Ain Shams U. Faculty of Engineering Mathematics and Engineering Physics Department Lecture 2 : Photoelectric Effect د. حاتم الرفاعي سبتمبر 2004

Ain Shams U.Faculty of Engineering

Mathematics and Engineering Physics Department

Lecture 2 : Photoelectric Effect

الرفاعي. حاتم د2004سبتمبر

Sept.-Dec. 2004 الفزياء الهندسية – أولى كهرباء

Contents

Websites Historical background Photoelectric phenomena Classical physics prediction Modern physics explanation


Websites

http://lectureonline.cl.msu.edu/~mmp/kap28/PhotoEffect/photo.htm

http://theory.uwinnipeg.ca/physics/quant/node3.html

http://www.colorado.edu/physics/2000/quantumzone/photoelectric.html


Historical background

1888, Hallwachs found that when a Zinc plat is illuminated with ultraviolet light, it becomes positively charged.

1899, Thomson showed that electrons were expelled from the plate.

1902, Lenard experiment. 1905, Einstein explanation.


Photoelectric effect

If the incident wave frequency (f) is smaller than a certain value (fo), there is no current flows.

If f> fo, the current flows instantaneously. Reverse biased connection

I2>I1

ForwardReverse


Photoelectric effect

With C forward biased, the electrons are accelerated faster resulting in an increasing current till a saturating value.

The higher the incident wave intensity, the higher the saturation current.

With C reverse biased, electrons are repelled from reaching C, and thus the current decreases.

At a certain reverse potential -Vo the current becomes zero independent on the incident wave intensity.

Reverse biased connection

I2>I1

ForwardReverse


Classical physics prediction

Electrons can be emitted regardless of the incident frequency, though it will take longer time for smaller incident wave amplitude.

There should be a time delay between the wave illumination and the emission of electrons.

The higher the wave intensity, the higher electron energy, and thus the higher the stopping voltage.


Modern physics explanation

The electromagnetic wave consists of many lumped energy particles called photons.

The energy of each individual photon is given by

hfE Jole



If N is the total number of photons incident during time interval T, then the total incident optical energy is

The incident energy per second (power) is given by

n=N/T is the number of incident photons per second.

NhfE

hfT

NP

Jole

Watt = J/Sec.



Interaction (absorption / emission) between the electromagnetic wave and matter occurs through annihilation ( على creation / (القضاء.energy (photon) (كمي) of a quantized (خلق)

In the photoelectric effect, each single absorbed photon gives its total energy (hf) to one single electron.

This energy is used by the electron to: Overcome the attraction force of the material. Gain kinetic energy when freed from the material.



Work function (): It is the minimum required energy required by an electron to be free from the attraction force of the metal ions.

Some of the electrons may need more energy than the work function to be freed.

Total Energy

Zer

o

-ve

+ve

The most energetic electrons in the material



Total Energy

Zero

-ve

+ve

The most energetic electrons in the material

hf

hf



Total Energy

Zero

-ve

+ve

The most energetic electron outside the material

2maxmv

2

1hf

hf



The electrons that need only the work function to be freed, will have the greatest kinetic energy outside the metal.

The electrons requiring higher energy to be freed, will have lower kinetic energy.

2maxmv

2

1hf



Thus, there is a minimum required photon energy (hfo) to overcome the work function of the material.

If the incident photon energy is less than the work function, the electron will not be freed from the surface, and no photoelectric effect will be observed.

ohf

=No photoelectric

current

If hf<

If f< fo



The most energetic electrons are stopped by the reverse biased stopping potential -Vo.

o2max eVmv

2

1

Kinetic energy of photoelectron

=

potential energy created by the reverse biased connection



The stopping potential doesn’t depend on the incident light intensity.

The stopping potential depends on the incident frequency.

oo ffheV

oo hfeVhf

2maxmv

2

1hf

oo ffe

hV

Slope = h/e

Documents

Ain Shams U. Faculty of Engineering Mathematics and Engineering Physics Department Lecture 2 : Photoelectric Effect د. حاتم الرفاعي سبتمبر 2004