# AQA A physics quantum phenomena (particle physics)

various bits of information to do with the particle physics topics in unit 1

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• Created by: Ravina
• Created on: 08-03-11 09:21

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PHYSICS Quantum Phenomena:
1. Photoelectricity:
A photon is a packet of energy.
Metal contains conduction electrons, which move
freely inside the metal. The electrons collide with each
other and with positive ions of the metal.
If you shine a light of high enough frequency onto the
surface of a metal, the metal will emit electrons. For most
metals the frequency falls in the UV range.
· Free electrons on the surface of the metal absorb
energy (photons) from the light making them
vibrate.
· If an electron absorbs enough energy, the bonds
holding the electron to the metal will break and the
electron is released. (one electron absorbs one
photon)
· This is known as the photoelectric effect and the
electrons emitted are called photoelectrons.
The energy of a photon is: plancks constant X frequency
E = hXf = hXc /
= wavelength(m) > c/f
Photon energies are usually measured in electron volts (eV)
1 eV= 1.6 X 10-19J <---- 1.6X10-19J X 1V
Therefore the higher the frequency of the electromagnetic radiation, the more energy
the photons carry.
Main conclusions on the emission of photoelectrons:
No photoelectrons are emitted if the radiation is below the threshold frequency of the
metal.
The number of photoelectrons emitted per second is proportional to the intensity of the
radiation, provided frequency is greater tha the threshold frequency.

## Other pages in this set

### Page 2

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Photoelectrons are emitted with a variety of kinetic energies ranging from zero to a
maximum value. This value of maximum kinetic energy increases with the frequency of
There is no time delay between absorbtion and emission.
WAVE THEORY CANNOT EXPLAIN THE PHOTOELECTRIC EFFECT
wave theory:
· emission should take place with waves of any frequency
· emission would take longer using low intensity waves than using high intensity waves.…read more

### Page 3

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This is proportional to the number of
electrons per second that transfer from the cathode to the anode.
· Photoelectric current = I/e (where is e is the charge of the electron)
· photoelectric current is proportional to the intensity of the light incident on the cathode.
· The intensity of the incident light does not affect the max. kinetic energy of a photoelectron.…read more

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· removing electrons=positively charged ion
· process of creating ions is known as
ionisation.
· A photon can be absorbed and cause
ionisation if its energy is greater than or equal to
the difference between the ionisation level and
ground state.
For example:
1)alpha, beta and gamma radiation create ions when
they pass through substances and collide with atoms of the
substance.
2)Electrons passing through a flourescent tube create
ions when they collide with atoms of the gas or

### Page 5

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Energy levels in atoms:
Electrons in atoms:
· electrons in atoms are trapped by the electrostatic force of attraction of the
nucleus.
· The move in shells surroundind the nucleus
· electron in an inner shell has less energy than an electron in an outer shell.
· Each shell can only hold a certain amount of electrons.
Energy levels:
· Lowest energy state of an atom is called its

### Page 6

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Deexcitation:
· Process by which electrons can 'relax' from
their excited state.
· When an atom deexcites it moves from a
high energy level to a lower energy level. In doing so
it releases a photon with the energy equal to the
energy lost by the atom.…read more

### Page 7

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An atom in an excited state can deexcite directly or indirectly to
the ground state. Therefore an atom can absorb photons or certain
energies and then emit photons of the same or lesser energies. As
shown below.
The above process explains why substances fluoresce/glow with visible light when they absorb UV radiation.
Atoms in the substance absorb UV photons and become excited. When the atoms de-excite
they produce visible photons. When the UV radiation is removed the substance stops glowing.…read more

### Page 8

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Since only certain photon energies are allowed, you only see the corresponding
wavelengths.
· No other element produces the same pattern of light wavelengths.
· This is because the energy levels of each type of atom is unique. Therefore the photons
emitted are characteristic of the atom.
6. Wave Particle Duality:
Dual nature of light:
· the wave-like nature of light is observed when the diffraction of light takes place. Eg.…read more

### Page 9

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Dual nature of electrons (MATTER WAVES):
· particlelike nature: electrons in a beam can be deflected by a magnetic field.
· wavelike nature: de broglie's hypothesis, that electrons can be diffracted.
De broglie's hypothesis and wavelength:
· de broglie's wavelength is related to the momentum, p, of the particle
= h / p
as momentum is defined as mass X velocity we also
get the equation:
= h/ m X v
Narrow beam of electrons in vacuum tube directed at thin metal foil.…read more