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HDR102 SCHOOL OF MEDICAL IMAGING FACULTY OF HEALTH SCIENCES PREPARED BY: MR KAMARUL AMIN BIN ABDULLAH CHAPTER 3 PHYSICS FOR RADIOGRAPHERS 1 CAPACITORS
HDR102
SCHOOL OF MEDICAL IMAGINGFACULTY OF HEALTH SCIENCES
PREPARED BY:MR KAMARUL AMIN BIN ABDULLAH
CHAPTER 3
PHYSICS FOR RADIOGRAPHERS 1
CAPACITORS
Slide 2 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
LEARNING OUTCOMES
At the end of the lesson, the student should be able to:-
Define the capacitor and its functions.
Briefly describe about the parallel and series.
Briefly explain the factors affecting capacitance including the dielectric
material and the distance between the plat area.
Briefly explain the charge and discharging capacitors including the
characteristics, exponential law, and time constant.
Briefly explain the capacitance with dielectrics.
Slide 3 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
TOPIC OUTLINES
INTRODUCTION
3.1 Capacitors 3.3 Charging and Discharging Capacitors
3.1.1 Mode of operation of capacitors 3.3.1 Charging
3.1.2 Capacitance 3.3.2 Discharging
3.1.3 Unit of Capacitance
3.1.4 Types of Capacitors 3.4 References
3.1.5 Factors Affecting Capacitors
3.2 Capacitors in Circuit
3.2.1 Parallel Circuit
3.2.2 Series Circuit
Slide 4 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
INTRODUCTION
IMPORTANT IN X-RAY CIRCUIT
EXAMPLE: MOBILE X-RAY MACHINE
An important component
in circuit for medical
equipment.
Slide 5 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
3.1 Capacitors
3.1.1 Mode of Operation of Capacitors
Capacitors consist of two conductors (electrodes) separated by an insulator.
A parallel-plate capacitor, has a thin layer of insulation (dielectric)
sandwiched between two flat metallic electrodes or plates.
One plate is charged positively and the other negatively.
Slide 6 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
3.1 Capacitors
The presence of positively charged plate makes easier to transfer negative
charges onto negative plate.
This is because the repulsion effect of charges already in negative plate is
weakened by attraction from opposite (positive) charges plate.
The closer the plates are placed, the more effective the second plate
becomes at easing the charging process.
Slide 7 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
3.1 Capacitors
The symbol representing a capacitor in an electric circuit looks like parallel
plates.
TWO functions of The Insulating Layer (Dielectric):
To prevent the two plates from touching each other.
To increase the capacitance of the capacitor.
Dielectric material
Plate 1
Plate 2
The dielectric material is an
insulator therefore no current flows
through the capacitor
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TOPIC
CHAPTER 3: Capacitors
TOPIC
3.1 Capacitors
-Q+Q
- + - + - + - +
- + - + - + - +
- + - + - + - +
- + - + - + - +
Figure 1: The electric field
between two plates has a
polarising effect on the
dielectric, creating alignment of
its molecular dipoles. The
positive charges on +ve plate
make it easier for negative
charges to be deposited on +ve
plate. A similar effect occur on
the –ve plate of the capacitor.
-ve
plate
+ve
plate
direction of electron moving
Click Here to enlarge
Slide 10 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
3.1 Capacitors
Capacitance is the quantity of charge which can be stored on a conductor per
unit potential.
If Q is the amount of charge a conductor is able to store at a potential V, its
capacitance C is given by:
C = Q/V
3.1.2 Capacitance
Slide 11 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
3.1 Capacitors
Capacitance also represents the amount of charge that can be transferred
either onto or off a conductor per unit change in its V resulting from the
addition/removal of an amount of charge Q, its capacitance (C) is given by the
same relationship.
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TOPIC
CHAPTER 3: Capacitors
TOPIC
3.1 Capacitors
Because the SI units of charge and potential are the coulomb and the volt,
respectively, the unit of capacitance is the coulomb per volt (C V-1), which is
known as the farad (F).
The farad is an extremely large unit, and in practice we usually use
microfarads (μF), where 1 μF = 10-6 F, or even in picofarads (pF),
where 1 pF = 10-12 F.
3.1.3 Unit of Capacitance
CLICK HERE TO SEE TABLE
Slide 14 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
3.1 Capacitors
Capacitors are one of the components found on printed circuit boards
and in integrated circuits:-
1. Printed circuit boards (PCBs) provide physical support for, and
electrical connection between, electronic components, using
conductive pathways etched from copper sheets laminated onto a
non-conductive substrate.
3.1.4 Types of Capacitors
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TOPIC
CHAPTER 3: Capacitors
TOPIC
3.1 Capacitors
2. Integrated circuits (ICs or “chips”) are miniaturized electronic
circuits consisting mainly of semiconductor devices, deposited in the
surface of a substrate of thin semiconducting material. A single
integrated circuit may contain more than a million electronic
components, including capacitors.
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TOPIC
CHAPTER 3: Capacitors
TOPIC
3.1 Capacitors
3.1.5 Factors Affecting Capacitance
There are three basic factors of capacitor construction determining
the amount of capacitance created.
These factors all dictate capacitance by affecting how much electric
field flux (relative difference of electrons between plates) will
develop for a given amount of electric field force (voltage between
the two plates):-
Slide 19 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
3.1 Capacitors
Plate Area
Plate Spacing
Dielectric Material
Three basic factors of capacitor construction determining the amount of
capacitance created.
BACK
Slide 20 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
BACK
3.1 Capacitors
Plate Area
All other factors being equal, greater plate area gives greater capacitance;
less plate area gives less capacitance.
Explanation: Larger plate area results in more field flux (charge collected on
the plates) for a given field force (voltage across the plates).
ere.
Figure 2: Plate area.
Slide 21 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
BACK
3.1 Capacitors
Plate Spacing
All other factors being equal, further plate spacing gives less capacitance;
closer plate spacing gives greater capacitance.
Explanation: Closer spacing results in a greater field force (voltage across the
capacitor divided by the distance between the plates), which results in a
greater field flux (charge collected on the plates) for any given voltage applied
across the plates
Figure 2
Figure 3: Plate spacing.
Slide 22 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
BACK
3.1 Capacitors
Dielectric Material
All other factors being equal, greater permittivity of the dielectric gives greater
capacitance; less permittivity of the dielectric gives less capacitance.
Explanation: Although its complicated to explain, some materials offer less
opposition to field flux for a given amount of field force. Materials with a
greater permittivity allow for more field flux (offer less opposition), and thus a
greater collected charge, for any given amount of field force (applied voltage).
Figure 3
NEXT
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TOPIC
CHAPTER 3: Capacitors
TOPIC
BACK
1.2. Topic
Dielectric Material
Figure 4: Dielectric material.
MAIN
Slide 24 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
3.2 Capacitors in Circuits
The used of capacitors can combined in the circuits with two types:-
Parallel Series
Slide 25 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
3.2 Capacitors in Circuits
3.2.1 Parallel Circuit
• Suppose there is a potential difference V
between a and b.
• Then q1 V = C1 & q2 V = C2
• We want to replace C1 and C2 with an equivalent
capacitance C = q V
• The charge on C is q = q1 + q2
• Then C = q V = (q1 + q2 ) V = q1 V + q2 V = C1 + C2
• This is the equation for capacitors in parallel.
• Increasing the number of capacitors increases
the capacitance.
C1 - q1
C2 - q2
a b
V
C = C1 + C2a b
C - q
Slide 26 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
3.2 Capacitors in Circuits
3.2.2 Series Circuit
• Here the total potential difference between a and b is V = V1 + V2
• Also V1 = (1/C1) q and V2 = (1/C2) q
• The charge on every plate (C1 and C2) must be the same (in magnitude)
• Then: V = V1 + V2 = q / C1 +q / C2 = [(1/C1) + (1/C2)] q
• or, V = (1/C) q
• This is the equation for capacitors in series.
• Increasing the number of capacitors decreases the capacitance.
C2C1
a b
V1 V2
+q-q a b
C
V
-q -q+q +q
1 / C = 1 / C1 + 1 / C2
Slide 27 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
3.3 Charging and Discharging Capacitors
When a Capacitor is connected to a circuit with Direct Current (DC) source,
two processes, which are called “charging” and “discharging” the Capacitor,
will happen in specific conditions.
.
Figure 5: The
process of charging
and discharging.
Slide 28 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
3.3 Charging and Discharging Capacitors
The Capacitor is connected to the DC Power Supply and current flows through
the circuit.
Both plates get the equal and opposite charges and an increasing Potential
Difference, (vc) is created.
Once the voltage at the terminals of the capacitor (vc), is equal to the Power
Supply Voltage, (vc = V) the capacitor is fully charged and the current stops
flowing through the circuit, the Charging Phase is over.
3.3.1 Charging
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TOPIC
CHAPTER 3: Capacitors
TOPIC
3.3 Charging and Discharging Capacitors
When the Capacitor disconnected from the Power Supply, the Capacitor is
discharging through the Resistor RD and the Voltage between the Plates drops
down gradually to zero, vc= 0.
3.3.2 Discharging
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TOPIC
CHAPTER 3: Capacitors
TOPIC
Answer the question.
ACTIVITY
The purpose of a capacitor is to
Quiz
produce electricity
store electrical charges
create a potential difference
create a resistance
Slide 35 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
SUMMARY
Capacitors consist of two conductors (electrodes) separated by an
insulator.
A parallel-plate capacitor, has a thin layer of insulation (dielectric)
sandwiched between two flat metallic electrodes or plates.
Capacitance is the quantity of charge which can be stored on a conductor per
unit potential.
The unit of capacitance is the coulomb per volt (C V-1), which is known as the
farad (F).
THREE factors affecting capacitance: Plate area, Plate spacing, Dielectric
material.
The used of capacitors can combined in the circuits with parallel and series.
Slide 36 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
NEXT SESSION PREVIEW
CHAPTER 4: ELECTRICITY
In chapter 4, students will learn about the origin of electricity
and how it occurs.
Slide 37 of 52
TOPIC
CHAPTER 3: Capacitors
TOPIC
3.4 References
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 3: Capacitors
TOPIC
APPENDIX
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-energy-
example.gif
Figure 4 http://www.petervaldivia.com/technology/energy/image/potencial-and-
kinetic.bmp
Figure 5 http://iws.collin.edu/biopage/faculty/mcculloch/1406/outlines/chapter%206/S
B7-2b.JPG
