Transcript photoelectric effect

PHOTOELECTRIC EFFECT
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At the end of this presentation we should be able to:
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explain the process of the photoelectric effect using wave and particle theories.
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calculate the energy of photons for a given frequency.
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define and explain the work function of a metal.
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explain how the energy supplied by the incident radiation is used in the photoelectric
effect process (work function, kinetic energy).
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explain the effect of intensity on the photoelectric effect process .
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solve problems using Einstein's Photoelectric equation.
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Prior knowledge
(What we need to know before we start talking about PE)
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Metals have a sea of free electrons
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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).
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Radio waves – Gamma rays
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Quantisation of energy
(Photons)
o In a beam of radiation there are discrete particles
called photons.
o The photons of the light beam have a characteristic
energy (energy of each photon) determined by the
frequency of the light.
o Each photon has energy E = hf, where h is Planck’s
constant.
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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
W=QV=1,6×10-19 ×1
W=1,6×10-19 J
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Photoelectric Effect
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What is it :
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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.
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How did it all start?
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Henrich Hertz was the first to discover this phenomena in 1887 when he
was investigating radio waves.
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In 1901 Max Planck showed that energy is quantized, E=hf.
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Albert Einstein explained the photoelectric effect in 1905.
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An investigation
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Let’s look at an animation:
◦ Observe
◦ Record and
◦ Draw conclusions from the results.
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Results of the Investigations
1.
Photoelectrons are emitted for a specific metal if the frequency of
radiation exceeds a certain limit (threshold frequency, fo).
2.
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.
3.
The emitted photoelectrons have kinetic energy ranging from zero to a
maximum.
4.
Maximum kinetic energy depends on frequency.
5.
The intensity of radiation has no effect on the kinetic energy of the
emitted photoelectrons.
6.
Emission starts as soon as the surface is irradiated with effective radiation.
7.
Photoelectric current depends on intensity.
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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.
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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.
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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)
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Einstein’s theory of the
Photoelectric effect
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EM radiation consists of small particles or lumps/packets of
energy called photons.
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Each photon carries energy proportional to its frequency.
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NB: There are free electrons in metals.
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When light is directed onto a metal surface a photon will
collide with a free electron.
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The interaction between a photon and an electron is a one
to one correspondence.
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The photon can then be reflected without a change in its
kinetic energy or it transfers all its kinetic energy to the
electron.
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Einstein’s Theory (continued)
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The electron gains all the kinetic energy
from the photon.
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If the energy gained is sufficient the electron
will escape from the metal surface. This is
the process of photoelectric effect.
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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.
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And more …..
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The minimum energy required to overcome the
forces is called the work function (W).
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The magnitude of this energy is a few electron
volts.
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The frequency that corresponds to this energy is
the threshold frequency (fo).
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The relation between the work function and the
threshold frequency is given by
W = hfo
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Electrons are only emitted if the frequency of
radiation greater than the threshold frequency
(hf > W)
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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 =
W=
the energy of each photon of frequency f
work function of the metal surface
½mv2 = maximum kinetic energy of the emitted
electrons
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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.
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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
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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.
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