Physics - Matter and Radiation

Physics - Matter and Radiation

1. Matter and Radiation

2. Atoms

   • Stable and Unstable Nuclei
     • Strong nuclear force
       • Stable = nucleus doesn't disintegrate
       • Strong N.F. overcomes electrostatic forces of repulsions between protons in nucleus. Keeps nucleons together in nucleus
       • Range = not more than 3-4 femtometers. roughly same as diameter of small nucleus
         • The electrostatic force between two charged particles has infinite range.
       • Has the same effect on two neutrons or p and n.
       • Attractive: 3-4 fm. Repel >0.5 fm to stop n and p being pushed into each other.
     • Radioactive decay
       • Alpha Radiation - α. 2p+2n. Helium nucleus.
       • Beta - fast moving electron. β. charge = -1. 0 mass. neutron -> proton and emits beta particle and anti-neutrino (no charge).
         • Lost certain amount of energy but beta particles varied up to a max. Introduced neutrinos and anti-neutrinos.
       • Gamma - γ. electromagnetic. emitted by nucleus with too much energy. after α or β.

3. Particle Interactions

   • Electro-magnetic Force
     • Single force produces a momentum. p=mv
       • Richard Feynman. Electromagnetic forces between two charged objects is due to exchange of virtual photons. virtual since they cannot directly detect them.
         • 2 people on skateboards. One throws a ball to other = repel. Boomerang = attract.
   • Weak Nuclear Force
     • Strong N.F. doesn't cause proton to form a neutron or vice versa. Not electrostatic either since neutron is uncharged. Weaker than strong nuclear force otherwise it would affect stable nuclei.
       • β- and β+ decay, no corresponding particle and anti particle pairs (positron/ electorn and neutino/ antineutrino).
         • neutrino + neutron -> proton + β- antineutrino + proton -> neutron + β+
           • These due to W bosons. Unlike photons. non-zero mass. very short range (0.001 fm). positively/negatively charged W+/W-.
   • Beta Decay
     • W boson meets neutrino or anti-neutrino changing them into β- and β+ respectively. if no (anti)neutrino present, W- decays into β- and antineutrino. W+ decays to β+ and neutrino.
   • Electron Capture
     • proton in proton-rich nucleus turn to neutron by interacting through weak interaction with inner-shell electron. W+ boson turns electron to neutrino.
       • same change for high spped collision of electron and proton. for electron with sufficeint energy, overall change = W- from electron to proton.
   • Force Carriers
     • photon and W boson are force carriers because they are exchanged when electromagnetic force and weak nuclear force act respectively.

4. Anti-Particles

   • Anti-matter
     • Positron Emitting Tomography (PET) used for brain scans. positron-emitting isotope goven to patient. tyo brain via blood. it will quickly find an electron and annihilate forming two gamma photons, whioch are sensed by detectors. image built up from detector signals
     • E=mc^2. mass of particle increases as it gets faster. m0 = rest mass corresponds to rest energy m0c^2. should be included in conservation of energy.
     • anti-particle= same mass but opposite charge.
     • Positron emission = proton turns to neutron and positron and neutrino. Happens in nucleus with too many protons. β+ decay. no mass. +1 charge.
       • β+ emitting isotope not natural. Place stable isotope (liquid/solid) in front of a beam of protons. Some absorb extra protons.
   • Annihilation and Pair production
     • 1 MeV = 1.6 x 10^-13 J. energy transferred when an electron is moved through p.d. of 1 volt.
     • Annihilation = particle + corresponding antiparticle meet and mass is converted into radiation energy (photons). 2 photons produced. One photon cannot ensure a total momentum of 0 after the collision. Minimum energy of each photon= 2hf(min) is equated to rest energy of particle and antiparticle. 2hf(min)=2E(0)
       • Minimum energy of each photon produced, hf(min) = E(0)
     • Pair Production = photon creates particle and antiparticle. rest energy 0 (particle and antiparticle) can be used to work out min E and freq photon must have.
       • Min E of photon, hf(min) = 2E(0)

5. Photons

   • In a vacuum, all electromagnetic waves travel at speed of light. wavelength = λ (nm). Freq. = f. λ=c/f
     • All E.M. waves consist of magnetic and electric waves. right angles to each other. In phase with each other.
   • EM waves emitted by charged particles when it loses energy.
     • Fast moving electron stopped (e.g. in X-ray tube), slows or changes direction. or, when an electron moves to different shell of lower energy.
   • Photon= packet of electromagnetic waves.
     • Photon energy E=hf.
       • h= planck constant (6.63 x 10^-34 Js)
     • Laser=photons of same freq. power= energy per sec transferred by photons.
       • Power of beam= nhf. n= no. of photons in beam passing a fixed point each sec. each photon has energy hf.

6. Atoms

   • Specific Charge
     • Charge/mass. (Ignore mass of electron.)
     • Charge of electron = 1.60 x 10^-19. Mass of proton = 1.67 x 10^-27
   • Isotopes= same no. of protons, different no. of neutrons.
   • Stable and Unstable Nuclei
     • Radioactive Decay

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