Transcript Energy

The photon
• A “particle” of light
• A “quantum” of light energy
• The energy of a given photon
depends on the frequency
(color) of the light
But light is also a wave!
• Travels at constant speed c
in a vacuum.
• c = lf
8
–c: 3 x 10 m/s
– l: wavelength (m)
– f: frequency (Hz)
Calculating photon energy
•E = hf
–E: energy (J or eV)
–h: Planck’s constant
• 6.62510-34 J s or 4.14 10-15 eV s
–f: frequency of light (s-1, Hz)
The “electron-volt” (eV)
is an energy unit
• Useful on the atomic level.
• If a moving electron is
stopped by 1 V of electric
potential, we say it has 1
electron-volt (or 1 eV) of
kinetic energy!
Converting eV to Joules (J)
1 eV =
-19
1.60210 J
Photoelectric Effect experiment
light
Collector (-)
At a certain voltage,
PhotoVs, the current can’t
Diode e- e- e- e- e- eflow anymore!
(+)
e-
e-
e-
e-
V
e-
ee-
e-
e-
Ae-
e-
Anomalous Behavior in
Photoelectric Effect
• Voltage necessary to stop electrons is
independent of intensity (brightness) of
light.
• Photoelectrons are not released below a
certain frequency, regardless of intensity
of light.
• The release of photoelectrons is
instantaneous, even in very feeble light,
provided the frequency is above the
cutoff.
Voltage current for different
intensities of light.
i
I3 > I2 > I1
I3
I2
I1
Vs
Stopping potential is
unaffected!
V
Voltage versus current for different
frequencies of light.
i
f3 > f2 > f1
f3 f2 f1
V
Vs,3 Vs,2 Vs,1 Stopping potential becomes
more negative at higher
frequencies!
Photoelectric Effect
• Ephoton = Kmax + Wo
–Ephoton = hf (Planck’s equation)
–Kmax: maximum kinetic energy of electrons
–Wo: binding energy or “work function”
• hf = Kmax + Wo
hf = Kmax+ Wo
Kmax = hf - Wo
y
= mx + b
Graph of Photoelectric
Equation
Kmax
slope = h
(Planck’s
Constant)
Cut-off frequency
Wo (binding energy)
f
Absorption Spectrum
Photon is
absorbed
and excites
atom to
higher
quantum
energy
state.
0 eV
DE
hf
Ground state
-10 eV
Absorption Spectrum
Absorption
spectra
always
involve
atoms going
up in
energy
level.
ionized
0 eV
-10 eV
Emission Spectrum
Photon is
emitted
and atom
drops to
lower
quantum
energy
state.
Excited state
0 eV
DE
hf
-10 eV
Emission Spectrum
Emission
spectra
always
involve
atoms going
down in
energy
level.
ionized
0 eV
-10 eV
A typical nucleus
12
Atomic mass:
protons plus neutrons
C
6
Element
name
Atomic number:
protons
Isotope
characteristics differ
238
235
92
92
U
U
Binding energy
• Energy released when a
nucleus is formed from
protons and neutrons.
• Mass is lost.
2
• E = mc
–where m is the lost mass
Nuclear Particles
1
p
1
1
n
0
• Nucleons
– Proton
• Charge: +e
• Mass: 1 amu
– Neutron
• Charge: 0
• Mass: 1 amu
Nuclear reactions
• Nuclear Decay
–Alpha decay
–Beta decay
• Beta Minus
• Positron
• Fission
• Fusion
Decay Particles
4
He •Alpha
2
0
e
-1
0
e
1
•Beta
•Positron
Alpha Decay
• Occurs only with very heavy
elements.
• Nucleus too large to be stable.
226
Ra
88
222
4
86
2
Rn
He
Beta Decay
• Occurs with elements that have
too many neutrons for the
nucleus to be stable.
40
40
19
20
K
Ca
0
e
-1
n
antineutrino
Positron Decay
• Occurs with elements that have
too many protons for the nucleus
to be stable.
2
2
2
1
He
H
0
e
1
n
neutrino
Neutrino and Anti-Neutrino
• Proposed to make beta and positron decay
obey conservation of energy.
• No mass, no charge.
• Energy and spin.
• Does not react easily with matter.
• Hard to detect.
Gamma Radiation, g
• Released by atoms which have
undergone a nuclear reaction.
• Results when excited nuclei
return to ground state.
• High energy! E = hf!
Fission
• Occurs only with very heavy
elements.
• Nucleus too large to be stable.
• Induced by neutrons.
239
1
92
144
94
0
38
56
Pu n
Sr
Ba
4
1
n
0
Fusion
• The largest amount of energy available.
• Energy produced in the sun.
• Fusion of light elements results in nonradioactive waste.
1
1
2
1
1
2
H
H
He
Summary of Wave-Particle Duality
Waves are particles and particles are
waves
Energy
• Particle
–E = K + U
• Photon
–E = hf
Momentum
• Particle
–p = mv
• Photon
–p = h/l
Wavelength
• Photon
– l = c/f
• Particle
– l = h/p
– deBroglie wavelength
Compton Scattering
• Proof of the momentum of
photons.
• High-energy photons collided with
electrons.
• Conservation of momentum.
• Scattered photons examined to
determine loss of momentum.
Davisson-Germer
Experiement
• Verified that electrons
have wave properties by
proving that they diffract.
• Electron diffraction