PHYSICS FOR RADIOGRAPHERS 1 (HDR102)
CHAPTER 2:
Electrostatics
PREPARED BY:
MR KAMARUL AMIN BIN ABDULLAH
SCHOOL OF MEDICAL IMAGING
FACULTY OF HEALTH SCIENCES
Chapter 2: ELECTROSTATICS
LEARNING OUTCOMES
TOPIC
At the end of the lesson, the student should be able to: Define the electrostatic, electrification, and electric charge.
 Briefly explain the properties of electric charge.
 Describe the law of electrostatic (Coulomb's Law)
 Briefly explain what is potential difference.
 Briefly explain what is electric field and its strength.
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Chapter 2: ELECTROSTATICS
TOPIC OUTLINE
TOPIC
INTRODUCTION
1.1 Electrostatics
1.1.1 Electric Charge
1.1.2 Electrostatics
1.1.3 Electrification
1.2 Electrostatic Laws
1.3 Electric Potential
1.4 Conducting Properties of Material
1.5 References
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Chapter 2: ELECTROSTATICS
INTRODUCTION
TOPIC
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Chapter 2: ELECTROSTATICS
1.1 Electrostatics
TOPIC
1.1.1 Electric Charge
 Matter has mass and energy equivalence. It means matter also may have
electric charge.
 Electric charge is a physical property of matter that causes it to experience
a force when near other electrically charged matter.
 Electrons and protons are the smallest units of electric charge.
 The electron has one unit of negative charge and the proton has one unit of
positive charge.
 The smallest unit of electric charge is electron. This charge is much too small
to be useful, so the fundamental unit of electric charge is the coulomb (c): 1C
= 6 x 1018 electron charges.
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Chapter 2: ELECTROSTATICS
1.1 Electrostatics
TOPIC
1.1.2 Electrostatics
 Electrostatics is the study of stationary electric charges.
 Because of the way atoms are constructed, electrons often are free to travel
from the outermost shell of one atom to another atom.
 Protons, are fixed in the nucleus of an atom and not free to move.
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Chapter 2: ELECTROSTATICS
1.1 Electrostatics
TOPIC
For example, on touching a metal doorknob after having walked across a
deep-pile carpet in winter, you get a shock (by contact). It is because
electrons are rubbed off the carpet onto your shoes causing you to
become electrified. An object is said to be electrified if it has too few or
too many electrons.
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Chapter 2: ELECTROSTATICS
1.1 Electrostatics
TOPIC
1.1.3 Electrification
 It is the process of electron charges being added to or subtracted from an
object.
 For instance, the outer shell electrons of some types of atoms are loosely
bound and can be removed easily.
 Removal of the electrons electrifies the substances from which they were
removed and results in static electricity.
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Chapter 2: ELECTROSTATICS
1.1 Electrostatics
TOPIC
For example, if you run a comb through your
hair, electrons are removed from the hair and
deposited on the comb.
The comb become electrified with too many
negative charges.
An electrified comb can pick up tiny pieces of
paper as though the comb were a magnet.
Because of its excess electrons, the comb repels
some electrons in the paper, causing the closest
end of the paper to become slightly positively
charged. This results in a small electrostatic
attractive force.
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Chapter 2: ELECTROSTATICS
1.2 Electrostatic Laws
TOPIC
 Laws of electrostatics describe how electric charges interact with each other
and with neutral objects.
 Associated with each electric charge is electric field.
 The electric field points outward from a positive charge.
 Uncharged particles do not have an electric field.
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Chapter 2: ELECTROSTATICS
1.2 Electrostatic Laws
TOPIC
 SIMILAR electric charges (-ve and –ve or +ve and +ve) – electric fields are in
opposite direction, repel to each other.
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Chapter 2: ELECTROSTATICS
1.2 Electrostatic Laws
TOPIC
 UNLIKE electric charges (-ve and +ve) – electric fields radiate in same
direction, attract each other.
 The attraction and repulsion between charges is due to electric field as it is
called electrostatic force.
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Chapter 2: ELECTROSTATICS
1.2 Electrostatic Laws
TOPIC
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Chapter 2: ELECTROSTATICS
1.2 Electrostatic Laws
TOPIC
 Coulomb's law states that the electrical force between two charged objects is
directly proportional to the product of the quantity of charge on the objects
and inversely proportional to the square of the separation distance between
the two objects.
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Chapter 2: ELECTROSTATICS
1.2 Electrostatic Laws
TOPIC
 In equation form, Coulomb's law can be stated as:-
 Q1 = the quantity of charge on object 1 (in Coulombs),
 Q2 = the quantity of charge on object 2 (in Coulombs),
 d = the distance of separation between the two objects (in meters).
 k = is a proportionality constant known as the Coulomb's law constant.
(9.0 x 109 N • m2 / C2 ) – medium= air
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Chapter 2: ELECTROSTATICS
1.3 Electric Potential
TOPIC
 Electric charges have potential energy.
 When it is positioned close to each other, like electric charges have electric
potential energy because they can do work when they fly apart.
 Electron bunched up at one end of wire create an electric potential because
the repulsive force causes some electrons to move along the wire so that
work can be done.
OFF
ON
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Chapter 2: ELECTROSTATICS
1.3 Electric Potential
TOPIC
 The unit of electric potential is the volt (V).
 Electric potential is sometimes called voltage (the higher the voltage, the
greater potential to do work).
 X-ray imaging systems usually require 220 V or higher.
 The volt is potential energy/unit charge, or joule/coulomb (1 V = 1 J/C)
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Chapter 2: ELECTROSTATICS
1.4 Conducting Properties of Materials
TOPIC
 Insulators are materials in which electric charge does not move easily
 They can be charged, but charge doesn’t move well
 Glass, rubber, plastic, wood, and paper are examples
 Conductors are materials in which electric charge moves easily
 When an area becomes charged, charge distributes itself over entire
surface
 Copper, aluminum, and silver are examples
 Charge will remain on conductor if you hold it with an insulator
 Semiconductors are materials that have electrical properties somewhere
between conductors and insulators
 Silicon and germanium are examples
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Chapter 2: ELECTROSTATICS
1.5 References
TOPIC
No.
REFERENCES
1
Ball, J., Moore, A. D., & Turner, S. (2008). Essential physics for
radiographers. Blackwell.
2
Bushong, S. C. (2008). Radiologic science for technologists. Canada:
Elsevier.
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Chapter 2: ELECTROSTATICS
SUMMARY
TOPIC
 Electric charge is a physical property of matter that causes it to experience a
force when near other electrically charged matter.
 Electrostatics is the study of stationary electric charges.
 Electrification is the process of electron charges being added to or subtracted
from an object.
 SIMILAR electric charges (-ve and –ve or +ve and +ve) – electric fields are in
opposite direction, repel to each other.
 UNLIKE electric charges (-ve and +ve) – electric fields radiate in same
direction, attract each other.
 Coulomb's law states that the electrical force between two charged objects
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Chapter 2: ELECTROSTATICS
NEXT SESSION PREVIEW
TOPIC
CHAPTER 3: CAPACITORS
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Chapter 2: ELECTROSTATICS
APPENDIX
TOPIC
FIGURE
SOURCE
Figure 1
http://www.actors.co.ke/en/news/Energy1.jpg
Figure 2
http://intechweb.files.wordpress.com/2012/03/shutterstock_77399518.jpg
Figure 3
http://www.solarenergybook.org/wp-content/uploads/2009/12/solar-energyexample.gif
Figure 4
http://www.petervaldivia.com/technology/energy/image/potencial-andkinetic.bmp
Figure 5
http://iws.collin.edu/biopage/faculty/mcculloch/1406/outlines/chapter%206/S
B7-2b.JPG
Figure 6
http://www.petervaldivia.com/technology/energy/image/potencial-andkinetic.bmp
Figure 7
http://www.physics4kids.com/files/art/motion_energy1_240x180.jpg
Figure 8
http://www.sciencebuilder.com/michigan/science/images/p/potentialenergy.j
pg
Figure 9
http://4.bp.blogspot.com/_V7DuEO3c2E8/Sb2PZfOXZI/AAAAAAAAADk/KKXoueyon2I/s1600/One-balanced-rock.jpg
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Chapter 2: ELECTROSTATICS
Activity
TOPIC
 Q1. What is the electrostatic charge of one electron?
 Q2. The electrostatic charge transferred between two people after one has
scuffed his feet across a nylon rug is one microcoulumb. How many electrons
are transferred?
 Q3. One Ampere is the flow of one Coulomb per second; therefore “mAs” is a
measure of what quantity?
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