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Slide 1
Slide 2
11.1 THE PHOTOELECTRIC EFFECT
Slide 3
Setting the stage for modern physics
Slide 4
Objectives WWBAT Describe the photoelectric effect Describe the
effect of changing light intensity or wavelength on the number and
energy of electrons emitted in the photoelectric effect Describe
the historical significance of the photoelectric effect on the
evolution of physical thought Calculate the kinetic energy, speed,
or stopping potential of an emitted electron, or the work function
of metal, or frequency or wavelength of an incident photon in the
photoelectric effect ( B Level )
Slide 5
The Photoelectric Effect Light shines on metal, light is
absorbed and electrons are emitted
Slide 6
Results from the Photoelectric Effect What Scientists
predictedWhat actually happened Increasing the intensity of light
would increase the kinetic energy of emitted electrons Electrons
would be emitted regardless of frequency as long as intensity was
great enough
Slide 7
Results from the Photoelectric Effect What Scientists
predictedWhat actually happened Increasing the intensity of light
would increase the kinetic energy of emitted electrons Increasing
intensity of light only increased the NUMBER of electrons emitted,
not energy Electrons would be emitted regardless of frequency as
long as intensity was great enough Electrons were emitted even at
the lowest intensities, but the light had to be greater than a
certain frequency The kinetic energy of the emitted electrons was
proportional to the frequency of the incident light
Slide 8
Check yourself Green light, when shone on a particular metal,
causes electrons to be released with little to no kinetic energy.
What would happen if the intensity of green light were
increased?
Slide 9
Check yourself Green light, when shone on a particular metal,
causes electrons to be released with little to no kinetic energy.
What would happen if the intensity of green light were increased?
More electrons would be released with the same amount of KE
Slide 10
Check yourself Green light, when shone on a particular metal,
causes electrons to be released with little to no kinetic energy.
What would happen if the intensity of green light were increased?
More electrons would be released with the same amount of KE What
would happen if red light was shone instead?
Slide 11
Check yourself Green light, when shone on a particular metal,
causes electrons to be released with little to no kinetic energy.
What would happen if the intensity of green light were increased?
More electrons would be released with the same amount of KE What
would happen if red light was shone instead? No electrons would be
emitted.
Slide 12
Check yourself Green light, when shone on a particular metal,
causes electrons to be released with little to no kinetic energy.
What would happen if the intensity of green light were increased?
More electrons would be released with the same amount of KE What
would happen if red light was shone instead? No electrons would be
emitted. What would happen if UV light was shone instead?
Slide 13
Check yourself Green light, when shone on a particular metal,
causes electrons to be released with little to no kinetic energy.
What would happen if the intensity of green light were increased?
More electrons would be released with the same amount of KE What
would happen if red light was shone instead? No electrons would be
emitted. What would happen if UV light was shone instead? Electrons
would be emitted with a greater kinetic energy.
Slide 14
A New Postulate Light behaves like a wave, but energy is
carried in discrete packets, like particles The amount of energy in
the packet depends on the frequency of light These packets,
representing the smallest discrete, measurable amount of
electromagnetic energy in light are called photons The smallest
measurable amount of any substance is called a quantum
Slide 15
Setting the stage for a new theory Light is not the only thing
that has a quantum and exhibits wave- particle duality Electrons
also exist as quanta and exhibit wave-particle duality This led to
the theory of Quantum Mechanics
Slide 16
Objectives WWBAT Describe the photoelectric effect Describe the
effect of changing light intensity or wavelength on the number and
energy of electrons emitted in the photoelectric effect Describe
the historical significance of the photoelectric effect on the
evolution of physical thought Calculate the kinetic energy, speed,
or stopping potential of an emitted electron, or the work function
of metal, or frequency or wavelength of an incident photon in the
photoelectric effect ( B Level )
Slide 17
Enter Albert Einstein In 1905, Einstein correctly,
mathematically described the photoelectric effect He won a Nobel
Prize in 1921 for his work All of this he discovered while working
on his theory of relativity, while working as an examiner at a
patent office
Slide 18
A Few Definitions Work Function ( ) : Minimum amount of energy
needed to eject an electron from an atom in metal Threshold
Frequency ( 0 ): Frequency of light that carries photons with the
amount of energy equal to the work function of a metal; will eject
an electron with zero kinetic energy Stopping Potential (V s ):
Voltage an ejected electron must move through before being
stopped
Slide 19
A Few Reminders h is Plancks constant = 6.63 x 10 -34 Js c is
the speed of light in a vacuum = 3.0 x 10 8 m/s e is the charge of
an electron = 1.6 x 10 -19 C v = , for light, c = KE = h W =
qV
Slide 20
A New Unit for Energy 1 electronvolt (eV) = 1.6 x 10 -19 J
Slide 21
Einsteins Equations = h 0 KE max = h KE max = work required to
stop electron = eV s
Slide 22
Sample Problem An electron with a maximum stopping potential of
4.0 V is ejected from a metal with a work function of 2.2 eV.
Determine the frequency of the incident wavelength that caused the
ejection of this electron.
Slide 23
Sample Problem An electron with a maximum stopping potential of
4.0 V is ejected from a metal with a work function of 2.2 eV.
Determine the frequency of the incident wavelength that caused the
ejection of this electron. KNOWN: V s = 4.0 V = 2.2 eV UNKNOWN: =
?
Slide 24
Sample Problem An electron with a maximum stopping potential of
4.0 V is ejected from a metal with a work function of 2.2 eV.
Determine the frequency of the incident wavelength that caused the
ejection of this electron. KNOWN: V s = 4.0 V = 2.2 eV x 1.6 x 10
-19 = 3.52 x 10 -19 J UNKNOWN: = ?
Slide 25
Sample Problem An electron with a maximum stopping potential of
4.0 V is ejected from a metal with a work function of 2.2 eV.
Determine the frequency of the incident wavelength that caused the
ejection of this electron. KNOWN: V s = 4.0 V = 2.2 eV x 1.6 x 10
-19 = 3.52 x 10 -19 J UNKNOWN: = ?KE max = h
Slide 26
Sample Problem An electron with a maximum stopping potential of
4.0 V is ejected from a metal with a work function of 2.2 eV.
Determine the frequency of the incident wavelength that caused the
ejection of this electron. KNOWN: V s = 4.0 VKE max = eV s = 2.2 eV
x 1.6 x 10 -19 = 3.52 x 10 -19 J UNKNOWN: = ?KE max = h
Slide 27
Sample Problem An electron with a maximum stopping potential of
4.0 V is ejected from a metal with a work function of 2.2 eV.
Determine the frequency of the incident wavelength that caused the
ejection of this electron. KNOWN: V s = 4.0 VKE max = eV s KE max =
1.6 x 10 -19 (4.0) = 6.4 x 10 -19 J = 2.2 eV x 1.6 x 10 -19 = 3.52
x 10 -19 J UNKNOWN: = ?KE max = h
Slide 28
Sample Problem An electron with a maximum stopping potential of
4.0 V is ejected from a metal with a work function of 2.2 eV.
Determine the frequency of the incident wavelength that caused the
ejection of this electron. KNOWN: V s = 4.0 VKE max = eV s KE max =
1.6 x 10 -19 (4.0) = 6.4 x 10 -19 J = 2.2 eV x 1.6 x 10 -19 = 3.52
x 10 -19 J UNKNOWN: = ?KE max = h 6.4 x 10 -19 = (6.63 x 10 -34 )
3.52 x 10 -19
Slide 29
Sample Problem An electron with a maximum stopping potential of
4.0 V is ejected from a metal with a work function of 2.2 eV.
Determine the frequency of the incident wavelength that caused the
ejection of this electron. 6.4 x 10 -19 = (6.63 x 10 -34 ) 3.52 x
10 -19 7.92 x 10 -19 = (6.63 x 10 -34 ) = 1.19 x 10 15 Hz
Slide 30
Check Yourself An photon with a wavelength of 200 nm is
incident on a photoactive metal with a work function of 1.2 eV.
Determine the maximum stopping potential of the ejected
electron.
Slide 31
Check Yourself An photon with a wavelength of 200 nm is
incident on a photoactive metal with a work function of 1.2 eV.
Determine the maximum stopping potential of the ejected electron.
KNOWN: = 200 nm = 200 x 10 -9 mc = 3.0 x 10 8 = (200 x 10 -9 ) =
1.5 x 10 15 Hz = 1.2 eV = 1.2 x 1.6 x 10 -19 = 1.92 x 10 -19 J KE
max = h = (6.63 x 10 -34 )(1.5 x 10 15 ) 1.92 x 10 -19 = 8.025 x 10
-19 J UNKNOWN: V s = ?KE max = eV s (8.025 x 10 -19 ) = (1.6 x 10
-19 )V s V s = 5.02 V
Slide 32
Objectives WWBAT Describe the photoelectric effect Describe the
effect of changing light intensity or wavelength on the number and
energy of electrons emitted in the photoelectric effect Describe
the historical significance of the photoelectric effect on the
evolution of physical thought Calculate the kinetic energy, speed,
or stopping potential of an emitted electron, or the work function
of metal, or frequency or wavelength of an incident photon in the
photoelectric effect ( B Level )