# Modern Physics

Modern Physics.

Electron properties: Small mass, orbits nucleus, negative charge.(Stoney named it, Millikan measured charge)

Thermionic emission: The giving off of electrons from the surface of a hot metal.

Photon: A packet of electromagnetic energy. Energy given by E = hf where f = frequency of the light.

Photoelectric effect: The giving off of electrons by a metal when light of high enough frequency falls on it.

Work function (Ф) : Minimum energy needed to release the loosest electron from the surface of a metal.

Photocell: Conducts electricity when light of suitable frequency falls on it. Light intensity increases current

Einstein's photoelectric law: The velocity of the emitted electrons ranges from zero to a definite maximum and the kinetic energy of the electrons increases with an increase in the frequency of the light. hf = ½ mv2 + Ф

Electron volt: W = eV 1.6X10-19.(1) = 1.6X10-19 j = one electron volt.

Cathode rays: Streams of high speed electrons moving from a cathode. (have kinetic energy, are invisible).

Cathode ray tube: 1.Element heats cathode 2. thermionic emission occurs 3. electrons are accelerated to anode by high voltage 4. deflection plates guide the cathode rays 5. the electrons give up their kinetic energy as light.

X rays: High frequency electromagnetic radiation, produced by cathode rays as they strike at target.

Discovered by Rontgen. X ray production is said to be the opposite of the photoelectric effect.

Atom: Nucleus containing protons and neutrons with electrons orbiting.

Rutherford experiment: Fired helium nuclei (alpha particles) at a gold foil. Most just passed through, a few were deflected and fewer still were deflected a 90 degrees. He concluded that atoms were mostly empty space with a positively charged area in the middle.

Energy levels: If an atom is given extra energy (eg: by heating) its electrons can jump to a higher energy level orbit. Very quickly they fall back to the original energy level and give out the extra energy as light..

Line spectrum: The name for the type of light given out by the electron that falls back from high Energy level.

Atomic number (Z): The number of protons an element contains.

Mass Number (A): The number of protons and neutrons an element contains.

Isotope: Atom's of the same element which contain different numbers of neutrons.

Ionising: Is the ability of radiation to knock electrons off an atom.

Radioactivity:Decay of certain atoms with the emission of one or more types of radiation.

Alpha: Fast moving helium nuclei ejected from the nucleus of radioactive atoms.

Positively charged. Low penetrating power stopped by paper, high ionising ability.

Beta: High speed electrons ejected from the nuclei of radioactive atoms.

Negatively Charged, medium penetrating power stopped by aluminium, medium ionising ability.

Gamma: High frequency electromagnetic radiation emitted from the nuclei of radioactive atoms.

No charge. High penetrating power stopped by lead, low ionising abiltiy.

If an alpha particle is lost from an atom it will become the element that is two places to the left on the periodic table. If a beta particle is ejected by an atom, it becomes the element one place to the right on the table.

Half life:The time taken for the activity of a radioactive element to decrease by half.

Detect radiation: Geiger Muller tube, works by ionization of argon gas. Solid State detector, a p-n junction.

Uses of Radioactive isotopes: Food irradiation, Carbon dating, Smoke detectors, Medical therapy & Imaging.

A mole of any substance is the amount of that substance that contains as many particles as there are in exactly 12 grams of Carbon. This number is 6.02X1023 and is known as Avogadro’s number.

An atomic mass unit (AMU) is 1/12th the mass of a carbon atom.

Fission: Splitting of large nucleus into two smaller nuclei with the release of energy.

When U235 is bombarded with neutrons (especially slow moving ones) it undergoes fission.

Fission reactor: Fuel = Uranium, Moderator = graphite, slows neutrons. Control rods = Boron, slows reaction. Shielding = Lead, stops radiation leaks. Coolant = removes heat to heat exchanger to drive turbine.

Fusion: The joining of two small nuclei to form a larger one with the release of energy.

Advantages over Fission: Less radioactive waste, can never have a meltdown, fuel easy to get (water).

In a nuclear reaction Mass-energy electric charge and momentum are conserved.

The neutrino: Wolfgang Pauli proposed this particle when it seemed that momentum and charge were not conserved. He said the neutrino took accounted for the missing momentum and charge.

Walton & Cockroft: 1st artificial splitting of atom: 7Li3 + 1H1 4He2 + 4He2 + Energy

They made the first linear particle accelerator which used high voltage to accelerate hydrogen atoms.

Walton & Cockroft were the first to verify experimentally Einsteins Equation E = mc2.

Antimatter: A particles anti particle has the same mass as the original particle and it also has the same charge but the charge is the opposite charge to the original particle.

The positron: Discovered by Carl David Anderson. He found a particle with the same mass as the electron but with a positive charge (equal but opposite charge to that of electron).

The positron is the antiparticle of the electron.

Pair Production: Creating of two particles from energy. In this process a particle and its antiparticle are produced. Momentum and electric charge are conserved.

This was first done by placing a lead plate into a cloud chamber and bombarding it with gamma rays. An electron and a positron were produced.

Pair production occurs when a gamma ray loses its energy (hf) is converted into an electron and a positron and the energy left over is the kinetic energy each particle has.

hf = 2mc2 + Ek1 + Ek2.

Pair Annihilation: An electron and a positron that lie close together and are almost at rest, join together and are annihilated producing energy. The matter they are made of disappears.

The momentum of the system before annihilation is zero and so must be zero after the annihilation. Thus two photons are produced and they travel in different directions to conserve momentum.

Pair annihilation e+ + e- 2hf.

The Four Fundamental Force’s of Nature.

1. Gravitational: Act’s on masses. Always an attractive force. Infinite range. A weak force.

2. Electromagnetic: Binds atoms and molecules together (can be attractive or repulsive) Much stronger than the gravitational force between the same particles. Infinite Range. 2nd strongest.

3. Strong Nuclear: Short range. Binds the nucleus. Electrons do not feel it. Strongest force.

4. Weak Nuclear: Force between particles not affected by or subject to the strong force. Beta decay occurs via the weak nuclear force. Short range. Stronger than gravitational only.

Elementary particle: A particle with no other particles inside. Eg Eletcron.

Lepton:A particle that does not feel the strong force. Eg: Electron.

Hadron: A particle that feels the strong force. Eg: Protons, Neutrons

Quarks: Elementary particles from which Baryons and Mesons are made.

Baryon: Made up of any three quarks or any three anti quarks.

Mesons: Made up of a quark and an anti quark.