11.1 THE PHOTOELECTRIC EFFECT

11.1 THE PHOTOELECTRIC EFFECT

Setting the stage for modern physics

ObjectivesWWBATDescribe 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)

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

Results from the Photoelectric EffectWhat Scientists predictedWhat actually happenedIncreasing the intensity of light would increase the kinetic energy of emitted electronsElectrons would be emitted regardless of frequency as long as intensity was great enough

Results from the Photoelectric EffectWhat Scientists predictedWhat actually happenedIncreasing the intensity of light would increase the kinetic energy of emitted electronsIncreasing intensity of light only increased the NUMBER of electrons emitted, not energyElectrons would be emitted regardless of frequency as long as intensity was great enoughElectrons were emitted even at the lowest intensities, but the light had to be greater than a certain frequencyThe kinetic energy of the emitted electrons was proportional to the frequency of the incident light

Check yourselfGreen 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?

Check yourselfGreen 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

Check yourselfGreen 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?

Check yourselfGreen 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.

Check yourselfGreen 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?

Check yourselfGreen 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.

A New PostulateLight 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

Setting the stage for a new theoryLight 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

ObjectivesWWBATDescribe 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)

Enter Albert EinsteinIn 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

A Few DefinitionsWork 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 (Vs): Voltage an ejected electron must move through before being stopped

A Few Remindersh is Plancks constant = 6.63 x 10-34 Jsc is the speed of light in a vacuum = 3.0 x 108 m/se is the charge of an electron = 1.6 x 10-19 C

v = , for light, c =

KE = h

W = qV

A New Unit for Energy1 electronvolt (eV) = 1.6 x 10-19 J

Einsteins Equations = h0

KEmax = h

KEmax = work required to stop electron = eVs

Sample ProblemAn 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.

Sample ProblemAn 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:Vs = 4.0 V

= 2.2 eV

UNKNOWN: = ?

Sample ProblemAn 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:Vs = 4.0 V

= 2.2 eV x 1.6 x 10-19 = 3.52 x 10-19 J

UNKNOWN: = ?

Sample ProblemAn 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:Vs = 4.0 V

= 2.2 eV x 1.6 x 10-19 = 3.52 x 10-19 J

UNKNOWN: = ?KEmax = h

Sample ProblemAn 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:Vs = 4.0 VKEmax = eVs

= 2.2 eV x 1.6 x 10-19 = 3.52 x 10-19 J

UNKNOWN: = ?KEmax = h

Sample ProblemAn 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:Vs = 4.0 VKEmax = eVsKEmax = 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: = ?KEmax = h

Sample ProblemAn 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:Vs = 4.0 VKEmax = eVsKEmax = 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: = ?KEmax = h 6.4 x 10-19 = (6.63 x 10-34) 3.52 x 10-19

Sample ProblemAn 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 1015 Hz

Check YourselfAn 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.

Check YourselfAn photon with a wavelength of 200 nm is incident on a photoactivemetal 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 108 = (200 x 10-9) = 1.5 x 1015 Hz = 1.2 eV = 1.2 x 1.6 x 10-19 = 1.92 x 10-19 JKEmax = h = (6.63 x 10-34)(1.5 x 1015) 1.92 x 10-19 = 8.025 x 10-19 J

UNKNOWN:Vs = ?KEmax = eVs (8.025 x 10-19) = (1.6 x 10-19)VsVs = 5.02 V

ObjectivesWWBATDescribe 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)