Session 4: Basic Electrical Concepts Unit I: Physics
Session 4: Basic Electrical Concepts Unit I: Physics Associated with Nuclear Medicine Instrumentation Part B CLRS 321 Nuclear Medicine Physics and Instrumentation 1 Objectives (Mostly from your text)
Describe behavior of electrons in an electric field. Identify basic components of an electric circuit. Distinguish between covalent and delocalized molecular bonding, and briefly describe the electrical conductivity of insulators, conductors, and semiconductors. Define and utilize basic terms and units of electricity including Coulomb, current, voltage, resistance, and capacitance. Diagram a RC circuit and discuss its uses in radiation detectors. Introduction
Instrumentation = Energy Transfer Usually EM gamma energy emitted from a source That energy is converted into light and/or electrical energy That electrical energy is used to make the source energy useful to the human mind. We use electrical concepts in nuclear medicine instrumentation Behavior of Electric Charges Charles-Augustin de Coulomb
http://www.nndb.com/people/777/000091504/ Coulombs Law Law of Electrostatic Force kQa Qb F 2 d Coulombs Law is the basis behind the concept of an electric field.
F=Electrostatic Force Qa=Charge on Object a Qb=Charge on Object b d=distance k=a proportionality constant Electric Field According to Coulombs Law, a force field is created around an electrically charged object The more charge you have the greater the electrostatic force
The greater the distance between charges, the lesser the electrostatic force. (Another inverse square law.) In instrumentation, we use electrostatic force to do work. Electric Fields http://buphy.bu.edu/~duffy/PY106/2e.GIF Electric Fields Benjamin Franklin established the
convention of having force arrows move away from the positive charge. In actuality, in electrical circuits, negatively charged electrons move towards a positive charge. http://www.loc.gov/loc/legacy/ concord.html
Electric Fields: Dipole Electric charges close enough that their electric fields interact http://www.rpdp.net/sciencetips_v2/images/P12B2_7.gif Molecular Bonding Electricity is a flow of electrons within a circuit. The molecular bonding that comprises the
materials making up the circuit impacts the ability of electrons to flow through the circuit. Molecular Bonding: Electrons and Protons Electrons are negatively charged Electrons are outside of the atomic nucleus Protons are positively charged relatively, positively charged things are stationary
Electrons can move Electrons move toward a positive charge When electrons are removed from a source that source becomes positively charged. The unit of charge is a Coulomb A Coulomb is 6.24 X 1018 moving electrons Molecular Bonding: Atomic Orbitals Covalent bonds Form two types of orbitals from valence (outer)
shell electrons Bondinglower energy state of electron Anti-bondinghigher energy state Bonding orbitals more pervasive in covalent bonds between two atoms since this requires less energy Common in organic compounds and many inorganic molecules Molecular Bonding: Atomic Orbitals
Delocalized Bonds In some materials, many atoms are bound together by sharing all electrons in a band of electrons. Happens often with metals The whole piece of metal is the molecule with delocalized bonding Valence band Holds bonding orbitals Conduction band Holds anti-bonding orbitals
Molecular Bonding: Atomic Orbitals For electrons to move from the valence band to the conduction band requires energy. http://www.vtaide.com/png/images/atom.jpg http://oldsite.vislab.usyd.edu.au/photonics/devices/semic/images/valcond.gif
Conduction Properties of Materials Conductors Full valence band Extra electrons in conduction band Materials with small forbidden gaps can become conductors. Insulators Full valence band 5eV forbidden gap or larger Difficult if not impossible to get electrons to conduction band
Semiconductors Full valence band Small forbidden gap (about 1 eV) Heat will jump electrons to conduction band Figure 03: Energy diagram showing bonding and antibonding orbitals in a delocalized molecular bonding situation Delocalized bonds form bands surrounding conducting-type materials. In conductors, the valence band is full and extra electrons are found in the conduction band. Materials with small energy requirements for electrons to jump from
the valence band to the conduction band make good conductors. Electrical Circuits Closed Loop Circuit Electrons moving through a conductor and exciting gas in a light bulb. Electrical Circuits: Voltage & Current Voltage = potential electrical energy (Joules/Coulomb) Current = movement of electrons over time (1 Coulomb/Second = Ampere)
When voltage is applied to a copper wire, current moves through it. Voltage is like the suction on a straw, if electrons are present, theyll get sucked up. The more suction, the more electrons, the more current. (Insulation keeps the current from moving outside the wire.) Electric Circuits: Resistance If we reduce the diameter of our wire, it will reduce the flow of electrons and thus the current. If we use a coffee stirrer instead of a
drinking straw to suck up electrons, we will suck up less electrons over a given period of time. This effect of reduction is called Resistance and is measured in Ohms ().). Ohms Law V=IR Georg Simon Ohm R=V
I Or I=V R VPotential (Volts) ICurrent (Amperes) RResistance ().) For a given voltage If you increase resistance, you decrease the current. http://www.stegen.k12.mo.us/tchrpges/sghs/aengelmann/OhmGeorgSimon2.htm
Electric Circuits: Capacitance Capacitor: Two conducting plates separated by an insulator Electrical potential builds up charge difference between plates Charge on plates limited to number of electrons that can be crowded on Electric field created between the plates Mathematically expressed as: CV Q
C is Capacitance in farads V is change in voltage Q is charge on one plate Uniform electric field (Or area of potential difference [V]) Figure B-4: Capacitor + pole - pole
Some Electrical Symbols Bushong, Stuart, Radiologic Science for Technologist, 8th Ed., (St. Louis: Mosby Inc. 2004), p. 83. Resistor-Capacitor Circuit Prekeges, J. Nuclear Medicine Instrumentation. 2011 Sudbury, MA. Jones & Bartlett. Fig B-5, p. 273
Electrical Units and Mathematical Relationships 1 Coulomb = the charge on 6.24 X 1018 electrons 1 Coulomb 1 ampere (A) = sec 1 joule 1 volt (V) = Coulomb 1 Coulomb
19 1 eV = 1V = 1.6X10 joules 18 6.24 X 10 electrons 1 Volt 1 ohm = Ampere
1 Coulomb 1 Farad = Volt Capacitance: Conversion of Charge to Voltage Voltage is easier to measure and manipulate than current Increasing resistance in an RC current results in a longer
voltage pulse compared to the charge imposed. This is often desirable in NM Instrumentation Prekeges, J. Nuclear Medicine Instrumentation. 2011 Sudbury, MA. Jones & Bartlett. p. 272 q (t ) V (t )
C q(t) is the charge on any plate C is the capacitance Next: Gas-filled detectors! http://www.aolcdn.com/uk_promo/homer_promo
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