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11.1 – THE PHOTOELECTRIC EFFECT

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Setting the stage for modern physics…

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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 )

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The Photoelectric Effect Light shines on metal, light is absorbed and electrons are emitted

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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

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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

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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?

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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

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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?

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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.

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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?

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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.

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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

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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

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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 )

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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

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A Few Definitions… Work Function (φ ) : Minimum amount of energy needed to eject an electron from an atom in metal Threshold Frequency (ƒ 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

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A Few Reminders… h is Planck’s constant = 6.63 x 10 -34 J·s 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 = ƒλ, for light, c =ƒλ KE = hƒ W = qΔV

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A New Unit for Energy 1 electronvolt (eV) = 1.6 x 10 -19 J

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Einstein’s Equations φ = hƒ 0 KE max = hƒ – φ KE max = work required to stop electron = eV s

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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.

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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 φ = 2.2 eV UNKNOWN: ƒ = ?

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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 φ = 2.2 eV x 1.6 x 10 -19 = 3.52 x 10 -19 J UNKNOWN: ƒ = ?

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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 φ = 2.2 eV x 1.6 x 10 -19 = 3.52 x 10 -19 J UNKNOWN: ƒ = ?KE max = hƒ – φ

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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 φ = 2.2 eV x 1.6 x 10 -19 = 3.52 x 10 -19 J UNKNOWN: ƒ = ?KE max = hƒ – φ

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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 φ = 2.2 eV x 1.6 x 10 -19 = 3.52 x 10 -19 J UNKNOWN: ƒ = ?KE max = hƒ – φ

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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 φ = 2.2 eV x 1.6 x 10 -19 = 3.52 x 10 -19 J UNKNOWN: ƒ = ?KE max = hƒ – φ 6.4 x 10 -19 = (6.63 x 10 -34 )ƒ – 3.52 x 10 -19

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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 )ƒ – 3.52 x 10 -19 7.92 x 10 -19 = (6.63 x 10 -34 )ƒ ƒ = 1.19 x 10 15 Hz

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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.

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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: λ = 200 nm = 200 x 10 -9 mc = ƒλ3.0 x 10 8 = ƒ(200 x 10 -9 )ƒ = 1.5 x 10 15 Hz φ = 1.2 eV = 1.2 x 1.6 x 10 -19 = 1.92 x 10 -19 J KE max = hƒ – φ = (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

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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 )

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