Transcript PHOTOELECTRIC DEVICES - Beacon Learning Center

PHOTOELECTRIC DEVICES
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 certain
light-sensitive materials,
 It may cause them to give electrons.
 It may change their ability to conduct
electricity.
 It may cause them to develop an electrical
potential, or voltage across two surfaces.
( These are called photoelectric devices.)
These devices that make use of the
photoelectric effect are often called
A photo electric cell
A photo cell, or
An electric eye
KINDS OF PHOTOELECTRIC CELLS
Photoemissive cells
Photoconductive cells
Photovoltaic cells
PHOTOEMISSIVE CELLS
 First explained by Albert Einstein
Photoemission
 Light, like all other radiation, is made up of
small particles called photons.
 The amount of energy the photons have
depends on the frequency of the light.
 The frequency of light is the number of
times that a light wave vibrates in a second.
 The higher the frequency, the more energy
the photons have.
…Photoemission
 If a photon hits an atom of a certain
material, it may be absorbed by an electron
of that material.
 However, if the photon has enough energy,
the electron is ejected, or emitted, from the
atom.
 In this way, light energy changes 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 Cells
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 CELLS
Results from light hitting a semiconductor
 Semiconductors contain free, negatively charged
electrons
 A free electron is an electron that has left its atom
and so is not bound to it
 When the electron leaves its atom, it creates a
positive charge in the atom
 The positive charge attracts an electron from
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.
Burglar alarms sometimes have
electric eyes that operate on the
principle of photoconductivity.
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.
Other examples of electric eyes :
Automatic door openers
Elevator doors
Supermarket scanners
PHOTOVOLTAIC CELLS
Is similar to photoconductivity.
 In this effect, light falls on two semiconductors, or
on a metal and a semiconductor sandwiched
together.
 A boundary develops between these two
substances.
 Free electrons build up along the boundary but
cannot flow across it.
 However, if the two substances are connected in 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.
Solar cells work on the same principle.
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.
Bibliography
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>.