1. PHOTOELECTRIC EFFECT
hhhhh
12/4/2018

2. At the end of this presentation we should be able to:
explain the process of the photoelectric effect using wave and particle theories.
calculate the energy of photons for a given frequency.
define and explain the work function of a metal.
explain how the energy supplied by the incident radiation is used in the photoelectric effect process (work function, kinetic energy).
explain the effect of intensity on the photoelectric effect process .
solve problems using Einstein's Photoelectric equation.
12/4/2018

3. Prior knowledge (What we need to know before we start talking about PE)
Metals have a sea of free electrons
Electromagnetic (EM) radiation
Waves consisting of oscillating electric (E) and magnetic (B) fields.
All EM waves travel at the speed of light.
Since all EM waves travel at same speed and using the wave equation v = fλ, we can see that the differences only arise in the frequency and the wavelength of EM waves.
EM waves range from radio waves (long wavelength& low frequency) to gamma rays (short wavelength & high frequency).
12/4/2018

4. Radio waves – Gamma rays
12/4/2018

5. Quantisation of energy (Photons)
In a beam of radiation there are discrete particles called photons.
The photons of the light beam have a characteristic energy (energy of each photon) determined by the frequency of the light.
Each photon has energy E = hf, where h is Planck’s constant.
12/4/2018

6. The Electron volt
When an electron is accelerated in a potential difference it gains kinetic energy.
When the electron is accelerated by a PD of 1 V the amount of kinetic energy gained is 1 electron-volt (1 eV).
Work = Charge (Q) x Potential difference (V).
When an electron is accelerated through a pd of 1 V the energy gained is given by
12/4/2018

7. Photoelectric Effect What is it :
When metal surfaces are exposed to electromagnetic radiation with sufficient energy they absorb the photons of energy and emit electrons. This process is called the photoelectric effect.
How did it all start?
Henrich Hertz was the first to discover this phenomena in 1887 when he was investigating radio waves.
In 1901 Max Planck showed that energy is quantized, E=hf.
Albert Einstein explained the photoelectric effect in 1905.
12/4/2018

8. An investigation Let’s look at an animation: Observe Record and
Draw conclusions from the results.
12/4/2018

9. Results of the Investigations
Photoelectrons are emitted for a specific metal if the frequency of radiation exceeds a certain limit (threshold frequency, fo).
The rate of photoelectron emission for a single frequency radiation beam is proportional to the intensity of radiation i.e. the more intense the radiation of the same frequency the more photoelectrons are emitted.
The emitted photoelectrons have kinetic energy ranging from zero to a maximum.
Maximum kinetic energy depends on frequency.
The intensity of radiation has no effect on the kinetic energy of the emitted photoelectrons.
Emission starts as soon as the surface is irradiated with effective radiation.
Photoelectric current depends on intensity.
12/4/2018

10. After these investigations there was a problem.
Q Wave theory :
An electromagnetic wave produces an electric field, which exerts force on the electrons on the surface of a metal. The force will push the electrons from the surface.
Higher intensity of electromagnetic radiation results in a high electric field which then produces a bigger electric force on the electrons. This force will push off the electrons with a higher speed.
Emission should take place at any frequency because the electrons would absorb energy from the incoming radiation until they have energy enough to escape “So why threshold frequency?”
A The Quantum Theory (particle nature of light) was the answer (Einstein, 1905)
12/4/2018

11. Einstein’s theory of the Photoelectric effect
EM radiation consists of small particles or lumps/packets of energy called photons.
Each photon carries energy proportional to its frequency.
NB: There are free electrons in metals.
When light is directed onto a metal surface a photon will collide with a free electron.
The interaction between a photon and an electron is a one to one correspondence.
The photon can then be reflected without a change in its kinetic energy or it transfers all its kinetic energy to the electron.
12/4/2018

12. Einstein’s Theory (continued)
The electron gains all the kinetic energy from the photon.
If the energy gained is sufficient the electron will escape from the metal surface. This is the process of photoelectric effect.
Part of the energy gained by the electron is used to release it from the surface (i.e. to overcome the force of attraction between the electrons and the metal ions) and the rest of the energy is the kinetic energy of the electron as it leaves the metal.
12/4/2018

13. And more …..
The minimum energy required to overcome the forces is called the work function (W).
The magnitude of this energy is a few electron volts.
The frequency that corresponds to this energy is the threshold frequency (fo).
The relation between the work function and the threshold frequency is given by
W = hfo
Electrons are only emitted if the frequency of radiation greater than the threshold frequency
(hf > W)
12/4/2018

14. Einstein’s PE Equation
Energy of incident photon =
work function of the metal + maximum kinetic energy of the released electrons.
hf = W + ½ mv2
where : hf = the energy of each photon of frequency f
W = work function of the metal surface
½mv2 = maximum kinetic energy of the emitted electrons
12/4/2018

15. Graph of Ek of photoelectrons vs frequency of em-radiation
Maximum kinetic energy is measured in electron volts eV.
The threshold frequency (f0) of this material is 6,4 x1014 Hz.
12/4/2018

16. WHY IS THE PHOTOELECTRIC EFFECT SO IMPORTANT?
It helped explain the particle nature of light.
It is the basis of the quantum theory.
It is used in photocells e.g. in solar calculators, alarms and automatic door openers
12/4/2018

17. Misconceptions to watch out for
Light can be a particle or a wave depending on time.
A photon is a particle containing a wave in it.
A photon is a wave containing a particle in it.
The higher the frequency of a photon the bigger the photon.
All photons have the same energy.
12/4/2018