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Chapter 3
Electricity
Objectives:
 Explain the atomic nature of electricity
 State the laws of electrostatics
 Using the Inverse Square law, calculate changes in intensity and distance
 Describe the three methods of electrification
 Describe insulators and conductors
 Describe the four basic factors of electrodynamics
 Differentiate between current flow and electron flow
 Explain current, potential difference and resistance
 Describe differences between series and parallel circuits
 Using Ohm’s law, calculate changes in voltage, amperage, and resistance in series and parallel
circuits.
Electricity concerns the distribution and movement of electrons.
ELECTROSTATICS
Define electrostatics: the study of the distribution of fixed charges, electrons at rest.
Objects can become charged just like atoms.
Define the term electrification:
the process of electron charges being added to or subtracted
from an object.
How does an object have a negative charge?
To have more electrons relative to another object
How does an object have a positive charge?
To have less electrons relative to another object
What is zero or ground potential?
The reference point for discussing charges, the earth has an infinite number of charges
making it neutral, Zero potential to perform work and release energy.
LAWS OF ELECTROSTATICS
Define each of the following five laws of electrostatics.
1) Repulsion-attraction: like charges repel, unlike
charges attract (electric field is
created, has direction)
2) The Inverse Square law: force is directly proportional to the product of the
magnitudes and inversely proportional to the distance squared between them
Coulomb’s Law (F = kq1q2 / D2) is more accurate. But use:
I1 / I2 = D22 / D12
3) Distribution: charges reside on the external surface of conductors and equally
throughout nonconductors
4) Concentration: concentration
of charge will be on the surface where curvature is
sharpest (cause ionization or static discharge)
5) Movement: only negative charges
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move along solid conductors
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ELECTRIFICATION
What are the three methods of electrification?
Friction, contact and induction
Friction
Define friction: electrification
when an object is rubbed against another
What are the ideal conditions for friction to occur?
During cold weather when low humidity removes stabilizing electrons from the air
Give an example of attraction friction:
rubbing a balloon against wool, electrons transfer to the
balloon and will then stick to a ‘positively’ charged wall
Give an example of repelling friction: comb
removing electrons from hair, leaving each strand
with a ‘positive’ charge, like charges repel, individual hair strands will seek to separate,
making hair stand on end
How does a humidifier in the darkroom eliminate electrostatic discharge that causes artifacts on
radiographic film?
Increases humidity, causes slight condensation on surfaces, moisture becomes a pathway
for the distribution of electrons
Contact/Conduction
Define electrification by contact: two
objects touch, electrons move from one object to the
other
What is resulting charge of both objects after contact electrification?
Both objects have a more equal distribution of charge
Give an example of contact electrification: touching metal
doorknob (contact) after walking
across a carpet (friction) and experiencing an ‘electric shock’
What is static discharge?
When oppositely charged objects are in close proximity, electrons jump the gap to
equalize electron distribution
How is excess energy released during a static discharge?
In the form of light photons
Why is this relevant to a radiographer?
The light forms an image on the film, degrades diagnostic quality of the film, repeat film
is needed.
Induction (electric fields)
Why is induction the most important method of electrification?
Because it is used in the operation of electronic devices (motors, solenoids, transformers)
Induction is the process of:
electrical fields acting on one another without contact
Every charged body is surrounded by a force field, which is called: an
electric field
When a strongly charged object comes near a weakly charged object the electric fields act on one another
before contact occurs.
ELECTRODYNAMICS
Electric current is defined as electrons that are moving in
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predominantly the same direction
Define vacuum: a space from
which air has been removed
Why is a vacuum important to the production of x-rays?
It permits electrons to reach the speed necessary to produce x-rays.
What is the most common conductor used in electricity?
Copper wire
Electrons move along a conductor is a similar fashion as: dominos
Electrons move at nearly the speed of: light
Materials, which allow electrons to flow, are called: conductors
(Cu, Al, H2O)/ superconductors
(Titanium)
Materials, which inhibit the flow of electrons, are called: insulators
(plastic, wood, rubber, glass)
Materials, which have the ability to act as insulators under certain circumstances and a conductor at other
times are called: semiconductors (silicon, germanium) temp. determines the ability: increase
of temperature causes a decrease in resistance.
Define an electrical circuit: a
pathway that permits electrons to move in a complete circle
from their source, through resisting devices and back to their source
What are some sources for an electrical circuit?
Batteries, generators, solar converters, and atomic reactors
What are the most efficient sources of electrical current?
Batteries and generators
Current Flow
Electrons move from the highest concentration to the lowest.
Electric current is described as going from positive to negative while electron flow is actually from
negative to positive.
What are the four most common terms to define current?
Quantity (of electrons), force, opposition/resistance (amount of), and direction (of travel)
What is direct current?
All electrons moving in the same direction, straight line
What is alternating current?
Electrons move first in one direction, then reverse and move in the opposite direction,
sine wave
Current
Current is sometimes defined as the number if electrons: flowing past
a given point per unit of time
What is the unit used to measure current?
Ampere = 1 Coulomb of charge / second
Milliamperage (mA) on x-ray equipment controls the: number
of electrons available to produce xray photons, in x-ray equipment mAs (a combination of kV, mA and sec) is used.
Potential Difference
The force, which electrons travel, is a function of the difference between the number of: electrons
excess at one end of the circuit and deficiency at the other end
The best term to describe the strength of movement behind electrons is: potential
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difference
in
An alternate term used to mean potential difference is: Electromotive
force (emf)
What is the unit of potential difference?
Volt
What is the SI unit for mechanical energy and work?
Joule
Resistance
Define resistance in relation to electricity.
The amount of opposition to current in the circuit
What is the unit of resistance?
Ohm
What are the primary factors which effect resistance in electrical circuits?
The ability to conduct electrons (number of valence electrons- better conductors have one
valence electron, and distance of electron from the nucleus), length of conductor, cross
sectional diameter and temperature.
How does the length of a conductor affect resistance?
As the length doubles, resistance doubles
How does the cross sectional diameter affect resistance?
It is inversely proportional, as cross-sectional diameter doubles, resistance will be halved.
How does the temperature affect resistance?
Increased temperature increases free electron collisions, which increases resistance
How does temperature affect semiconductors?
Semiconductors become more conductive as temperature increases
Ohm’s law
Ohm’s law describes the relationship between which factors?
Current, potential difference and resistance
What is the formula including resistance?
V = IR
where V = potential difference
I = current
R = resistance
What is the unit of power?
Watt
What is the formula for calculating power?
P = IV
What is the power-loss formula? What is it used to calculate?
Shows the importance of changes in resistance to heat output. Power.
How does current affect heat power loss?
Double current, power loss increases by a factor of 4 (P = I2R)
SERIES AND PARALLEL CIRCUITS
What are the formulas for calculating current, voltage and resistance in a series circuit?
It = I1 = I2 = I3
Vt = V1 + V2 + V3
Rt = R1 + R2 + R3
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What are the formulas for calculating current, voltage and resistance in a parallel circuit?
It = V1 + I2 + I3
Vt = V1 = V2 = V3
1/Rt = 1/R1 + 1/R2 + 1/R3
Which factor in a series circuit is constant (equal throughout circuit)?
Current
Which factor in a parallel circuit is constant (equal throughout circuit)?
Voltage
Parallel circuits are not broken when a single resistance is interrupted.
Which type of circuit is preferred in electrical wiring in buildings? Why?
Parallel, because failure of device does not break the electrical supply to the other devices
In a parallel circuit, what happens to resistance, amperage and voltage if more resistors are added to the
circuit?
Resistance decreases, current increases and the voltage remains unchanged
What is a disadvantage of a parallel circuit?
Increase in current can short circuit entire system, which is a fire hazard
What device is used to prevent the wires from becoming too hot and causing a fire? How do these devices
work?
Circuit breaker: permits breaking of circuit before a dangerous temperature is reached
Fuse: metal tab melts when dangerously heated, breaking circuit
Define rheostat: resistor
that permits a variable contact to slide along a series circuit of
resistance coils
Explain how a rheostat operates: when
rheostat is connected in a series circuit, an increase in
resistance will result in a decrease in voltage and vice-versa if the current remains
unchanged
What is the disadvantage of rheostats?
Cause energy waste in heat
Inverse Square law
What will the new intensity be at 30” if an object has an intensity of 100 mR at 60”?
I1 = 100mR
I2 = ?
D1 = 60”
D2 = 30’
I1 / I2 = D22 / D12
100mR / ? = 900 / 3600
I2 = 400mR
If the intensity measures 200mR at 50”, what new distance will be needed to decrease the intensity to
50mR?
I1 = 200mR
I2 = 50mR
D1 = 50”
D2 = ?
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I1 / I2 = D22 / D12
200mR / 50mR = ? / 2500
D2 = 100”
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Ohm’s law
What is the amperage in a circuit of 10 volts and 10 ohms?
I=?
V = 10 volts
R = 10 ohms
I = V/R
I = 10 volts / 10 ohms
I = 1 amp
What is the current in a circuit with 100 volts and 15 ohms?
I=?
V = 100 volts
R = 15 ohms
I=V/R
I = 100 volts / 15 ohms
I = 6.67 amps
What is the voltage in a circuit with 25 ohms and 50 amperes?
V=?
I = 50 amperes
R = 25 ohms
V = RI
V = 25 ohms * 50 amperes
V = 1250 volts
What is the voltage in a parallel circuit that has 20 amperes and resistance of 6, 10, and 15 ohms?
V=?
I = 20 amps
1/Rt = 1/R1 + 1/R2 + 1/R3
1/Rt = 1/6 + 1/10 + 1/15
R = 3 ohms
V= IR
V = 20 amps * 3 ohms
V = 60 volts
What is the amperage in a series circuit of 110 volts with resistance of 5, 10, and 15 ohms?
I=?
V = 110 volts
Rt = R1 + R2 + R3
Rt = 5 + 10 + 15
R = 30 ohms
I=V/R
I = 110 volts / 30 ohms
I = 3.67 amperes
What is the potential difference in a circuit of 20 amperes and 10 ohms?
V=?
I = 20 amperes
R = 10 ohms
V = IR
V = 20 amperes * 10 ohms
V = 200 volts
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What is the power rating of an x-ray generator capable of 1000mA at 110kV?
P=?
I = 1000 mA = 1 ampere
V = 110 kV = 110,000 volts
P = IV
P = 1 ampere * 110,000 volts
P = 110,000 watts
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