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Quantum Phenomena 1 Photoelectricity

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The photoelectric effect is the emission of electrons from the surface of a ... on a gold leaf electroscope, the electroscope's gold leaf would be initially deflected. ... – PowerPoint PPT presentation

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Title: Quantum Phenomena 1 Photoelectricity


1
Quantum Phenomena 1Photoelectricity More
about photoelectricity
  • Unit 1.2a1
  • Breithaupt chapters 3.1 3.2
  • pages 30 to 33

2
AS Specification
  • Work function f,
  • Photoelectric equation hf f Ek
  • The stopping potential experiment is not required
  • Breithaupt chapters 3.1 3.2 pages 30 to 33

3
The photoelectric effect
  • The photoelectric effect is the emission of
    electrons from the surface of a material due to
    the exposure of the material to electromagnetic
    radiation.
  • For example zinc emits electrons when exposed to
    ultraviolet radiation.
  • If the zinc was initially negatively charged and
    placed on a gold leaf electroscope, the
    electroscopes gold leaf would be initially
    deflected.
  • However, when exposed to the uv radiation the
    zinc loses electrons and therefore negative
    charge.
  • This causes the gold-leaf to fall.

Phet Photoelectric effect NTNU Photoelectric
effect
4
Experimental observations
  • Threshold frequency
  • The photoelectric effect only occurs if the
    frequency of the electromagnetic radiation is
    above a certain threshold value, f0
  • Variation of threshold frequency
  • The threshold frequency varied with different
    materials.
  • Affect of radiation intensity
  • The greater the intensity the greater the number
    of electrons emitted, but only if the radiation
    was above the threshold frequency.
  • Time of emission
  • Electrons were emitted as soon as the material
    was exposed.
  • Maximum kinetic energy of photoelectrons
  • This depends only on the frequency of the
    electromagnetic radiation and the material
    exposed, not on its intensity.

5
Problems with the wave theory
  • Up to the time the photoelectric effect was first
    investigated it was believed that electromagnetic
    radiation behaved like normal waves.
  • The wave theory could not be used to explain the
    observations of the photoelectric effect in
    particular wave theory predicted
  • that there would not be any threshold frequency
    all frequencies of radiation should eventually
    cause electron emission
  • that increasing intensity would increase the rate
    of emission at all frequencies not just those
    above a certain minimum frequency
  • that emission would not take place immediately
    upon exposure the weaker radiations would take
    longer to produce electrons.

6
Einsteins explanation
  • Electromagnetic radiation consisted of packets or
    quanta of energy called photons
  • The energy of these photons
  • depended on the frequency of the radiation only
  • was proportional to this frequency
  • Photons interact one-to-one with electrons in the
    material
  • If the photon energy was above a certain minimum
    amount (depending on the material)
  • the electron was emitted
  • any excess energy was available for electron
    kinetic energy
  • Einstein won his only Nobel Prize in 1921 for
    this explanation. This explanation also began the
    field of Physics called Quantum Theory, an
    attempt to explain the behaviour of very small
    (sub-atomic) particles.

7
Photon energy (revision)
  • photon energy (E) h x f
  • where h the Planck constant 6.63 x 10-34 Js
  • also as f c / ? E hc / ?
  • Question Calculate the energy of a photon of
    ultraviolet light (f 9.0 x 1014 Hz)
  • E h f
  • (6.63 x 10-34 Js) x (9.0 x 1014 Hz) 5.37
    x 10-19 J

8
The photoelectric equation
  • EKmax hf f
  • where
  • EKmax maximum kinetic energy of the
    photoelectrons
  • hf energy of the photons of electromagnetic
    radiation
  • f work function of the exposed material
  • Work function, f
  • This is the minimum energy required for an
    electron to escape from the surface of a material

9
Threshold frequency f0
  • As EKmax hf f
  • If the incoming photons are of the threshold
    frequency f0, the electrons will have the minimum
    energy for emission
  • and EKmax will be zero
  • therefore 0 hf0 f
  • and so f0 f / h

10
Question 1
  • Calculate the threshold frequency of a metal if
    the metals work function is 1.2 x 10 -19 J.
  • f0 f / h
  • (1.2 x 10-19 J) / (6.63 x 10-34 Js)
  • threshold frequency 1.81 x 1014 Hz

11
Question 2
  • Calculate the maximum kinetic energy of the
    photoelectrons emitted from a metal of work
    function 1.5 x 10 -19 J when exposed with photons
    of frequency 3.0 x 1014 Hz.
  • EKmax hf f
  • (6.63 x 10-34 Js) x (3.0 x 1014 Hz) - (1.5 x
    10-19 J)
  • 1.989 x 10-19 - 1.5 x 10-19
  • 0.489 x 10-19 J
  • maximum kinetic energy 4.89 x 10 - 20 J

12
Question 3
  • A metal emits photoelectrons with a maximum
    kinetic energy of 1.0 x 10-19 J when exposed with
    photons of wavelength 200 nm. Calculate the work
    function and threshold frequency of the metal.
  • EKmax hf f
  • becomes EKmax hc / ? f
  • the term hc / ? (6.63 x 10-34 Js) x (3.0 x 108
    ms-1) / (2.0 x 10-7 m)
  • 9.945 x 10-19 J
  • EKmax hc / ? f
  • becomes (1.0 x 10-19) (9.945 x 10-19) - f
  • and so f (9.945 x 10-19) (1.0 x 10-19)
  • work function 8.95 x 10-19 J
  • f0 f / h
  • 8.95 x 10-19 J / 6.63 x 10-34 Js
  • threshold frequency 1.35 x 1015 Hz

13
The vacuum photocell
  • Light is incident on a metal plate called the
    photocathode.
  • If the lights frequency is above the metals
    threshold frequency electrons are emitted .
  • These electrons passing across the vacuum to the
    anode constitute and electric currrent which can
    be measured by the microammeter.

The photocell is an application of the
photoelectric effect
Phet Photoelectric effect NTNU Photoelectric
effect
14
Obtaining Plancks constant
  • By attaching a variable voltage power supply it
    is possible to measure the maximum kinetic energy
    of the photoelectrons produced in the photocell.
  • The graph opposite shows how this energy varies
    with photon frequency
  • EKmax hf f
  • has the form y mx c
  • with gradient, m h
  • Hence Plancks constant can be found.

15
Notes from Breithaupt pages 30 31
  • What is the photoelectric effect?
  • Explain how the observations made from
    photoelectric experiments contradict the wave
    theory of electromagnetic radiation.
  • Show how the photoelectric equation follows from
    Einsteins explanation of the photoelectric
    effect.
  • Define (a) threshold frequency (b) work
    function. Give the relationship between these two
    quantities.
  • A metal emits photoelectrons with a maximum
    kinetic energy of 2.0 x 10-19 J when exposed with
    photons of wavelength 300 nm. Calculate the work
    function and threshold frequency of the metal.
  • Try the summary questions on page 31

16
Notes from Breithaupt pages 32 33
  • Explain why Einsteins photon model was
    revolutionary.
  • What is a quantum?
  • Draw a diagram and explain the operation of a
    vacuum photocell.
  • Describe how the value of Plancks constant can
    be found from measurements made with a photocell.
  • Try the summary questions on page 33

17
Answers to the summary questions on page 31
18
Answers to the summary questions on page 33
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