Photoelectric Effect

Basically, the photoelectric effect is the ejecting of electrons from a metal by shining light of a particular frequency on it.

It doesn’t work for all wavelengths/frequencies

It was during 1887 that Hertz was conducting the experiment in which the photoelectric effect was discovered.Light was shone onto the cathode, which at certain frequencies emitted electrons, which were detected by the ammeter.These emitted electronswere called photoelectrons.

Named to show that lightwas responsible for emittingthem.

The flow of these photoelectrons is called the photocurrent.In the initial set up of the experiment, the cathode is made negative and the anode is made positive.When the photoelectrons are released from the metal, they will naturally go to the positively charged anode.

A sensitive ammeter will measure this photocurrent flow

With this arrangement, the maximum current flowing will be recorded by the ammeter.

All of the photoelectrons flowing across the gap will have a certain amount of kinetic energy.

The amount depends on the location of the metal that the electron came from

At or close to the surface – high energyWell below the surface – low energy

When the anode and cathode had zero potential, there was still a current being recorded flowing across the gap.

Though not as high as when the anode was made to be positive.

For electrons to be emitted, the light had to have a certain frequency.The minimum frequency that would cause this is called the threshold or cut-off frequency, f0

Anything below this, would not provide enough energy to eject electrons.

The German physicist Lenard found out the following things:

The rate at which electrons are emitted depends on the intensity of the light

High intensity – high currentLow intensity – low current

Cont’d..Metals would only emit electron above certain frequencies

Threshold frequency, f0

Electrons were emitted without any apparent time delay

The time delay is actually about 10-9s

The intensity of light doesn’t affect the threshold frequency.

Only frequency affects it

Lenard continued his investigations to find out more about the kinetic energy of the electrons being emitted.

In particular the maximum kinetic energy of the released electrons

This was done by reversing the potential around the circuit

The anode was made more and more negative

As this occurred, the current being recorded was observed to decrease.

This showed that a decreasing number of photoelectrons had enough energy to overcome the opposing electric potential of the anode.

Electrons with little or no KE were stopped as soon as the anode becomes negative

Once a certain potential was reached, no more photoelectric current was recorded.

This point is called the stopping voltage, V0

The photoelectrons with the most KE being stopped by Vo volts

Light with a higher frequency, was found to have a higher stopping voltage.

Red light (low frequency)

No electrons emitted

i = 0A

+

Bright Red light

No electrons emitted

i = 0A

+

Green light. Electron emission starts

μA

Small current

e-+

Brighter Green light. Electron emission starts

e-μA

Bigger current

+

Violet light. Electron emission continues

μA

Small current

e-+

Brighter Violet light. Electron emission continues

μA

Bigger current

e-+

Dim Light

Small current

e-

+

-

Small reversed voltage

Dim Light

No current

e-

More negative reversed voltage

+-

Photoelectric effect animation

http://www.ifae.es/xec/phot2.html

A convenient unit of energy when dealing with electrons is the electron volt (eV).An electron gains 1eV of energy when it is accelerated across a potential difference of 1 volt.

1 eV = 1.6 × 10-19 J

To convert from J to eV:divide the energy (in Joules) by 1.6 10-19.

Since an electron gains 1eV of energy when it is accelerated across a potential difference of 1 volt, it can be said that:

An electron accelerated through a potential difference of 50V, will gain 50eV of kinetic energy

Should a stopping voltage of 10V be required, the maximum kinetic energy of any electron would be 10eV.

Convert 10eV into J:

Convert 650J into eV & MeV:

The voltage at which the current is reduced to zero, the ‘Stopping voltage’ Vo, gives a measure of the maximum kinetic energy of the emitted photoelectrons.

Ek max = ½mvmax2= eVo

e - Charge on an electronV0 – stopping voltagem – mass of an electron = 9.11 x 10-31kg vmax – maximum speed of the electron

A photoelectron is released though doesn’t make it to ‘the other side’ as the stopping voltage is 6.0V.

What is the maximum kinetic energy of this photoelectron and at what speed must it be travelling?If the stopping voltage is 6.0V, the maximum kinetic energy must be 6.0eV

After further work by Philip Lenard between 1899 and 1902, Albert Einstein concluded that light does in fact behave like a particle.

The wave model could not explain the photoelectric effect.