Transcript File - The Physics Doctor

A –Level Physics:
Waves and Quanta:
Polarisation and the Photoelectric Effect
Objectives:
70. be able to use the equation intensity of radiation
92. understand that the absorption of a photon can result in the
emission of a photoelectron
93. understand the terms threshold frequency and work function
and be able to use the equation
94. be able to use the electronvolt (eV) to express small energies
95. understand how the photoelectric effect provides evidence for
the particle nature of electromagnetic radiation.
Additional skills gained:
• Practical Planning
• Integrating GCSE content
Starter: Explanations
Using standing waves as examples; explain
constructive and destructive interference
(6 marks)
You
have
10mins
Polarisation
What does the word ‘polarising’ mean?
“restricting the vibrations of a transverse wave
wholly or partially to a specific direction”
Draw what you
believe an EM
wave looks like
What would it
look
Challenge
like ‘side
on’?
When waves come from a luminous source the waves will
reach us in various directions so can be considered to look
like ‘E’ shown on the diagram below
Copy the diagram down and then explain what is
happening. Use the words ‘restricting’ and ‘polarisation’
What commonly used object is made to have this
function and why?
Have a try!
Grab one of the
filters from the
box and put it up
to see through it
to the window.
1) Turn it, does
anything change?
2) Put a second one
behind it and
slowly turn it,
what happens?
3) Why is this
happening?
Exam Practice: 1
Time for a history lesson
Grab one of the
text books and
turn to page
274. Read with
us through the
history of wave
particle duality
10m
Gold-Leaf Experiment
Draw this set-up in your notes and then we will add the
charges!
Gold-Leaf Experiment
1) In this experiment, the metal
cap (and as such, the stem
and gold leaf) were given a
negative charge
2) This caused the gold leaf to
______ because
_________________
3) Different colours of light were
then shone onto the metal
cap
4) Red light had no effect, but
ultraviolet caused the gold
leaf to go flat again
Why is it important that the
gold leaf be situated in a
vacuum?
Why did ultraviolet have
this effect? [2 marks]
Photoelectric Effect
“the emission or release of electrons from a negatively
charged metal when exposed to high frequency radiation”
The energy of the incoming light,
E = hf
Where h= Plank’s constant and f= frequency
One ‘packet of energy’ (photon)
would allow the emission of one electron (photoelectron)
Exam Practice: 2
5b) Explain why the following observations may be
understood by using a photon model of light, rather than a
wave model.
• Light above a certain frequency causes the emission of
electrons from the surface of a metal. This emission
occurs instantaneously.
• Light below a certain frequency will not result in the
emission of electrons however long it illuminates the
surface.(5 marks)
..............................................................................................
...............................................
Work Function
The electrons in different metals require different amounts
of energy to escape. This is the work function of the
metal.
This is shown by the symbol ‘phi’
So: The energy coming in
will be used to
overcome work and the
left over would be the
kinetic energy of the
particle!
What would happen if the
work function was greater
than the ‘hf’?
If I kept the type of metal constant, what could I do to
increase the kinetic energy of the electrons emitted?
Kinetic Energy
You could increase the intensity of incoming light by moving
it closer to the metal. (Intensity=power/area). What effect
would this have on the energy of the electron emitted?
So what would it affect?
Increase intensity= increase NUMBER of
electrons emitted
Increase frequency= increase the KINETIC
ENERGY of the electrons
Calculating Work Function
REMINDER!
What was an ‘electronvolt’?
It is “the amount of energy an electron gains after
being accelerated by a p.d. of 1V.”
eV
Charge
on a
single
electron
Voltage
NOTE: to convert joules
into electronvolts, divide
joules by the charge of
an electron, which is about
1.602×10−19
Exam Practice: 3
An electron is accelerated from rest
through a potential difference of 5.0 kV.
The kinetic energy gained by the
electron is
A
8.0 × 10−16 J
B
8.0 × 10−19 J
C
3.2 × 10−20 J
D
3.2 × 10−23 J
Phototube
A phototube consists of a anode that is radiated to release
electrons that would hit the cathode and read as current
Using a variable
resistor, a p.d can
be created across the
system in the
reverse direction.
This would result in
work being done
against the flow
from anode to
cathode
Phototube
The p.d is increased until no current is recorded. thus the
Work done=K.Emax. This is the stopping voltage (Vs)
We do this to find out the
kinetic energy of the
emitted electrons!
1
2
eVs = mv2
Phototube
So with our new value of kinetic energy (Ek) we go back to
our original equation!
Rearrange this equation
into ‘y=mx+c’
DON’T FORGET!
When you’re
dealing with the
KE, you use
joules!
Phototube
So on a graph, the gradient would represent plank’s
constant and the y-intercept would represent a negative
value of the WORK FUNCTION! (phi)
The threshold
frequency is the
mimumum
frequency of incident
radiation that would
cause an electron to
be emitted. It would
be when the
work
function=
incoming
energy
Exam Practice: 4
3. When electromagnetic radiation is incident on a metal plate,
electrons may be emitted.
(a) State what is meant by threshold frequency. (1)
....................................................................................................
.........................................
(b) Calculate the threshold frequency for a metal with a work function
of 2.28 eV. Charge on an electron=1.6x10-19C and Plank’s constant =
6.63×10−34Js (3)
Threshold frequency = ....................................
CLICK FOR ANSWER
Exam Practice: 5
Ultraviolet radiation of wavelength 2.00 × 10−7 m is shone onto a zinc
plate.
Calculate the maximum speed of the electrons emitted from the plate.
work function of zinc = 6.88 × 10−19 J
(4)
Maximum speed of electrons = ...........................................
CLICK FOR ANSWER
Independent Study
.