Transcript Quantum Physics 2 - More About

More About Photoelectricity
Quantum Physics Lesson 2
Learning Objectives
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State and use the photoelectric equation.
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Explain why electrons leave with a range of
kinetic energies.
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Plot the results from the vacuum photocell to
determine Planck’s constant and the work
function.
THE ‘ULTRAVIOLET CATASTROPHE’
1900 - Rayleigh
This was a CLASSICAL
prediction, first made in the late
19th century, that an IDEAL
BLACK BODY at thermal
equilibrium will emit radiation
with INFINITE POWER.
Max Planck resolved this issue by
postulating that electromagnetic
energy did not follow the classical
description, but could only
oscillate or be emitted in
DISCRETE PACKETS OF
ENERGY proportional to the
frequency. He called these packets
‘QUANTA’.
E  hf
Note:
h  6.626 x1034 J .s
The Photoelectric Effect
1905 - Einstein
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The emission of electrons
from a surface (usually
metallic) upon exposure to,
and absorption of,
electromagnetic radiation.
The photoelectric effect was
explained mathematically by
Einstein who extended the
work on QUANTA as
developed by Planck.
KE  hf  
Homework
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Complete Past Paper Question – may need to
look up answer to part (b)!
Complete worksheet but not questions that are
crossed out – don’t need to know that bit!
I will post a link to some useful online notes
over the weekend on Unit 1 page.
I will collect and mark next Thursday.
Definitions (From Past Papers)
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The Work Function:minimum energy to remove an electron from the
surface of a metal
The Threshold Frequency:minimum frequency of electromagnetic radiation
required to eject photoelectrons from a metal
surface
Photon Energy
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Recall from Particle Physics – Lesson 3 – Photons:The energy of an incoming photon is given by
E  hf 
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hc
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Where E is the Energy of Photon in Joules (J)
f is the Frequency of the radiation in Hertz (Hz)
λ is the wavelength of the radiation in metres (m)
h is Planck’s constant = 6.63 × 10-34 Js
More Equations
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The process of tearing an electron loose takes up an
amount of energy called the work function,Φ, and
the rest is converted into kinetic energy, EK(max)
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So when emission occurs we use Einstein’s
equation:Photon Energy (J)  Work Function (J)  Maximum Kinetic Energy(J)
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Or in Symbols:-
hf    EK (max)
Analogies
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If you’re stuck down a well you can’t get out
unless you have enough energy to jump out in
one go – same for an electron.
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Coconut Shy – can fire 1,000 ping pong balls at
a coconut – but they’re just ping pong balls, not
going to knock the coconut off!
It only takes one bullet though...that does have
enough energy and momentum
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What’s going on?
More Equations
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When the light incident on the metal is at
exactly the threshold frequency the photons
have just enough energy to free the electrons
(i.e. the work function)
Work Function (J )  Photon Energy (J)
In Symbols:-
  hf 0
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where f0 is the threshold frequency.
Range of KE of Released Electrons
Graph of Emax against freq.
Equation of a Straight Line
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The graph plotted is a straight line in the form of:
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Where m is the gradient and c is the y-intercept.
Comparison with the straight line equation:-
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y  mx  c
E K ,max  hf  
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We can see that a graph of EK,max vs f, will result in a
straight line with gradient = h and intercept = -φ