Transcript Photoelectric effect: Graphical Analysis

Q:
Describe the observations made of the
photoelectric effect and how this
supports a particle model of light.
Observations
1.
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4.
No photoelectrons are emitted if the frequency of
the incident light is below a certain value known as
the threshold frequency.
Photoelectric emission occurs at the instant that
light of a suitably high frequency is incident on the
metal surface.
The photoelectrons have a range of kinetic energies
from zero up to a maximum value
The number of photoelectrons emitted from the
metal surface per second is proportional to the
intensity of the incident radiation
Photoelectric effect
 Light hits a metal surface
 Each photon of light is absorbed by a single
electron
 If the energy of the photon is equal to or
greater than the work function of the metal,
the electron will be released
 Any extra energy will be carried away as the
kinetic energy of the electron
Work function and threshold
frequency
 Work function is the minimum amount of
energy required to release a photoelectron
from a metal.
Observation
Wave model
Even weak, lowintensity light can
release electrons
instantly
Weak light waves should
not provide enough
energy to do this
Increasing intensity of
light increases rate at
which electrons leave
metal
Greater intensity means
more energy, so more
electrons released
Increasing intensity has
no effect on kinetic
energies of electrons
Greater intensity should
mean electrons have
more energy
A minimum (threshold)
frequency of light is
needed
Low frequency light
should work – electrons
just released more
slowly
Increasing frequency of
light increases kinetic
energy of electrons
It should be increasing
intensity, not frequency,
that increases electron
energy
Particle model
Observation
Wave model
Particle model
Even weak, lowintensity light can
release electrons
instantly
Weak light waves should
not provide enough
energy to do this
Low intensity means fewer photons, but
each photon can still release an electron
Increasing intensity of
light increases rate at
which electrons leave
metal
Greater intensity means
more energy, so more
electrons released
Higher intensity  More photons per
second  More electrons per second
released
Increasing intensity has
no effect on kinetic
energies of electrons
Greater intensity should
mean electrons have
more energy
Higher intensity means more photons
per second but each one only has the
same energy
A minimum (threshold)
frequency of light is
needed
Low frequency light
should work – electrons
just released more
slowly
If each photon does not have enough
energy (the work function energy) then
no electrons can be released
Increasing frequency of
light increases kinetic
energy of electrons
It should be increasing
intensity, not frequency,
that increases electron
energy
Higher frequency  More energy per
photon  More kinetic energy per
electron
Match em up:
Maximum kinetic
energy of an emitted
photoelectron
Planck’s constant
Ek  hf  
Frequency of incident
radiation
Minimum energy
needed to emit
an electron
Measured in Joules
Measured in Hertz
Work function
The vacuum photocell – annotate your
diagram
The vacuum photocell
 Ek(max)=hf – f
 How can this be modified to take into account
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the work done by the electron against the
applied voltage?
Ek(max)= hf – f  eV
And at the stopping potential Vs…
Ek(max)= hf – f  eVs = 0
eVs = hf - f
Rearrange to Vs= and compare to a straight
line graph
Plot a graph
Colour
l /nm
Vs /V
Yellow
578
0.66
Green
546
0.76
Blue
436
1.35
Violet
405
1.53