Quantum+Physics

= Quantum Physics = Quantum Physics led to the discovery of the electron, which was previously called a cathode ray. J. J. Thomson was the first to measure the charge to mass ratio of the cathode ray/electron, by using a strange apparatus, where cathode rays accelerated by a high voltage and then pass between a pair of parallel plates built into the tube. The voltage that is applied to the plates produces an electric field, and a pair of coils produces a magnetic field. Thomson discovered that when he applied Newton's second law to the cathode ray/electron, when the magnetic field is equal to the mass times the acceleration, the equation is: evB = m(v^2/r) and turns into e/m = v/Br. e/m was later found out to be 1.76 x 10^11 C/kg.

Robert A. Millikan performed the famous oil drop experiment that led to the to discovery of the charge of an electron and that the charge is quantized. The experiment was that tiny droplets of mineral oil that were carrying an electric charge were allowed to fall under gravity between two parallel plates. The electric field between the plates was adjusted so that the drops would be suspended in midair. In that situation, mg = qE. The mass of the droplet was found by measuring the terminal velocity when the electric field was not present. With all that information, the charge of the electron was found to be 1.6 x 10^-19 C. Using the charge of the electron, and the charge to mass ratio, the mass of an electron was found to be 9.11 x 10^-31 kg.

In 1900, Max Planck proposed a new theory, that the energy from the vibrations of atoms cannot be just anything, it must be a multiple of the value which happens to be related to the frequency of oscillation. The equation used for this purpose was E = hf, where h = 6.63 x 10^-34 J*s. h was called Planck's constant.

There is also a phenomenon called the photoelectric effect, where light is shined on a surface, (metals work best) electrons are knocked loose from the metal by way of either electric fields or photons. The electric field theory says that if the intensity of the light is increased, then the number of electrons should increase, and so should their maximum kinetic energy. It also says that the frequency of the light that is being shined would not affect the kinetic energy of the ejected electrons, only the intensity. The photon theory, as proposed by Einstein, says that the electron is ejected from the metal by the photon, but since the electron is restricted to the metal by attractive forces, a certain amount of work is needed to break the grip of these attractive forces. This work is called the Work Function, Wo, and is just a few electron volts for most metals.

Quantum Physics also found that momentum of a photon is not p =mv, but rather p = E/c = hf/c = h/lambada

Compton Effect: Scattered light has less energy, longer wavelength and lower frequency compared to incident light, because scattered light loses energy during a collision/reflection.

De Broglie Wavelength: Louis de Broglie thought that the wavelength of a particle is related to momentum in the same way as for a photon: p = h/lambada.