Transcript Photoelectric Effect

Scientific Models
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Models
“Scientific models are created to explain observations.
Good models provide clear explanations for all known
data and make predictions for new observations.
New observations either support and strengthen the model
or refute it. Models that fail to explain an observation
are wrong and must be modified or replaced with better
models—the old model might still be useful in a limited
way but ultimately it has failed and must be replaced.”
Modeling in Sci 10: Black boxes, jumping shampoo, and through Alice and Bob videos.
Photoelectric Effect
Background Info
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PEE
The photoelectric effect is an important focus in the Grade 12 Modern Unit.
This unit is worth 12% of the total grade.
More importantly, the photoelectric effect helped us
realize the wave model of light does not account for all
the behavior of light.
PEE – Where Does it Fit?
I start the Modern Unit with the investigation we’ll do today.
From this investigation, students realize
a) The wave model of light cannot account for
all the behavior of light
b) Light can be considered as a “packet of energy”
We then look at the atomic spectra from the discharge tubes.
This reinforces that light is a packet of energy. ΔE = hf
Modern Outline
PEE – Where Does it Fit?
Since light can be modeled as a packet of energy, we might be able to model it as a particle.
The Compton Effect shows that light scatters
off of electrons and the electrons have
momentum after the interaction with light.
So light must have momentum.
Using the an investigation into special
relativity, we see that E = mc2.
Knowing that “waves” can act like “particles”
might mean that “particles” can act like “waves”.
This is the de Broglie wavelengths.
This leads to the conclusion of wave-particle duality.
Modern Outline
PEE
The data for the photoelectric effect uses some of the relationships discussed
in the Electromagnetism unit.
Namely, that a charged object will experience a force when placed in an electric field.
The electric field is uniform when it exists between two parallel plates.
PEE
The electric field exists because the plates are charged. This also means that
the plates are at different potentials.
The potential difference between the
plates, ΔV, is the difference across the
battery.
When a negatively charged object travels from
the negative plate to the positive plate, it loses
potential energy and gains kinetic energy.
Using the Conservation of
Energy
The electric field exists because the plates are charged. This also means that
the plates are at different potentials.
The potential difference between the
plates, ΔV, is the difference across the
battery.
Or, if the negatively charged object is
traveling through a space in the positive plate
and moves toward the negative plate, it will
slow down.
This is just like a ball being thrown into the air. It goes from a point of
low potential to high potential and thus will lose kinetic energy.
PEE
We will use this transferring of energy from one from to another in
the collection of data for the photoelectric effect.
The potential energy of a negative charge at the negative plate is:
EQ = ΔV*q
The charge on the object (C)
The charge on an electron is 1.6 x 10-19 C
(Electric) Potential Energy (J)
Now we’re ready to start!
Investigating the PEE
Answers
1a) List three properties of waves.
Examples:
1) Interfere with other waves as they pass
through each other.
2) Diffract around objects.
3) Can cause resonance.
4) Can create standing waves
5) Reflect/Refract
1b) Light is how nature transfers energy
without transferring mass. List three
experiments (or observations) that
demonstrate light is a wave.
1) Young’s double slit
2) Thin film interference
3) Microwaves ovens using microwaves to
resonate the water molecules in food. Standing
waves are set up in the microwave.
Investigating the PEE
Answers
3) The light is incident on a metal plate. Even though there
is not a completed circuit, current flows. Make a hypothesis
about what is happening. Some things to consider are: what
is electricity? This effect does not happen when light hits an
insulator, like wood. What happens to the light after it hits
the metal? Discuss these questions with a partner.
Electricity is the flow of moving charges. The charges in this
case are probably electrons since the protons in the metal are
more or less stationary. The metal has a “sea of electrons”. The
light could hit the metal and the electrons are ejected.
Investigating the PEE
Answers
Investigating the PEE
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Investigating the PEE
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7) How does this amount of potential energy
compare to the electron’s initial kinetic
6) We can find the energy of the emitted
energy?
electrons, or photoelectrons. If we make the This potential energy would be the initial
collecting plate negative enough, the
kinetic energy of the electrons.
photoelectrons will be repelled back to the
metal. To do so, we create a potential
8) Predict what will happen to the stopping
difference between the plates. What is the
potential energy of a photoelectron if it just potential when we increase the intensity of
the wave. Remember, the intensity of a wave
stops at the negative plate, after traveling
is proportional to the energy of the wave.
though a potential difference of ΔV?
EQ = ΔVq
An increase in intensity means the wave is more
energetic. Therefore more energy is transferred
to the electrons. The electrons have more energy
and so a higher stopping potential is required to
stop them.
Investigating the PEE
Answers
9) How does the
stopping potential
depend on the intensity?
It doesn’t depend on the
intensity.
10) What can we conclude about the kinetic
energy of the photoelectrons? Justify with
reference to your answers in 6-9.
The kinetic energy of the electrons can be
measured by using the stopping potential. This
kinetic energy is not dependent on the intensity.
11) In this section, we are trying to discover why increasing the
intensity of the light leads to a higher current reading. In question 5,
you hypothesized about the increased current being due more
energetic electrons. If the increased current is not due to more
energetic electrons, what is causes it?
Increasing the intensity does not increase the energy of the electrons. It
simply means more electrons must be emitted from the metal, which
explains the increased current.