellsPhotovoltaic cellsPhotovoltaic cells

PHOTOEMISSIVE CELLSPHOTOEMISSIVE CELLS

First explained by Albert EinsteinFirst explained by Albert Einstein

PhotoemissionPhotoemission

Light, like all other radiation, is made up Light, like all other radiation, is made up of small particles called photons.of small particles called photons.

The amount of energy the photons have The amount of energy the photons have depends on the frequency of the light.depends on the frequency of the light.

The frequency of light is the number of The frequency of light is the number of times that a light wave vibrates in a times that a light wave vibrates in a second.second.

The higher the frequency, the more The higher the frequency, the more energy the photons have.energy the photons have.

……PhotoemissionPhotoemission

If a photon hits an atom of a certain If a photon hits an atom of a certain material, it may be absorbed by an material, it may be absorbed by an electron of that material.electron of that material.

However, if the photon has enough However, if the photon has enough energy, the electron is ejected, or energy, the electron is ejected, or emitted, from the atom.emitted, from the atom.

In this way, light energy changes In this way, light energy changes into electrical energy.into electrical energy.

If wires are attached to a photoemittive material, the electrons can flow along the wires, forming an electric current.

Photoemission cells are used in motion picture projectors to read film sound tracks.

Example of Photoelectric Example of Photoelectric CellsCells

The photocell on this photographer’s slave flash unit uses the photoelectric effect. When a nearby flashgun goes off, it immediately fires the slave flash.

PHOTOCONDUCTIVE PHOTOCONDUCTIVE CELLSCELLS

Results from light hitting a semiconductorResults from light hitting a semiconductor Semiconductors contain free, negatively Semiconductors contain free, negatively

charged electronscharged electrons A free electron is an electron that has left A free electron is an electron that has left

its atom and so is not bound to itits atom and so is not bound to it When the electron leaves its atom, it When the electron leaves its atom, it

creates a positive charge in the atomcreates a positive charge in the atom The positive charge attracts an electron The positive charge attracts an electron

from another atomfrom another atom

This process repeats itself, creating an electric current. When light falls on certain semiconductors, the number of electrons leaving their atoms increases. This increases the current.

A streetlight is a good example of an application of photoconductivity.

As daylight fades, the electrical current in the streetlight’s semiconductor comes to a stop. This activates a switch that turns the streetlight on.

When the light source to the semiconductor is interrupted by a person walking through the beam of light, a switch is activated that triggers the alarm.

Burglar alarms sometimes have electric eyes that operate on the principle of photoconductivity.

Other examples of electric eyes :

Automatic door openers

Elevator doors

Supermarket scanners

PHOTOVOLTAIC CELLSPHOTOVOLTAIC CELLS

Is similar to photoconductivity.Is similar to photoconductivity. In this effect, light falls on two semiconductors, or In this effect, light falls on two semiconductors, or

on a metal and a semiconductor sandwiched on a metal and a semiconductor sandwiched together.together.

A boundary develops between these two A boundary develops between these two substances.substances.

Free electrons build up along the boundary but Free electrons build up along the boundary but cannot flow across it.cannot flow across it.

However, if the two substances are connected in However, if the two substances are connected in a circuit, then the electrons can flow across it.a circuit, then the electrons can flow across it.

The voltage generated from a single photovoltaic cell is typically a fraction of a volt. By connecting many thousands of individual cells together, however, as in modern solar batteries, more than 1 kilowatt of electric power can be generated.

The energy efficiency of most present-day photovoltaic cells is only about 7 to 11 percent—that is, only that fraction of the incoming radiant energy is converted to electric energy. Since the intensity of solar radiation is low—about 125 watts per square foot (1,350 watts per square meter) above the atmosphere and less at the Earth's surface—huge and costly assemblies of such cells are required to produce even moderate amounts of power.

Photovoltaic cells are used in exposure meters for photography.

Light falling on the meter causes a current to flow. The size of the current depends on the amount of light. The light can be measured indirectly by measuring the current.

Consequently, photovoltaic cells that operate on solar light (using solar cells or solar batteries) have so far been used primarily for very low-power applications. These applications include their use as power sources for calculators, watches, and cameras, for example. Larger units have been used to power space satellites and experimental airplanes and automobiles.

Solar cells work on the same principle.

BibliographyBibliography

Sharpe, Diane. Illustrated Science Encyclopedia.

“Photoelectric Devices.” Austin, Texas: Raintree Steck-Vaughn, 1997.

“Photoelectric Device.” Microsoft Internet

Explorer. 13 Nov.2001<http://search.ebi.eb.

com/ebi/article/0,6101,36464,00.html>.