work and energy - xraykamarul electric current exists whenever electric charge flows through a...
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HDR102
SCHOOL OF MEDICAL IMAGINGFACULTY OF HEALTH SCIENCES
PREPARED BY:MR KAMARUL AMIN BIN ABDULLAH
CHAPTER 4
PHYSICS FOR RADIOGRAPHERS 1
ELECTRICITY
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CHAPTER 4: Electricity
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LEARNING OUTCOMES
At the end of the lesson, the student should be able to:-
Explain the concept of electric current and EMF.
Describe the direct current and alternating current.
Explain the Band Theory.
Explain the resistance and factors affecting it.
Define the Ohm’s Law.
Describe the resistors.
Explain the series and parallel circuit.
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CHAPTER 4: Electricity
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TOPIC OUTLINES
INTRODUCTION
4.1 Electricity 4.8 Series and Parallel Circuits
4.2 Types of Current 4.9 Resistors in Series and Parallel
4.3 Units of Measuring Electricity 4.10 Factors Affecting Resistance
4.4 Band’s Theory 4.11 References
4.5 Resistance (Ohm’s Law)
4.6 Resistors
4.7 Resistivity
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CHAPTER 4: Electricity
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INTRODUCTION
Electricity is important to supply the
medical equipment…
BUT…
Does we know how it happens?
Through the chapter, you will find out…
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CHAPTER 4: Electricity
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4.1 Electricity
4.1.1 Electric Current
An electric CURRENT exists whenever electric charge flows through a region,
e.g., a simple light bulb circuit.
The magnitude of the current is measured in AMPERES (Amps/A), where
1 ampere = 1 coulomb/second
I = ΔQ/ Δt
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CHAPTER 4: Electricity
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4.1 Electricity
Figure 1: A simple basic circuit with using a battery.
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CHAPTER 4: Electricity
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4.1 Electricity
4.1.2 Electromotive Force (emf) or Voltage (V)
Is the voltage produced by the energy supplied in an electrical circuit.
The unit is the volt (V).
A difference in charge between two points in a material can be created by an
external energy source such as a battery or generator.
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CHAPTER 4: Electricity
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4.1 Electricity
This causes electrons to move so that there is an excess of electrons at one
point and a deficiency of electrons at a second point.
This difference in charge is stored as electrical potential energy known as
emf. It is the emf that causes a current to flow through a circuit.
HIGH LOW
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CHAPTER 4: Electricity
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4.2 Types of Current
4.2.1 Direct current:
electrons that flow in the same
direction in a wire. (DC)
from batteries.
Figure 2: A diagram shows a direct current.
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CHAPTER 4: Electricity
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4.2 Types of Current
4.2.2 Alternating current:
electrons that flow in different
directions in a wire. (AC)
from generators.
transformers change AC to DC.
Figure 2: A diagram shows an alternating
current.
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CHAPTER 4: Electricity
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4.3 Units of Measuring Electricity
4.3.1 Current
Measured in Amperes or amps (A)
4.3.2 Voltage
Measured in volts (V)
Higher voltage, the more work the electrons can do.
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CHAPTER 4: Electricity
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4.3 Units of Measuring Electricity
4.3.3 Resistance
The force opposing the flow of electrons.
Measured in ohms
Symbol is Greek letter omega (Ώ)
Thicker wire - less resistance, Longer wire - more resistance
Conductors - low resistance, Insulators - high resistance
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CHAPTER 4: Electricity
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4.4 Band’s Theory
Band’s theory is an extension of molecular orbital theory that describes
bonding in solids.
Bands of orbital that are filled or partially filled by valence electrons are
called valence bands.
Higher-energy unoccupied bands in which electrons are free to migrate are
called conduction bands.
The energy gap between the valence and conduction bands is called the band
gap.
It does explain the differences in conductor, semiconductor, and insulator.
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CHAPTER 4: Electricity
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4.4 Band’s Theory
4.4.1 Conductor
As a potential difference is
applied across the sample, the
electrons in the partly filled band
can accelerate and gain energy
because there are unoccupied
states of higher energy available
and also no band gap.
It is therefore easy for a current
to flow, making it a good
conductor.
Valence Band
Conduction Band
Figure 3: A conductor
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CHAPTER 4: Electricity
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4.4 Band’s Theory
4.4.2 Insulator
As a potential difference is
applied across the sample, the
electrons in the valence band
cannot accelerate and increase
their energy since there are no
empty states readily available
and also there is a large band
gap.
Such materials are therefore
insulators.
Band Gap
Figure 4: An insulator
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CHAPTER 4: Electricity
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4.4 Band’s Theory
4.4.3 Semiconductor
The band structure of pure
(intrinsic) semiconductors is
similar to that of an insulator
except that the valence and
conduction bands are separated
by a smaller band gap Eg.
When a potential difference is
applied across the semiconductor
sample, the electrons in the
conduction band result in a
current flow.
Band Gap
Figure 5: A semiconductor
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CHAPTER 4: Electricity
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4.4 Band’s Theory
Valence band
Conduction bandBand gap
Band gap
Figure 6: The diagram shows the conductor, insulator and
semiconductor with regards to band’s theory.
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CHAPTER 4: Electricity
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4.5 Resistance (Ohm’s Law)
The magnitude of the electric current that flows through a closed circuit
depends directly on the voltage between the battery terminals and
inversely to the circuit resistance.
The relationship that connects current, voltage and resistance is known as
OHM'S LAW and is written as follows:
I = V/R or V = IR
The current is measured in amperes, the voltage in volts and the resistance in
ohms (Ώ).
V
I R
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CHAPTER 4: Electricity
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4.6 Resistors
Resistors are used to control the amount of current flowing in a circuit.
Resistors have resistances from less than 1 ohm to millions of ohms.
The two main types of resistors:-
Figure 7: Wire-wound (coil of fine
wire)resistors
Figure 8: Composition (carbon) resistors
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CHAPTER 4: Electricity
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4.6 Resistors
Symbol on a schematic diagram
For the color code, the first two colors represent the first two digits in the
value of the resistor, the third represents the power of ten that it must be
multiplied by, and the fourth is the tolerance.
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CHAPTER 4: Electricity
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4.6 Resistors
Figure 9: A resistor and list of color coding.
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CHAPTER 4: Electricity
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4.7 Resistivity
RESISTIVITY is actually when electric charge flows through a circuit it
encounters electrical RESISTANCE.
The resistance of a metal conductor is a property which depends on its
dimensions, material and temperature.
At a specific temperature, the resistance (R) of a metal wire of length (L) and
cross-sectional area (A) is given by:
R = ρ L/A
ρ is a constant of proportionality called the RESISTIVITY.
The unit of resistance is the ohm (Ώ) and the unit of resistivity is ohm-meter
Ώm.
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CHAPTER 4: Electricity
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4.8 Series and Parallel Circuits
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CHAPTER 4: Electricity
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4.9 Resistors in Series and Parallel
In Series the total resistance is:-
R = R1 + R2 + R3
R1
R2
R3
R1 R2 R3
In Parallel the total resistance is:-
1 = 1 + 1 + 1
R R1 R2 R3
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CHAPTER 4: Electricity
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4.10 Factors Affecting Resistance
Resistance depends on:-
1. Temperature
2. Material of conductor
3. Length
4. Cross-sectional area
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CHAPTER 4: Electricity
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4.10 Factors Affecting Resistance
4.10.1 Temperature
a) The resistance of a metallic
conductor increases as the
temperature increases e.g.
copper.
b) The resistance of a
semiconductor/insulator
decreases as the temperature
increases e.g. thermistor.
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CHAPTER 4: Electricity
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4.10 Factors Affecting Resistance
4.10.2 Length
a) Resistance of a uniform conductor is
directly proportional to its length.
i.e. R L
4.10.3 Cross-sectional area
a) Resistance of a uniform conductor is
inversely proportional to its cross-
sectional area.
i.e. R 1
A
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CHAPTER 4: Electricity
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4.10 Factors Affecting Resistance
4.10.4 Material
a) The material also affects the resistance of a conductor by a fixed
amount for different materials. This is known as resistivity ().
R = L = constant of proportionality
A Unit: ohm meter m
= Rd 2 (For a wire with circular cross-sectional area)
4L
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CHAPTER 4: Electricity
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Answer the question.
ACTIVITY
When there is an electric current passing through a wire, the particles moving are _____________.
Quiz
protons
electrons
atoms
ions
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CHAPTER 4: Electricity
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SUMMARY
An electric CURRENT exists whenever electric charge flows through a region,
e.g., a simple light bulb circuit.
Electromotive force (emf) or Voltage (V) Is the voltage produced by the
energy supplied in an electrical circuit.
There are TWO types of current: Direct Current (DC) and Alternating Current
(AC).
Band’s theory is an extension of molecular orbital theory that describes
bonding in solids.
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CHAPTER 4: Electricity
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SUMMARY
Ohm’s Law states The magnitude of the electric current that flows through a
closed circuit depends directly on the voltage between the battery
terminals and inversely to the circuit resistance.
RESISTIVITY is actually when electric charge flows through a circuit it
encounters electrical RESISTANCE.
Resistance depends on temperature, material used, length, and cross
sectional area.
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CHAPTER 4: Electricity
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NEXT SESSION PREVIEW
CHAPTER 5: ELECTROMAGNETISM
In chapter 5, students will be taught the basic concept and theory
of electromagnetism.
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CHAPTER 4: Electricity
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4.12 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 4: Electricity
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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
Figure 6 http://www.petervaldivia.com/technology/energy/image/potencial-and-
kinetic.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/S-
b2PZfOXZI/AAAAAAAAADk/KKXoueyon2I/s1600/One-balanced-rock.jpg