Thursday 15 April

 

Atomic and Nuclear Physics

 

The lowest energy state of an atom is called the ground state.

 

(Photo Electric Effect - PEE) Increasing light frequency increases the kinetic energy of the emitted photoelectrons.

 

Increasing light intensity increases the number of emitted photoelectrons but not their kinetic energy. (Photo Electric Effect - PEE)

 

The kinetic energy of the ejected electrons is the energy of the incident radiation minus the work function of the surface.
 

Below a certain frequency, called the threshold frequency no electrons are
emitted no matter how intense the light beam. (Photo Electric Effect - PEE)

 

Stopping potential is the value of the difference in potential between two plates that is just sufficient to stop the most energetic photoelectrons emitted.


De Broglie proposed that all objects have wavelengths related to their momentum.
 (h = lr)


Emission spectra are photons leaving the atom as electrons come down energy levels. Absorption spectra are photons being absorbed as electrons move up energy levels from the ground state.
http://www.wwnorton.com/college/chemistry/gilbert/overview/ch3.htm#Bohr
 

The diagram below is an energy level diagram for the hydrogen atom.  Possible transitions between energy states are shown for the first six levels.  The dashed line for each series indicates the series limit, which is a transition from the state where the electron is completely free from the nucleus (n = infinity).  The energies shown are the ionisation energies for electrons in each energy level.  This is the energy that must be supplied to remove the electron in a given energy level from the atom.  These energies are thus written as negative values. 
 

Alpha particles are the same as helium nuclei: 2 protons, 2 neutrons. Beta particles are electrons, and gamma “particles” are photons.


The mass of a nucleus is always less than the sum of the masses of the nucleons. This mass defect is converted into binding energy. (E=mc2).
 

One amu of mass is equal to 931 MeV of energy.

 

Nuclear fusion is a reaction in which two nuclei are combined to form a large nucleus. Fusion is the source of energy in stars.

 

Nuclear fission is a reaction in which a nucleus is split.

 

 

 

 

Classical Experiments
 J.J. Thomson
Experimentally measured the charge to mass ratio of cathode rays.

 Ernest Rutherford
The scattering of alpha particles by a thin sheet of gold foil. This experiment demonstrated that atoms consist of mostly empty space with a very dense core, the nucleus.

 Neils Bohr
Bohr's planetary model of the atom correctly describes the spectra of hydrogen.

 R.A. Millikan
Millikan's oil drop experiment confirmed that the fundamental electric charge is quantized.


 Hertz and Einstein
The photoelectric effect was first observed by Hertz and later explained by Einstein. Einstein obtained a Nobel Prize for the mathematical description of the photoelectric effect.

 A.H. Compton
The scattering of x-rays photons provided the final confirmation of the validity of Planck's quantum hypothesis that electromagnetic radiation came in discrete massless packets (photons) with energy proportional to frequency.


Davisson-Germer
Their experiment of the diffraction of electrons demonstrated the wave nature of the electron, confirming the earlier hypothesis of de Broglie.