Activity Photoelectric Effect Physics Electromagnetic Spectrum n c l Practical applications of the photoelectric effect solar cells smoke detectors security systems Many metals emit electrons when a light shines on their ID: 662129
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Slide1
CH 221 Computer Simulation Activity
Photoelectric EffectPhysicsElectromagnetic Spectrumn = c/lSlide2
Practical applications of the photoelectric effectsolar cellssmoke detectorssecurity systemsSlide3
Many metals emit electrons when a light shines on their surface. The emitted electrons are called photoelectrons.Slide4
Photoelectric EffectLight
ElectronsMetal’s electrons near the surfaceSlide5
Photoelectric effect: Many metals emit electrons when a light shines on their surface.
http://www.youtube.com/watch?v=jAt4Liq3bgcWhat to look for during the experiment if light has wave behavior:1. when the wavelength of light is made shorter, more electrons should be ejected. Why?2. when the light wave’s intensity made brighter, more electrons should be ejected. Why?3. when the light is dim, there should be a lag time in the ejection of electrons. Why?Slide6
Photoelectric Effect
In order to eject an electron, the force of attraction (work function or binding energy) between the protons and the electron must be overcome: Coulomb’s Law.Slide7
Coulomb’s Force Law
A law of physics describing the electrostatic interaction between charged particles.Force of attraction between a proton and an electron. Slide8
Two unlike charges are separated by a distance, which set of charges experiences a greater force of attraction?
A.B.Slide9
Two unlike charges are separated by a distance, which set of charges experiences a greater force of attraction?
A.B.Slide10
The Photoelectric EffectClassic wave theory attributed this effect to all of the energy of light E = hv
being transferred to an outer electron in the metal.According to this theory, if the wavelength of light hitting the metal is shorter, or the light wave’s intensity made brighter, more electrons should be ejected.the energy of a wave is directly proportional to its amplitude and its frequency. The more energy light has, the more energy it should have to knock off electrons.Wave theory also predicts if a dim light were used there would be a lag time
before electrons were
emitted. Dim light as waves were predicted to eject electrons if enough time was given for the electrons
to absorb enough
energy to break away from the force of attraction of the protons in the nucleus.Slide11
E = nhν
An atom has only certain quantities of energy… the energy is quantized.The smallest energy change is when Δn = 1n = 1n = 2
n
= 3
n
= 4
n
= 5
ΔE =
Δnh
ν
ΔE =
h
ν
a quantum of energySlide12
Electrons exist in discrete energy levelsSlide13
Classical View Compare to Quantum Mechanical View
Classical ViewQuantum Mechanical ViewAny energy is possibleOnly a few energy states are allowedSlide14
Energy and Frequency
A solid object emits visible light when it is heated to about 1000 K and above. This is called blackbody radiation.Smoldering coal
Lightbulb
filament
Electric heating element
The
color
(and the
intensity)
of the light depend only on the temperature and not the identity of the substance.
What is the source of this light and what is the relationship between the frequency of the radiation (i.e., the color of the light) and the temperature of the body?Slide15
Historically, light was thought of as a stream of particles until Young’s experiments proved light has wave-like properties.Planck was working with the wave notion of light when he related the energy of blackbody radiation to the frequency of emitted light. E = hvEinstein began to consider the particle viewpoint again when trying to explain the photoelectric effect.
Max Planck and Albert EinsteinBerlin, June 1929Slide16Slide17
Photoelectric Effect ExperimentWork the Questions of the Activity Worksheet posted on the Canvas web site
Download and work the computer simulationhttp://phet.colorado.edu/en/simulations/category/physicsSlide18
18
Electrons
Test metal
Photoelectric effect experiment apparatus.
if
light
follows
classical
wave theory, we predict
that
any incoming light will put
energy into
the surface e-s
Also
is should take
time
for the surface e-s to be ejected.
When the light
on longer,
more
e
’
s
should be ejected
Any color
light
(wavelength) should eject e-s,
only
an increase in the intensity of light should increase the KE of the e-s.Slide19
19
I
e
’
s
First
experiment
Keep the intensity of light constant
Start with IR or Red light, increase the frequency of light until electrons are ejected.
Note the threshold frequency (and wavelength)
Observe what happens, write
down what happens
Select sodium metalSlide20
20
I
e
’
s
First experiment Part 2
Select two different metals
Design an experiment to determine the threshold frequency of each metal.
What variables will you keep constant?
Observe what happens, write
down what happensSlide21
21
I
e
’
s
Second experiment
Select a frequency of light below the threshold frequency.
Increase the intensity of light
Select a frequency of light
above
the threshold frequency.
Increase the intensity of light
Make note of the speed and number of ejected electrons.
Observe what happens, write
down what happens
Select sodium metalSlide22
22
I
e
’
s
HIGH intensity
LOW intensity
Observation: When we dim the light,
fewer
electrons
are ejected from the metal.
Choose a
frequency
above
the threshold frequency,.
Keep the frequency and wavelength of the light the same,
vary the intensitySlide23
23
I
e
’
s
Third experiment
Work with the two other different metals from Experiment 2
- Determine the threshold frequency of each metal
Observe what happens, write
down what happensSlide24
24
Select one of your metals.
Predict
what happens to the initial KE of the electrons as the
frequency
of light changes? (Light intensity is constant)
Predict shape of the graph
I
e
’
s
0 Frequency of light
Initial KE
look at sim for few different
colors, small forward VSlide25
25
I
e
’
s
Third experiment
Work with the two other different metals from Experiment 2
- Determine the threshold frequency of each metal
Determine if there is a relationship between the # of e-s ejected and the intensity of light
Determine if there is a relationship between the KE of the ejected electrons and the frequency of light.
Observe what happens, write
down what happensSlide26
26
0 Frequency
Initial KE
0 Frequency
Initial KE
0 Frequency
Initial KE
0 Frequency
Initial KE
A
B
C
D
E. something different
What is the relationship between KE and frequency?Slide27
27
I
e
’
s
0 Frequency of light
Initial KE
As the
frequency
of light increases (shorter
l!
), the KE of electrons being popped off increases.
(it is a linear relationship)
There is a minimum frequency below which the light cannot kick out electrons…
even if wait a long time
Correct answer is D.
do sim showing graph
what happens if change metal?
GOTO the simulation
do the experimentSlide28
Threshold Frequency (Energy)Slide29
Each Metal Has a Different Threshold Frequency (Energy)Slide30
Which Metal Requires the Greatest (Minimum) Energy to Begin to Eject Electrons? What does this mean with respect to high tightly electrons are bound to an atom?Slide31
Photoelectric EffectLight
ElectronsMetal’s electrons near the surfacePhoto-The theory of the photoelectric effect must explain the experimental observations of the emission of electrons from an illuminated metal surface. For a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons are emitted.Slide32
32
Summary of Photoelectric experiment results.
(play with
sim
to check and thoroughly understand)
1.
# of e
-
s ejected
linearly proportional to
intensity of light.
2. # of e
-
s
ejected appears
with no delay.
3. Electrons only emitted if frequency of light exceeds
a threshold. (same as
“
if wavelength short enough
”
).
4. Maximum energy that electrons come off with
increases linearly with frequency (=1/wavelength).
5. Threshold frequency depends on
the type
of
metal - - which is an indication of how tightly the surface e
-
s are held.
how do these compare with classical wave predictions?
http://phet.colorado.edu/new/simulations/sims.php?sim=Photoelectric_EffectSlide33Slide34
34
Classical wave predictions vs.
experimental observations
Increase intensity, increase
# of electrons.
experiment matches
Wavelength of
light does not matter, only intensity.
experiment shows strong dependence on
wavelength
Takes time to heat up
⇒
# of e-s coming off start low
and
should increase with time (energy added).
experiment:
with the threshold frequency electrons
come
off the surface
immediately, no time delay to heat upSlide35
35
Summary
If
light can kick out
electrons,
then even smallest intensities of that light will continue to kick out electrons. KE of electrons does not depend on
the intensity of the light of a specific wave length.
(Light energy must be getting concentrated/focused somehow)
2.
At lower frequencies, initial
KE of the ejected e-s is low.
KE
of
the ejected e
-s changes
linearly
with the frequency of the incoming light.
(This concentrated energy is linearly related to frequency)
3.
There is a minimum
frequency below which light won
’
t kick out electrons.
(Need a certain amount of energy to free electron from metal)
(Einstein)
Need
“
photon
”
picture of light to explain observations:
- Light comes in chunks (
“
particle-like
”
) of energy (
“
photon
”
)
- a photon interacts only with single electron
- Photon energy depends on frequency of light, …
for lower frequencies, photon energy not enough to free an electronSlide36
Einstein’s Model of Light: Photon TorpedoesLight can be represented as separate, discontinuous quanta called photons. Light energy comes in packets
.Each photon has an energy of E = hvLight interacts with matter as a stream of particle-like photons. Light travels as a wave. Light is complex – it has a wave-particle duality behavior.Slide37
Photoelectric EffectEnergy In > Energy Out What absorbed the missing energy?
ThreeThreeEnergy InEnergy OutMetal SurfaceSlide38
Photoelectric Effect Experiment: Sodium metal has a threshold wavelength of 232 nm. Calculate the frequency and the energy of the incoming light.Slide39
Photoelectric Effect Experiment: Sodium metal has a threshold wavelength of 232 nm. Calculate the frequency and the energy of the incoming light.Slide40
Einstein proposed that light consists of particles, or quanta, of energy called photons.Energy of one photon =
hν Energy of n photons = E = nhν Photons hit electrons on the surface of a metal (like billiard balls).Photon energy is transferred completely to the electron.The electron must spend some energy breaking away. This is the “binding energy” or “work function”, Φ.Excess energy remains in the emitted electron as kinetic energy.
or,
E
incoming
photon
=
Φ
+
KE
ejected
photoelectron
=
h
ν
Light below a minimum “threshold frequency” will not cause the photoelectric effect.Slide41
Photoelectric EffectLight
ElectronsMetal’s electrons near the surfacePhoto-Energy In = hvEnergy Out= Kinetic Energy
Binding Energy
Photoelectron Energy = Light Energy In – Binding Energy
KE
photoelectron
=
h
ν
–
Φ
Slide42
Photoelectric Effect: Calculate Binding EnergyLight
ElectronsPhoto-If Energy In = 25 = E = hv If Energy OutKE = 5
Binding Energy
Photoelectron Energy = Light Energy In – Binding Energy
5
=
25
–
Φ
Φ
= 25 – 5 = 20
Slide43
Photoelectric EffectElectrons
Photo-Energy In = hv h v = h x 1.09 x 1015 cyc/sec Binding Energy = minimum energy needed to eject an electron from the metal
surface
Molybdenum metal must absorb radiation with a minimum frequency of 1.09 x 10
15
cyc
/sec before it can eject an electron from its surface. Calculate the energy (units of J) to eject an electron.Slide44
Photoelectric EffectElectrons
Photo-Energy In = hv v = 1.09 x 1015 cyc/sec Molybdenum metal must absorb radiation with a minimum frequency of 1.09 x 1015 cyc/sec before it can eject an electron from its surface. What is the wavelength, l ,
of this radiation? What type of radiation is it?
l =
c
/
v
Binding EnergySlide45
Photoelectric EffectElectrons
Photo-Energy In = hv l = 120.0 nmIf Energy OutKE = ?Binding Energy
Photoelectron Energy = Light Energy In – Binding Energy
?
=
hv
–
Φ
If Molybdenum is irradiated with EMR of wavelength 120.0 nm, what is the maximum possible kinetic energy of the emitted photoelectrons?
v
=
c
/lSlide46Slide47
47
Photoelectric effect experiment: Apply Conservation of Energy
Inside
metal
Electron Potential
Energy
work function (
) =
energy needed to kick
highest electron out of metal
Energy in = Energy out
Energy of photon = energy needed to kick + Initial KE of electron
electron
out of metal
as
exits
metal
Loosely stuck electron, takes least energy to kick out
Tightly stuck, needs more energy to escape
Outside
metalSlide48
48
Typical energies
Each photon has: Energy = Planks constant * Frequency
(Energy in Joules)
(Energy in eV)
E=hf=(6.626*10
-34
J-s)*(f s
-1
)
E=hf= (4.14*10
-15
eV-s)*(f s
-1
)
E=hc/
l
= (1.99*10
-25
J-m)/(
l
m)
E= hc/
l = (
1240 eV-nm)/(
l
nm)
Photon Energies:
Work functions of metals (in eV):
Aluminum
4.08 eV
Cesium
2.1
Lead
4.14
Potassium
2.3
Beryllium
5.0 eV
Cobalt
5.0
Magnesium
3.68
Platinum
6.35
Cadmium
4.07 eV
Copper
4.7
Mercury
4.5
Selenium
5.11
Calcium
2.9
Gold
5.1
Nickel
5.01
Silver
4.73
Carbon
4.81
Iron
4.5
Niobium
4.3
Sodium
2.28
Uranium
3.6
Zinc
4.3
Red Photon: 650 nm
E
photon
=
1240 eV-nm
= 1.91 eV
650 nm