Post on 24-May-2015
Quantum Phenomena
Electron-volt Photons
Energy levels
Wednesday 12 April 2023
The electronThompson was studying the conductivity of gases in fluorescent tubes
(neon) when he discovered the electron.
1) An electric current flows through a low pressure gas when a high potential difference is applied between two electrodes
2) The Cathode (negative electrode) gave off some invisible rays (Cathode rays)
3) These rays could be deflected by electric fields they were negatively charged particles called ELECTRONS.
AnodeCathode
Low pressure gas
Thermionic emissionIt is possible to produce electron emission from metals using low
voltage between the anode and the cathode.
1) The cathode has to be heated up to high temperatures (by a current flowing through a filament)
2) Electrons will escape from the filament (Cathode)
Cathode
Electrons
Thermionic emissionWhat would happen to the electrons if an anode (positive electrode) is
placed near the cathode?
1) The emitted electrons are attracted by the anode
2) The anode exerts a force on each electron the electrons gain K.E.
Cathode Anode
Vacuum
Electron deflectionSo, how did Thompson realised that the “cathode rays” are streams of
negative particles?The cathode rays could be deflected by electric, or magnetic fields.
Cathode Anode
Electrodes
Draw the path of the electrons that go through the hole in the anode and between the positive and negative electrodes.
Electron-voltWe can measure the Ek (Kinetic Energy) of a charge which is accelerated
across a potential difference using this formula:
QVEk Kinetic Energy (J) = Charge (C) x Voltage (V)
Kinetic Energy (J) = Charge (C) x Voltage (V)
The charge of the electron (elementary charge) is e = 1.60 x 10-19 C
So, we can define a new unit of energy, the ELECTRONVOLT:
JeV 191060.11 One electronvolt is the Kinetic Energy gained by an electron
when it is accelerated through a potential difference of one volt.
Neon LampsBut, why do fluorescent tubes emit light of different…
The gas that fills the tubes is different, so it emits different colour light when an electric current flows through it.
COLOURSAnd why do different gases emit different colours?
To answer this question we must understand the nature of light and electromagnetic radiation, and the structure of the atom.
Neon LampsShine the light from a light bulb and different gas lamps through a
prism. Then look at the spectra. What do you notice? What is the difference between the spectrum from the light bulb and the gas lamps?
• The light bulb gives a Continuous Spectrum.
• The gas lamps give a Line Spectrum.
• Each gas lamp gives different lines in their spectrum.
Continuous Spectrum
Line Spectrum
The Hydrogen SpectrumAll elements have their own line spectrum emitted when an electric
charge is passed through their vapour. For an hydrogen discharge tube this is the line spectrum we would obtain:
The lines on the spectrum are the wavelengths of the light produced by the discharge through a hydrogen gas.
What is this light made of?
656 nm
400
nm
500
nm
600
nm
700
nm
486 nm434 nm410 nm
The Photon: a massless particleWe’ve always thought of light as a wave, because it behaves like a wave
in many cases (e.g. refraction, reflection, diffraction…). However, Einstein discovered that in some instances light behaves like a particle. He called these “particles” PHOTONS. His observations extend to all electromagnetic waves.
What are they made of?
What are they made of?
What do they carry?
What do they carry?
How is the energy carried affected?
On what does their energy depend?
Representing a photonSo, why does a photon behave like a particle?
1) It is a packet of electromagnetic energy gives the idea of an “item” occupying a certain space, and not a continuum like a wave propagating in space
2) It travels in one direction only. So, a light bulb emits photons in all possible directions, with each photon travelling in one direction only.
3) The energy of a single photon is “quantized” and measurable. So, if a single photon hits a surface, it is a bit like a ball hitting a wall.
Photons emitted by filament lamp
Energy of a photonWe can measure the energy of a photon using Einstein’s equation:
h = 6.63 x 10-34 Js Planck constant
f = frequency of photon/electromagnetic radiation
c = 3 x 108 m/s speed of light in a vacuum
= wavelength of photon/electromagnetic radiation
hc
hfE
Energy LevelsSo, what causes the hydrogen in the discharge tube to emit just four
wavelengths of light?• The electrons in the atom can only orbit at certain distances from
the nucleus, i.e. the radius of the orbit is “quantized”
• Each “orbit” represents the energy of the electrons. So, electrons on the lower level (orbit) is at Ground State and have the lowest potential energy, while the other states are called Excited States because the electrons are at a higher potential energy
• The electric discharge gives the electrons the energy to jump on to higher energy levels
• Eventually (pretty quickly), these electrons will jump down to a lower energy level, or even the ground level
• In jumping down the electrons emit a photon of energy equal to the difference in the energy between the two levels
Energy Levels and Photon Emission
Calculate the energy of all the photons that can be emitted by this atom. E is the energy gap between two energy levels. Which of them is not part of the visible spectrum?
E = 0.66 eV
E = 1.90 eV
Ground State
1st Exited State
2nd Exited State
1st to ground state 2nd to 1st state 2nd to ground state
Answer: 1st to ground level• The energy of a photon is E = hf
• The energy of the emitted photon equals the energy gap between the two energy levels.
• E = 1.90 eV, so:
)(1059.4
1063.6
1060.190.190.190.1
114
34
19
Hzsf
Js
J
h
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Answer: 2nd to 1st level• The energy of a photon is E = hf
• The energy of the emitted photon equals the energy gap between the two energy levels.
• E = 0.66 eV, so:
)(1059.1
1063.6
1060.166.066.066.0
114
34
19
Hzsf
Js
J
h
eVfhfeVE
Answer: 2nd to ground level• The energy of a photon is E = hf
• The energy of the emitted photon equals the energy gap between the 2nd and the ground energy level.
• E = 1.90 + 0.66 = 2.56 eV, so:
)(1018.6
1063.6
1060.156.256.256.2
114
34
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Js
J
h
eVfhfeVE