chapter 28 a photoelectric effect when light (ultraviolet...
TRANSCRIPT
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CHAPTER 28 a
PHOTOELECTRIC EFFECT
When light (ultraviolet) falls on a metal
surface electrons are observed coming off
with various
velocities.
To study the effect we need an apparatus
like
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What we find is
The voltage necessary to stop all electrons
is called the stopping voltage, V0
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The photocurrent depends on the intensity
of the light.
The stopping voltage is the same for all
intensities of light.
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Change the frequency
The photocurrent is the same for all
frequencies.
The stopping voltage is more negative for
higher frequencies.
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Plot stopping potential times e for various
frequencies of light, f
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Cannot explain with electromagnetic wave.
Einstein Solution
1. Light made up of pulses or quanta of
energy called photons
2. Photons appear or disappear in units of 1
or more.
3. Each photon has energy ℎ�
Photon has energy ℎ�and gives this energy
to one electron in metal. Electron uses
certain amount of energy to escape. Left
over energy goes into kinetic energy
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Note: �� ≝ �
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Think: Conservation of Energy
Energy available from photon less energy to
remove electron from metal equals energy
of escaping electron.
ℎ� − � = � = 1
2��� = ���
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The electrons with the highest velocity will
have come from the top of the energy
levels in the metal – near the Fermi level.
Note in figure �� ≝ �
The energy needed to escape will be the
work function.
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Therefore
ℎ� − � = ���
Also:
��� = ℎ� − �
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Einstein theory explains all data.
Increase intensity – number of electrons
increases and current increases
Increase frequency – energy of photons is
greater and give more energy to electrons
and need larger ��.
No time delay – photon gives all of energy
to one electron.
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WHAT IS LIGHT
WAVE?
PARTICLE?