The Electron
• Named by Stoney (Irish contribution )
• Indivisible quantity of charge measured by
Millikan in the oil drop experiment
• Negative
• Orbits the nucleus
• Very small mass
Thermionic effect The emission of electrons
from a hot metal.
Photoelectric effect The emission of
electrons due to electromagnetic radiation of
the correct frequency.
Photon A bundle of electromagnetic energy.
Threshold frequency The lowest frequency
that causes photo emission.
Einstein’s photoelectric law
mv
hf = Φ + 12
2
Thermionic Emission
• This is the release of electrons from the
surface of a hot metal.
• Cathode rays are beams of released
electrons.
The cathode ray tube-found in TVs and computer monitors
The Photoelectric Effect
The Photoelectric Effect
This is the release of electrons from the
surface of a metal due to electromagnetic
radiation of suitable frequency.
No electrons are released until the
frequency increases above a certain
threshold value. f0
To Demonstrate the photoelectric
Effect
• Attach a negatively charged
zinc plate to an electroscope
• Note that the leaves diverge.
• Shine ultraviolet light on the
zinc plate.
• The leaves collapse due to
the release of electrons
caused by the photoelectric
effect.
(What happens if the electroscope is
positively charged?)
The Electromagnetic Spectrum
The Photocell
• Filters can be used to
examine the effect of
frequency on
photocurrent.
• The light intensity can
be varied by changing
the distance from the
photocell to the light
source.
Effect of intensity and frequency
As the frequency
increases above the
threshold, the energy
of the electrons
Current
increases.
As the light intensity
increases the
photocurrent
increases.
high intensity
low intensity
Frequency of incident light
Einstein's explanation of
the photoelectric effect
• Light consists of bundles of
electromagnetic energy called photons.
• The energy of a photon depends on its
frequency. E=hf where h=Plancks
constant and f=frequency.
• Each photon liberates one electron.
• Increasing the intensity of the light
increases the number of photons.
The Work Function
Φ=hf0
•
•
•
•
This is the energy needed to just free an
electron but not give it any kinetic
energy.
This will take a minimum frequency
called the threshold frequency.
This threshold frequency is different for
different metals.
This is the frequency below which no
electrons are released.
Applications of the photoelectric
effect
Solar panels
Automatic doors
The Threshold Frequency
This is the frequency below which no
electrons are released.
We use the same colour of light, the same target
metal, but we use a brighter and brighter light.
What
is the
difference
A more
intense
light
between
lightmore
of low
sourceahas
intensity,
and one of
photons..
high intensity?
More photons free
more electrons from
Why does a more
the metal.
intense light cause a
larger current to flow?
Increased intensity frees
more
they
How electrons
come the but
stopping
have
all theissame
energy
voltage
the same
aseven
the frequency
of the
when the light
is
light brighter?
is the same.
Einstein's Photoelectric Law
E    mv
1
2
2
E    mv
1
2
1
2
2
E
= the energy of the photon =hf

= the work function = hf0
2
mv = the kinetic energy of the electron
Xrays
• Discovered by Wilhelm Rontgen 1895.
• They are high frequency electromagnetic
radiation.
• They cannot be deflected by electric or
magnetic fields.
• They can penetrate matter.
• They ionise matter.
• They cause fluorescence
Uses of X-rays
• Destroy cancerous cells.
• Detect cracks in pipes.
• Determining thickness of materials.
How X-rays are produced.
•
•
•
•
A heated filament releases electrons from
the cathode by thermionic emission.
A cooling system removes unwanted heat
from the target.
If the filament current is increased, more
heat is released, therefore more electrons
and as a result more x-rays are released.
(intensity)
If the voltage between cathode and anode is
increased, the electrons are accelerated to
higher speeds and so a higher energy
photon or a more penetrating x-ray is
produced.
Why is X-ray production often
described as the inverse of the photo
electric effect?
X-rays which are electromagnetic radiation are produced
when high energy electrons strike a metal target whereas
the photoelectric effect is the release of electrons by
electromagnetic radiation.