Transcript Electricity - people.vcu.edu

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/
Coulomb’s Law
• Law of Electrostatic Force
kQa Qb
F
2
d
Coulomb’s 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 Coulomb’s 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/conco
rd.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
• Bonding—lower energy state of electron
• Anti-bonding—higher 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, they’ll 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 (Ω).
Ohm’s Law
V=IR
Georg Simon Ohm
R=V
I
Or I=V
R
V—Potential (Volts)
I—Current (Amperes)
R—Resistance (Ω)
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
+ pole
Uniform electric field
(Or area of potential difference [∆V])
Figure B-4: Capacitor
- 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: TEST and then…
Gas-filled detectors!
http://www.aolcdn.com/uk_promo/homer_promo