3.2 More about photo electricity

The easiest electrons to eject are on the metals surface

And will have maximum kinetic energy

Other electrons need more energy than the work function to eject them.

They will have less kinetic energy

Photoelectrons are emitted with a range of kinetic energies

hc

EENERGYPHOTON

the visible spectrum

λ

frequency

violet light light 400 nm

red light light 700 nm

uv light < 400 nm

Blue photon Red photon

Which photon has the most energy ?????

BLUE !!!

Radiation

uA

Anode +ve Cathode -ve

electrons

The electromagnetic radiation releases electrons from the metal cathode. These electrons are attracted to the anode

and complete a circuit allowing a current to flow

vacuum

Using a vacuum photocell

If the polarity is reversed, the pd across the tube can be increased until even the most energetic electrons fail to cross the tube to A. The microammeter then reads zero.

uA

A C

Radiation

electronselectrons

The p.d. across the tube measures the maximum kinetic energy of the ejected electrons.

V

Increase the repelling voltage ( stopping potential Vs)until zero current flows. Then: PE gained = KE lost and eVs = ½ mv2 ( V = W so W = VQ ) Q

+

-

For electron emission, the photon's energy has to be greater than the work function .

The maximum kinetic energy the released electron can have is given by:

EK = hf - Φ For every metal there is a threshold frequency, f0, where hf0 = Φ ,that gives the photon enough energy to produce photoemission.

EK = photon energy – the work function.

Maximum EK emitted electrons / J

Frequency f / Hz

metal A

Work function, Φ

Threshold frequency f0

metal B

EK = hf - Φ

Gradient of each graph = Planck’s constant, h.

f / Hz 1014

0 5 10 15

Max Ek / eV

1

2

Potassium Magnesium Aluminium

If the receiving electrode is made positive,

it gives maximum help to all the photo electrons

to get there.

But does not cause any more photo electrons to be emitted.

This is saturation.

Vs

For both radiations the electrons are emitted

with the same maximum KE

Since the stopping voltages Vs are the same

Vs

Here the red and blue light have the same intensity (w/m2).

Vs is greater for blue light because blue photons

have more energy

Here the red and blue light have the same intensity (w/m2).

Vs is greater for blue light because blue photons

have more energy

Saturation current for blue light is less

because blue light arrives with each photon having

more energy.

( one photon one photo electron emitted ! )

Summary

For any metal there is a minimum threshold frequency, f0, of the incident radiation, below which no emission of electrons takes place, no matter what the intensity of the incident radiation is or for how long it falls on the surface.

Electrons emerge with a range of velocities from zero up to a maximum. The maximum kinetic energy, Ek, is found to depend linearly on the frequency of the radiation and to be independent of its intensity.

For incident radiation of a given frequency, the number of electrons emitted per second is proportional to the intensity of the radiation.

Electron emission takes place immediately after the light shines on the metal with no detectable time delay .