Principles of Imaging Science II
(RAD 120)
Electricity
Electricity
• X-ray tube converts electrical energy to
EM energy
• Principles of Electrostatics
– Refers to nonmoving electrical charges
– Charges are a property of all matter
• Electrons – Negative
• Protons - Positive
• Neutrons - Neutral
– An electric charge is not a particle but it is a
property of particles
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Attraction & Repulsion
• Opposite charges attract
– Proton attracts an electron
• Similar charges repel
– Electron repels an electron
– Protons repel each other in the nucleus
• Protons remain fixed within the nucleus,
electrons are moved from the outer shell
(ionization)
Electrification
• Abnormal state of an
object due to the
movement of
electrons
– Objects become more
negatively charged
because of the
additional electrons
– Examples:
• Bad hair day
• Static on radiographs
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Methods of Electrification
• Created by:
– Friction – rubbing e- off one object depositing on
another
– Contact – charged object touches another object
moving e- to that object
– Induction – charged object attracts the opposite
charges in another object (lightning)
• Process by which one charged object attracts the opposite
charges in another object and thereby electrifies the other
object. A negatively charged object repels the electrons of
another object, leaving that object with a positive charge.
Induction occurs as a result of the interaction of the electric
fields around two objects that are not in contact.
Electrostatic Charge
• Coulomb (C)
– Fundamental unit of electric charge
– 1 C = 6.3 X 1018 electron charges
– Represents a negative number for a negative
charge
– The number of electrons represented by
6.25 X 1018 is more than 6 billion billion. This
is approximately the number of electrons that
flows through a 100-watt light bulb in 1
second!
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Electrostatic Laws
• Describe how electric
and neutral charges
interact
• Electric fields
(electrostatic force)
– Radiate out from a
+ charge, toward
on a – charge
– Opposite charges
attract, like repel,
unlike do not
possess an electric
field
Electrostatic Laws
• Coulomb’s Law
– Describes the magnitude of
electrostatic force
– Follows premise of
Inverse Square Law
– The electrostatic force
is directly proportional
to the product of the
electrostatic charges
and inversely
proportional to the
square of the distance
between them
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Electrostatic Laws
• Electrical charges
are evenly
distributed
throughout the
object OR on the
surface
• This is an attempt
to repel each other
– Wire conductor vs.
thunder cloud
Electrostatic Laws
• Electrical charges
are concentrated
on the surface with
the greatest
curvature
– Round anodes vs
cattle prod
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Electrodynamics
• Electricity - movement of electrical
charges
– Electric current is the flow of electrons from
negative to positive
– An electric current will occur in a conductive
metal when an electric potential exists
– Electric potential is the difference between the
charge at the – end and the + end
– Electricity does work when the electrons flow
in an electric current along a wire
Electric Current
• Conditions necessary
1. Electric potential
difference exists between
two objects
2. Conducting pathway
Electron flow is – to +
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Electric Current
• Electric potential is
related to potential
. energy
• Greater the electric
potential, as the electric
field is increased, the
greater the potential
energy and the greater
the electric current once it
starts flowing.
TYPES OF ELECTRICAL
MATERIALS
• Materials possess different abilities to
conduct electricity
• There are four classes of materials:
– Conductors
– Insulators
– Semiconductors
– Superconductors
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TYPES OF ELECTRICAL
MATERIALS
Electrical States of Matter
The arrows demonstrate the difference between the energy levels
of conduction and valence bands in an insulator, semi-conductor,
and conductor
The conduction and valence bands overlap in a good conductor
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Electric Circuits
• Pathway of an electric current
– Factors:
• Voltage (V) – unit of potential difference in charges
• Current (I) - # electrons flowing/sec (ampere)
• Resistance(R) - resists, reduce or impede flow of
e- (Ohm)
THE UNITS OF ELECTRICITY
• Ampere (A)
– Unit to measure
electric current
• Rate of electric charge
that flows past a
particular point in 1
second.
• One ampere equals 1
coulomb flowing by in 1
second:
• Volt (V)
– Unit to measure
electrical pressure or
potential difference
– Represents the energy
in a circuit
– 1 kV = 1000 V
1 A = 1 C/s
– mA = 1/1000 A
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THE UNITS OF ELECTRICITY
• OHM (Ω)
– Unit to measure resistance (R)
• Opposes current flow in a circuit
– Four factors that affect the resistance of a
conductor:
•
•
•
•
The material used
Length
Cross-sectional diameter
Temperature
Factors that affect Resistance
• Conductivity of material
• Length
– Direct relationship
• Cross-sectional area
– Indirect relationship
• Temperature
– Increased temp impedes electron flow
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FACTORS AFFECTING
RESISTANCE
FACTORS AFFECTING
RESISTANCE
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FACTORS AFFECTING
RESISTANCE
Electric Circuit Symbols
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THE RELATIONSHIP BETWEEN
CURRENT, VOLTAGE, AND RESISTANCE
• First discovered by Georg Ohm and is
represented by Ohm’s Law:
– V = IR
I = V/R
R = V/I
• V = Voltage, I = Current, R = Resistance
Electric Circuit - Schematic
• The solid lines
represent wires in the
circuit. Shown are a
battery and a circuit
with two resistors in it.
A switch opens and
closes the circuit.
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Series Circuit
• All circuit elements are interconnected along the
wire in the conducting pathway
• Electrons travel along all resistors
• Rules:
– Rt = R1 + R2 + R3…
– It = I1 = I2 = I3…
– Vt = V1 + V2 + V3
Parallel Circuit
• Bridges exist along
the conducting
pathway creating
multiple avenues for
the e- to travel
• Rules:
– It = I1 + I2 + I3…
– Vt = V1 = V2 = V3
– 1/Rt = 1/R1 + 1/R2 +
1/R3…
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OHM’s Law
OHM’s Law Current
(Series/ParallelCircuit)
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OHM’s Law Voltage
(Series/ParallelCircuit)
OHM’s Law Resistance
(Series/ParallelCircuit)
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OHM’s Law Applied
POWER
• Power (P): The rate energy is used
• Unit: Watt (W)
– 1 W = 1 joule/second
– 1 watt is produced by one ampere of current
flowing with an electrical pressure of 1 volt
• Formula: P = IV
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Power Formula Calculations
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