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Chapter 3 Quantum Phenomena (3.1 (Einstein (Light is made of wavepackets…
Chapter 3
Quantum Phenomena
3.1
Photoelectric effect
- electrons are emitted from the surface of a metal when EM radiation above a certain frequency was aimed at it
Problems
Won't take place below threshold frequency (depends on metal). Wavelength of incident light must be less than speed of light divided by threshold frequency
(λ=c/f)
Number of electrons per second is proportional to intensity of incident radiation
Photoelectric emission happens as soon as incident radiation is directed at the surface
Einstein
Light is made of wavepackets
(photons)
Light incident on a metal surface, electron absorbs 1 photon (energy- to hf) where hf is energy of a light photon
Electron can leave surface if hf> work function
Φ
of metal, excess energy because kinetic energy
Work function
- min energy to escape
Stopping potential
Stopping potential (Vs)
- minimum potential needed to prevent photoelectric emission
At this potential KE is reduced to 0 as each emitted electron bust do extra work equal to e x Vs to leave metal
KE min= e x Vs
3.3
Ionisation
Ion
- charged atom (proton number doesn't equal electron number
Ion formed when electrons are added or removed from an uncharged atom
Ionisation
- creating ions
Alpha, beta and gamma radiation all create ions
Electrons passing through a fluorescent tube create ions when they collide with atoms in the gas or vapour
Electron volt
Unit of energy equal to the work done when an electron is moved through a PD of 1V
Work done to move an electron through a PD of 1V =
1.6x10 -19J
(1 eV)
Excitation by collision
Excitation
- when an atom absorbs energy without being ionised as a result of an electron inside an atom moving from an inner shell to an outer shell
If a colliding electron loses all its kinetic energy when it causes excitation the current due to the flow of electrons through the gas is reduced
If the colliding atom doesn't has sufficient energy to cause excitation it's deflected by the atom with no overall loss of kinetic energy
Excitation energies
- energy values at which an atom absorbs energy
Can calculate excitation energies of atoms in a gas-filled tube by increasing the PD between filament and anode and measuring the PD when the anode current falls
Excitation energy is less than ionisation energy because the atomic electron isn't removed completely from the atom when excited
3.4
Electrons in atoms
They're trapped by the electrostatic force of attraction of the nucleus
Electron closer the the nucleus has less energy than an electron further away
Ground state
- lowest energy state of an atom
When an atom in ground state absorbs energy one of its electrons move to a higher energy level- atom is excited
Energy level diagram
-Shows the allowed energy values of the atom
De-excitation
Electron configuration of an excited atom is unstable because an electron that moves to an outer shell leaves a vacancy in the original shell
Vacancy eventually filled- the electron emits a photon and the atom moves to a lower energy level
Excitation using photons
Photon energy must be exactly equal to the difference between final and initial energy levels for excitation to occur
Smaller or bigger and it won't be absorbed
Fluorescence
Fluoresce
- glow with visible light
Atoms absorb UV photons and become excited- when the de-excite they emit visible photons- when the source of UV radiation is removed they stop glowing
Fluorescent tube
- glass tube with a fluorescent coating on its inner surface
Tube contains mercury vapour at low pressure
It emits visible light when the tube is on because
Ionisation and excitation of mercury atoms can occur as they collide with each other and the electrons in the tube
Mercury atoms emit UV photons, visible photons and photons of less energy when they de-excite
UV photons are absorbed by the atoms of fluorescent coating- excitation
Coating atoms de-excite and emit visible photons
3.5
A Colourful spectrum
If you use a tube of glowing gas as a light source when you split light you get a spectrum of discrete lines of different colours
Measure the wavelength of the lines of a line spectrum to work out the element that produces the light
Each line of a line spectrum is due to light of a certain colour/wavelength
Photons producing each line all have the same energy- different from the energy of the photons producing any other line
Each photon is emitted when an atom de-excited due to one of its electrons moving to an inner shell
If electron moves from energy level E1 to a lower energy level E2
energy of emitted photon (hf) = E1-E2
3.6
Dual nature of light
Light part of EM spectrum of waves
Light has a dual nature- can behave as particles or waves depending on the circumstances
Wave-like
- when diffraction of light takes place- light passes through a narrow slit and spreads out like water waves do
Particle-like
- Photoelectric effect- light is directed at a metal surface, electron at the surface absorbs a photon of frequency f, KE of the electron is increased by hf
Matter waves
De broglie- matter particles have a dual wave-particle nature- wave-like behaviour is characterised by a wavelength
de brolie wavelength
(relates to the momentum of the particle)
Electrons
Particle-like
- Deflected in a magnetic field
Wave-like
Beam of electrons can be diffracted
Thin sheets of metal have a regular lattice of ions that represent a 2D diffraction grating- electrons can pass through the gaps and produce an interference pattern (ring pattern) on the other side
Beam produced by attracted electrons from a heated filament wire to a positively charged metal plate with a small hole at its centre
Increase PD between filament and metal plate increases the speed of electrons= smaller diffraction rings (smaller de Broglie wavelength)
3.2
Quantum world
Energy is quantised
- only certain levels of energy allowed
Only multiples of Planck's constant (hf)
Atom absorbs or emits radiation when it moves up or down a level
Conduction electrons
Average kinetic energy of a conduction electron depends on the temperature of the metal
Work function of a metal
- minimum energy needed by a conduction electron to escape from the metals surface when the metal is at zero potential
When a conduction electron absorbs a photon its KE increase by the energy of the photon
Energy of photon exceeds work function the electron can leave metal
If it doesn't leave metal it collides with other electrons and positive ions and it quickly loses its extra KE
Vacuum Photocell
Vacuum photocell= glass tube containing a metal plate (photocathode) and a smaller metal electrode (anode)
Light of freq. greater than threshold freq. directed at photocathode, electrons are emitted and attracted to anode- microammeter used to measure current, proportional to number of electrons per second transferred
Photoelectric current (photoelectrons per second) ∝ intensity of light
Intensity of light doesn't effect max KE of a photoelectron
Can measure max KE using a photocell
- Vacuum Photocell
