PHOTOELECTRIC EFFECT

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PHOTOELECTRIC EFFECT
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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, Ehf.
• Albert Einstein explained the photoelectric
  effect in 1905.

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The effect of light on a metal surface

• The photo-electric effect can be demonstrated by
  means of an ultraviolet lamp, a zinc plate, an
  electroscope and two ordinary light bulbs of 40 W
  and 200 W.

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Photoelectric effect 5
An electroscope can be charged by induction by
holding a charged acetate rod near the top
plate. Mobile negative charge in the metal plate
is repelled down to the leaf. The leaf and
central pole piece now have the same type of
charge so the leaf rises.
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Photoelectric effect 6
With the rod still nearby, the plate is touched
so more charge moves to the plate through the
person. The finger is pulled away and then the
charged rod is removed. This is called CHARGING
by INDUCTION You can charge the electroscope
positively by using a polythene rod instead.
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Photoelectric effect 7
The process can be repeated whilst a polished
zinc plate is placed on top of the
electroscope. The same effect will be achieved
and the leaf will have been left in a raised
position. It will fall slowly over time but not
appreciably during a short demonstration.
ZINC PLATE
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Photoelectric effect 8
When the rod is removed, the extra negative
charge redistributes itself evenly all over the
leaf, central pole and zinc plate. Why does it do
this?
ZINC PLATE
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Photoelectric effect 9
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Start with an electroscope that is charged
negatively. The U-V light causes photoelectrons
to be emitted. These are repelled by the surface
and escape. Charge is lost by the electroscope so
the leaf falls.
Polished zinc
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Photoelectric effect 10
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When all the extra charge has gone, the leaf has
fallen to its resting position. No further
electrons will escape because the surface will
not repel any liberated electron.
Polished zinc
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Photoelectric effect 9
The electroscope is charged negatively.
Polished zinc
The white light does not cause photoelectrons to
be emitted.
Charge is not lost by the electroscope. The leaf
does not fall no matter how bright(intense) the
light is or for how long it is shone onto the
zinc.
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Photoelectric effect 12
Why?
Why do ultraviolet photons liberate
photoelectrons whilst visible light photons do
not?
Answer none of the photons in white light
has enough energy to release even
one photoelectron
The energy of a photon is given by E hf where
h is Planck's constant and f is the frequency.
Also E hc because c fl (the wave equation)
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This means that the higher the frequency, the
greater the energy. Visible light contains
frequencies that are too low for photoelectric
emission. Alternatively, the shorter the
wavelength, the greater the energy of the
photons.
Visible wavelengths are too long.
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Observations

• Ultraviolet light causes a negatively charged
  electroscope to discharge the leaves of the
  electroscope collapse when UV light shines on it.
• White light does not release e- from the zinc
  plate even when irradiated with light of a much
  higher intensity or for a longer period.
• When the electroscope is positively charged
  nothing happens because it is much more difficult
  to remove e- from a positive object.

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• When a glass plate is placed between the
  ultraviolet source and the zinc plate, the
  electroscope stops discharging.
• CONCLUSION
• 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.

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• 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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Threshold frequency

• Each specific metal has a minimum frequency
  called the threshold frequency for which
  electrons will just be released from the metal.
  The frequency of the incident light must be equal
  to or greater than the threshold frequency before
  electrons can be released.
• More e- are liberated from a metal if light with
  a higher frequency than that of the threshold
  frequency of the metal strikes the metal surface
  an increase in intensity of this light
  increases the number of e- that are liberated per
  second.

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Plancks Quantum Theory

• In 1901, Max Planck, suggested that the radiation
  of energy was not a continuous process. Planck
  made the following assumptions
• Energy is radiated in packages or quanta.
• Each quantum consists of a specific amount of
  energy, E, which is directly proportional to the
  frequency of the radiation
• A fraction of a quantum can never be radiated nor
  absorbed, only whole numbers of quanta.

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After these investigations there was a problem.

• 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)

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Einsteins 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.

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• 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.
• 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.
• 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 gt W)

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Energy of photon

• 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

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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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Graph of KE of electron and frequency of incident
light on metal
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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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The Dual Nature of Light

• What is light a wave or a particle?
• The wave theory cannot explain all the known
  facts in connection with light.
• Diffraction and interference can only be
  explained by the wave theory.
• The quantum hypothesis offers an excellent
  explanation for the photo-electric effect but use
  the concept of frequency to calculate the energy
  of a photon.
• Light has both a wave- and particle nature.
• The wave nature predominates during the
  propagation of radiation, while the particle
  nature predominates during the interaction with
  matter.

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Applications of the photoelectric effect

• The photoelectric effect has many practical
  applications which include the photocell,
  photoconductive devices and solar cellsA
  photocell A photocell is usually a vacuum tube
  with two electrodes. One is a photosensitive
  cathode which emits electrons when exposed to
  light and the other is an anode which is
  maintained at a positive voltage with respect to
  the cathode. Thus when light shines on the
  cathode, electrons are attracted to the anode and
  an electron current flows in the tube from
  cathode to anode. The current can be used to
  operate a relay, which might turn a motor on to
  open a door or ring a bell in an alarm system
• .

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• The system can be made to be responsive to light,
  as described above, or sensitive to the removal
  of light as when a beam of light incident on the
  cathode is interrupted, causing the current to
  stop. Photocells are also useful as exposure
  meters for cameras in which case the current in
  the tube would be measured directly on a
  sensitive meter.
• The photocell is at the centre of the many
  applications of the photoelectric effect. It
  consists of a curved emitter and a rod as
  collector, so as not to inhibit light from
  reaching the emitter.

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• The structure of a typical photocell is shown
  below  The flash of a camera uses the
  photoelectric effect

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• Photocells are used in garage door openers. An
  example is shown in the diagram below

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• Spacecraft
• The photoelectric effect will cause spacecraft
  exposed to sunlight to develop a positive charge.
  This can get up to the tens of volts. This can be
  a major problem, as other parts of the spacecraft
  in shadow develop a negative charge (up to
  several kilovolts) from nearby plasma, and the
  imbalance can discharge through delicate
  electrical components. The static charge created
  by the photoelectric effect is self-limiting,
  though, because a more highly-charged object
  gives up its electrons less easily

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• Closely related to the photoelectric effect is
  the photoconductive effect which is the increase
  in electrical conductivity of certain non
  metallic materials such as cadmium sulfide when
  exposed to light. This effect can be quite large
  so that a very small current in a device suddenly
  becomes quite large when exposed to light. Thus
  photoconductive devices have many of the same
  uses as photocells.
• Solar cells, usually made from specially prepared
  silicon, act like a battery when exposed to
  light. Individual solar cells produce voltages of
  about 0.6 volts but higher voltages and large
  currents can be obtained by appropriately
  connecting many solar cells together.