Resident Physics Lecture

Resident Physics Lecture http://www.radiology.mcg.edu/radiologyphysics/ • Christensen, Chapter 1 Radiation George David Associate Professor Department of Radiology Medical College of Georgia

Whoops, I think I just lost an electron Are you sure? Atom #1 Atom #2 Yeah, I’m positive groan Atom #1 Atom #2 Physics Can Be Fun George David Associate Professor Department of Radiology Medical College of Georgia

Quicky Science Review

Abbreviations Memorize this. That’s an order! 109 giga G (billion) 106 mega M (million) 103 kilo K (thousand) 10-1 deci d (tenth) 10-2 centi c (hundredth) 10-3 milli m (thousandth) 10-6 micro m (millionth) 10-9 nano n (billionth) 10-12 pico p (millionth millionth) Angstrom = A = 10-10 m

Energy Aside • Kinetic Energy • Energy of an object by virtue of its speed • K.E. = (1/2) X mv2 • m  mass • v  velocity • Potential Energy • Energy of an object by virtue of its position

What’s the Smallest Thing that is Sugar? • Divide, divide, divide • The smallest entity that is still sugar is the sugar molecule

But What’s in that Sugar Molecule? • Different color balls? • No! Atoms

Interesting Fact You Already Knew • There are only 92 naturally occurring types of atomsHOWEVER • There are zillions of different types of molecules That’s way cool.

Ever Seen This?

Composition of the Atom + • Protons • Neutrons • Electrons -

Protons + • Positive charge • Live in nucleus

Neutrons • No charge (free?) • Live in nucleus • Ever-so-slightly more mass than proton • Better than oldtrons?

Electrons • Negative charge • Found outside the nucleus • Exist only in designated shell locations • Weighs only 1/1836th as much as proton - - +

Atomic Number • # protons • Defines element & its properties • Color • State • Helium is helium because it has 2 protons • # neutrons does not affect chemistry Helium Also Helium + + - + - + - -

Atomic Weight • # protons + # neutrons • # nucleons • A specific element often found with multiple atomic weights (isotopes) • Always the same # protons • Different # neutrons • For a particular element, some isotopes may be stable, others unstable (radioactive) HeliumAtomic Weight=4 HeliumAtomic Weight=3 + + - + - + - -

Atomic Mass Unit (amu) • Nominally • 1 amu = the weight of a proton or neutron • Officially • 1 amu = 1/12 the weight of a carbon-12 atom • Atomic # = 6

4 He 2 Atomic Symbol Atomic Weight(# protons + # neutrons) + + - Atomic #(# protons) -

How Many Electrons? • In a neutral atom (not negative or positive)# electrons = # protons + + - -

Charge Theory Like charges repel Unlike charges attract + + + -

Coulomb Forces + - k q1 q2 F = ------------ r2 • Equation F = Coulomb force q’s = charges of the two objects k = constant r = distance between objects

Coulomb Equation Story k q1 q2 F = ------------ r2 • Force proportional to the magnitude of the charges + + - + + + + - +

Coulomb Equation Story k q1 q2 F = ------------ r2 • Force falls off with the square of distance • Twice as far: one quarter the force • Three times as far: one ninth the force + + + + + + - - + + + + + + + + + -

Orbital Electrons • Electrons “-” charge very small mass compared with protons / neutrons • Electrons reside only at certain energy levels or Shells Designations start at K shell K shell closest to nucleus L shell next closest Shells proceed up from K, L, M, N, etc. Except for K shell, all shells contain sub-shells L K + - + + + X - -

Electrons & Shells • Atom mostly empty space • If atom were a baseball stadium nucleus would be size of baseball • Nucleus contains almost all of atom’s mass • Electron shells determine element’s chemical properties

Shell Capacities

Binding Energy • Negative electrons attracted to positive nucleus • more binding energy for shells closer to nucleus • K shell has highest binding force • higher atomic # materials (higher Z) result in more binding energy • more positive charge in nucleus • energy required to remove orbital electron from atom L - K + + + + - - -

Electron Shells • electrons attempt to reside in lowest available energy shell L K + + + + - - -

Electron Shells • electrons attempt to reside in lowest available energy shell L - K + + + + - -

* The Shell Game • Electrons can move from shell to shell • to move to higher energy shell requires energy input equal to difference between the binding energy of the two shells L Requiresenergyinput! K - + + + + - -

The Shell Game L • An atom with a gap in a lower shell is unhappy (unstable) • Electrons will attempt to drop to lower shells to fill the gapBUT • to move to a lower energy shell requires the release of energy equal to the difference between shells • characteristic x-rays - K - + + + + - Energyreleased

Electromagnetic Radiation • Transport of energy through space • Properties of EM are combination of • electric fields • magnetic fields • X-rays are one form of electromagnetic radiation • No transport medium required

Electromagnetic Radiation • Examples • x-rays • radio waves • microwaves • visible light • radiant heat

Electromagnetic Radiation • EM sometimes act like particles, sometimes like waves • Particle concept explains • radiation interactions with matter • Wave concept explains • refraction • diffraction • polarization

Particle concept (cont) • X-rays are discrete bundles of energy • quantum or photon • Photon Energy proportional to frequency • higher frequency = higher energy • energy measured in electron volts (eV) • energy gained by electron accelerated by 1 volt potential Energy = Planck’s Constant X Frequency E = hn

Wave Properties of EM • Wavelength • distance between successive waves • Frequency • number of waves passing a particular point per unit time • Velocity (“c”) of light / x-rays • 186,000 miles/second OR • 3 X 108 meters/second • Wavelength & frequency • inversely proportional Velocity = Wavelength X Frequency c = l X n

Wavelengths and EM Highest wavelength = lowest frequency Radio Infrared Visible light Ultraviolet Soft x-rays Diagnostic x-rays Therapeutic x-rays & gammas Low energy High energy Lowest wavelength = highest frequency Velocity = Wavelength X Frequency c = l X n

Energy vs. Wavelength as Equations Energy = Planck’s Constant X Frequency E = hn but Frequency = Speed of Light / Wavelength n = c / l so E = hc / l Energy (keV) = 12.4 / Wavelength (in Angstroms) E = 12.4 / l

Resident Physics Lecture