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STATIC ELECTRICS
[ELECTROSTATICS]
Standard Competency:Applies the concept of electricity and magnetism in someproblem solvings and technology products
Base Competency:Formulates the electric force, electric field strength, flux,
electric potential, electrical potential energy and their
applications on parallel plate
Learning Objectives:After completing this chapter, students should be able to
1 Describes electrostatic force (Coulombs law) on pointcharge
2 Applies the Coulombs and Gausss laws to find electricfiled for continuum charge distribution
3 Formulates electrical potential energy and its connection
to electric force/field and elelctric potential4 Formulates the principle work of parallel plate capacitor
References:Textbooks
[1] John D Cutnell and Kenneth W. Johnson (2002). Physics 5thEd
with Compliments. John Wiley and Sons, Inc. pp 586-615[2] Sunardi dan Etsa Indra Irawan (2007). Fisika Bilingual SMA/MA
untuk SMA/MA Kelas X. CV Yrama Widya pp 405-468
Internet
http://www.cliffsnotes.com/WileyCDA/CliffsReviewTopic/Electrostatics.topicArticleId-10453,articleId-10431.html
http://www.physics.sjsu.edu/becker/physics51/introduction.htm
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ELECTROSTATICS
Electrostatics is the physics term for static charge. Electro
means charge, and of course staticmeans stationary or notmoving.
Charge is a property of
matter. Two opposite types
of charge exists, namedpositiveand negative. It is
proposed by Benjamin
Franklin
Franklin tries to prove his founding by set an experiment
below
It was found that
Likecharges repelone another
Unlikecharges attracteach another
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ELECTRIC CHARGE
Charge is conserved. A neutral object has no net charge. If
the plastic rod and fur are initially neutral, when the rodbecomes charged by the fur, a negative charge is
transferred from the fur to the rod.
The net negative charge on the rod is equal to the net
positive charge on the fur.
A conductoris a material through which electric charges
can easily flow. An insulatoris a material through which
electric charges do not move easily, if at all.
An electroscopeis a
simple device used to
indicate the existence
of charge.
CHARGING METAL SPHERE BY INDUCTIONCharges are free to move in a conductor but are tightly bound in an insulator.The earth (ground) is a large conductor having many free charges.
Anelectroscopereports thepresence ofcharge
Charging anelectroscopeby induction
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ATOMIC THEORY SUPPORTS FRANKLINSFOUNDING
What was purposed by BenjaminFranklin, is now supported by the
establisment of Atomic Theories.
Firmly state by Neils-Bohr, it reveals
that all matter consist of chargedparticles, namely proton (positive),
electron (negative) and neutron
(neutral charge particle).
Atoms within molecule show electric
charge properties.
PROTONSare massive and are heldinside the nucleus. They do not
move from place to place in an
object.
ELECTRONSare not as massive and generally can move fromone object to another. This is the way electric charge istransferred from one object to another: one object loses
electrons and the other gains electrons
LITHIUM (Li) ELEMENTAtom: electrically neutral
3 protons and 3 electrons.
Positive ion: missing oneelectron so netcharge is
positiveNegative ion: has added
electron so netcharge is
negative
Protonscarry a +echarge
Electronscarry a echarge
The fundamental unit of electric charge is
e= 1,6 .1019coulomb
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CONDUCTORS: materials that have freely moving electronsthat respond to an electric field.
INSULATORS: materials that have fixed, immobile electrons
that are not easy to move.
Metal ball is charged negatively as shown in A
Copper is a good conductor of electricity; Glass and nylon are
good insulators
COULOMB'S LAW
Forcebetween two charges is given byCoulomb's law. It
gives the magnitude of the electrostatic force (F) between
two charges:
whereq1and q2are the charges,ris the distance between them, and
kis the proportionality constant; k= 9.0 10
9
N m
2
/C
2
The SI unit for charge is the coulomb
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The Purpose of Coulombs Law in Force Calculation
Coulombs Law lets us calculate the FORCE between TWO
ELECTRIC CHARGES.
Examples of Electrostatic Force Problems
Coulombs Law lets us calculate the force between MANY
CHARGES. We calculate the forces one at a time and ADD
them as vectors. This is called superposition.
The force on q3caused by q1and q2.
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Whats make two or more charges repel or attract (being
interaction) each other though they separate apart of
distance r?
Compare with type of interaction where all objects
would attract down to earth!
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A charged body creates an electric field. Coulomb force ofrepulsion between two charged bodies at A and B, (having
charges Qand qorespectively) has magnitude:
2r
qQkF
o=
where we have factored out the small charge qo.
We can write the force in terms of an electric field E:
2r
QkE =
Coulombs Law:
2r
qQkF
o=
Rearranged:
=
2r
QkqF
o
Gives us:
EqF orr
=
where the electric
field Eis:
2
r
QkE =
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Co u l o m b s La w v e c t o r p r o b l e m
Net force on charge Qis the vector sum of the forces by theother two charges.
Electric fieldat point a and
c set up by charges q1and q2
Calculate E1, E2, and ETOTALat points a & c:
Point (a)E1= 3.0 (10)
4N/C
E2 = 6.8 (10)4N/C
Ea = 9.8 (10)4N/C
Point (c)
E1= 6.4 (10)3N/C
E2= 6.4 (10)3N/C
Ec= 4.9 (10)3N/C
(in the +xdirection)
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Electric Charge on a Continous Charge Distribution
Electric fieldatPcaused by a line of charge along the y-axis.
(Consider symmetry! Ey= 0)
Electric Dipole Moment (p)
= r x F
= p x E
Net forceon an
ELECTRIC DIPOLE is
zero, but torque() isinto the page.
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ELECTRIC FIELDS AND LINES OF FORCE
The electric fieldis defined as the force per unit charge
exerted on a small positive test charge (qo) placed at thatpoint.
ELECTRIC FIELD LINES START AND END AT ELECTRIC
CHARGES
GAUSS'S LAW
Gauss's law provides a method to calculate any electric field;
however, its only practical use is for fields of highly symmetricdistributions of fixed charges.
The law states that the net electric flux through any real orimaginary closed surface is equal to the net electric charge
enclosed within that surface divided by . As a result, if no
charge exists with a given closed surface, then there are asmany flux lines entering the surface as there are leaving it.
The imaginary surface necessary to apply Gauss's law is called
the gaussian surface. Algebraically,
or in integral form,
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where is the angle between the direction of Eand theoutward direction of normal to the surface and is the
permittivity constant.
If the electric field is perpendicular to the gaussian surfaceand directed outward, is 90 degrees, and cos = 1.
Gauss's law is
Substitute in the area of a sphere, and the left side reduces to
or
which is the same expression obtained from Coulomb's law
and the definition of electric field in terms of force.
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DYNAMIC ELECTRICS
[ELECTRODINAMICS]
Standard Competency:Applies the concept of electricity and magnetism in someproblem solvings and technology products
Base Competency:- Using electrical measuring device- Formulate any electrical quantities in simple closed-circuit
(single loop)- Identify the application of AC and DC electric in daily life
Learning Objectives:After completing this chapter, students should be able to1 Using voltmeter, ammeter and ohmmeter in electric
circuit2 To formulate quantities of current and resistance in
simple circuit using Ohms law
3 To formulate quantity of voltage using Kirchoffs law4 To identify the application of AC and DC electric in daily
life
References:Textbooks
[1] John D Cutnell and Kenneth W. Johnson (2002). Physics 5th
Ed withCompliments. John Wiley and Sons, Inc. pp 586-615
[2] Sunardi dan Etsa Indra Irawan (2007). Fisika Bilingual SMA/MA untukSMA/MA Kelas X. CV Yrama Widya pp 405-468
[3] Douglas C. Giancolli (1985). Physics: Principles with Applications, 2ndEdition. Prentice Hall, Inc. pp 395-396, 403, 412-416, 424-426
Internethttp://www.kpsec.freeuk.com/index.htm
http://www.physics.sjsu.edu/becker/physics51/overview.htm#Slideshttp://hyperphyiscs.phy-astr.gsu.edu/hbase/electric/ecircon.html
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ELECTRO-DYNAMICStudy of electrical physical properties notably when
electric charges are in flow
[1] Electrical Measuring Device A device used to measure electrical physical
properties, i.e., current, voltage and resistance Familiar such device is known as multimeter or avo-
meter which are available in digital as well as analog.
Digital Multimeter and its readingscale
Analog multimeter and its
reading scale, less readable
Connecting metersIt is important to connect meters the correct way round:
The positive terminalof the meter, marked + or
coloured redshould be connected nearest to + on the
battery or power supply.
The negative terminalof the meter, marked - or
coloured blackshould be connected nearest to - on thebattery or power supply.
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Analogue displayAnalogue displays have a
pointer which moves over a
graduated scale.
For example the scale in the
picture has 10 small divisions
between 0 and 1 so each small division represents 0.1. Thereading is therefore either 1.2 Vor 1.3 V
The maximum reading of an analogue meter is called full-
scale deflectionor FSD
Taking accurate readings of analog display
Correct
reflection hidden
Wrong
reflection visible
Digital displayValues can be read directly from digital
displays so they are easy to read
accurately.
It is normal for the least significant digit (on the right) to
continually change between two or three values, this is afeature of the way digital meters work, not an error! Digitsafter point show precision of a measurment
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[2] Physical Description of Electro-dynamic
Also known as electric quantities, i.e., electric current (I),
voltage (v) and resistance (R)
ELECTRIC CURRENT
Electric current defined as
flow of electric chargeswithin a conductor per unit
time from higher to lower
electric potential
t
qI=
I, electric current (ampere)q, electric charge (coulomb)t, time (second)
CONDUCTOR WITH CURRENT MOVING FROM HIGH ELECTRICAL POTENTIAL(VOLTS) TO LOW POTENTIAL
12 Volts
0 Volts
Analogy of electron motion in a conductor, which is why acurrent could flows. The bars represent resistance of flows
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In microscopive view, electric
current is describe as the flow of
positive charges particles within
closed-circuit passes through anarea
The current through a
cross-section area A is thenet rate (dQ/dt) at which
charge passes through the
area. If the movingcharges are positive the
drift velocity is in the
same direction as the
field, as shown.
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[Potential Different; v(volt) ]
example
V1= -4 VoltsV2= +3 Volts
E = e(V1V2)= -1.6 10-19 (-7)
= 1.1 10-18Joules
Potential Diff Potential Difference Description
70mV the voltage across the inside and outside of a human nerve
1.5v the voltage of a walkman battery
6v the voltage of a moped battery
12v the voltage of a car or motorcycle battery
24v the voltage of a 50 seater coach battery
110v mains voltage in the USA & some continental countries
240v nominal mains voltage in the UK
1000 of volts voltages in amateurs' antennas whilst transmitting
10000 of volts voltages in overhead power cables
VOLTAGE
Defined as quantity to describe electric potential energy asthe work required of an external force to move charge
againts electrical field
qPEV =
PE, change in electric potential energy (joule)q, electric charge (coulomb)V, voltage (joule/coulomb or volt)
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Potential difference: difference electric potential energy valuebetween higher and lower terminal
qWVVV baabba ==
"Electromotive force" or emf () is an external force that driveelectric charges to flow within a closed-circuit. It represents
energy per unit charge (voltage) which has been made
available by the generating mechanism and is not a "force".
Voltage is the Cause, Current is the EffectVoltage attempts to make a current flow, and current will flowif the circuit is complete. Voltage is sometimes described as
the 'push' or 'force' of the electricity, it isn't really a force but
this may help you to imagine what is happening.
Voltage and CurrentThe switch is closed making a
complete circuit so current can
flow.
Voltage but No CurrentThe switch is open so the circuit
is broken and current cannot
flow.
No Voltage and No CurrentWithout the cell there is no
source of voltage so current
cannot flow.
CURRENT-VOLTAGE RELATION
(a) a resistor obeys Ohms Law. Constant slope = 1/R(b) a vacuum tube diode(c) A semiconductor diode
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RESISTANCEOr electric
resistance is an
internal properties
of a conductorwhich opposite the
flow of electric
current. Thisinternal properties
known as resistivity
().
A conductor of Llength and cross sectional surface areaAwill
have a proportional value so-called resistance R
A
LR
A
LR =
In some cases, the value of resistance is depend on
temperature change according to
)1( TRR ot +=
Rt, resistance after being heated or cooledRo, resistance in standard temperature (), temperature coeficient of resistance (Co)1
T, temperature change (oC)
W h y b i r d d o n o t e x p e r i e n ce
a n e l e ct r i c s h o ck w h i l s t s t a yo n a v e r y h i g h v o l t a g e w i r e ? (such wire has a current of 1500
A and has a resistance of 1,8 .105 /m.Birds feet separate in 3 cm away)
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RESISTOR IN SERIES AND PARALLEL
The combination rules for any number of resistors in series or
parallel can be derived with the use three way of Ohm's Law
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ELECTRIC POWERFor a resistor in a D C Circuit, the electric power is given by
the product of applied voltage and the electric current:
P = VI
The details of the units are as follows:
POWER DISSIPATED IN RESISTOR
Convenient expressions for the power dissipated in a resistorcan be obtained by the use of Ohm's Law. These relationships
are valid for AC applications also if the voltages and currents
are rms or effective values.
ELECTRIC (POTENTIAL) ENERGY
Suppose there is a potential difference Vacross this element,and in a time ta charge Qpasses by.
The work Wdone by the electric field in moving this charge
is given by, W= V Q. Thus, the work done per unit time,
or the powerP, is
VIt
QV
t
WP =
=
=
the units of power are J/s, or Watts (W).
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Electrical utilities normally bill on the basis of kilowatt-hours(kWh), which is the amount of energy the consumer has used
in a given time period.
One can convert kWh to J by the following formula:
1 kWh = 1000 Wh
= 1000 J.h/s= 1000 J.h/s x 3600
= 3.6 x 106J
CIRCUIT ENERGY AND POWER
P = VabI = I2R = Vab
2/ R
I= rate of conversion of non-electrical (chemical) energy toelectrical energy within the source
I2r= rate of electrical energy dissipation in the internal
resistance of the source (battery)
I - I2r= the rate at which the source delivers electricalenergy to the load (lamp)
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Kirchhoffs Laws
If complex circuits cannot be reduced to series parallel
combinations.
So use Kirchhoffs Rules:
1.Ij= 0 (junction rule, valid at anyjunction);conservation of charge
2.(Vj)= 0 (loop rule, valid for anyclosed loop);conservation of energy
Use Kirchhoffs junction rule at point a to reduce the number
of unknown BRANCH currents from three to two.
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Exercises
[Electric Current]
A steady current of 2.5 A flows in a wire connected to abattery. After 4 minutes, the current suddenly ceased
because the wire is disconnected. How much chargespassed through the circuit? q = I t = (2.5 C/s )(240 s)
= 600 C
A service station charges a battery using a current of 5.5
A for 6 hours. How much charge passes through the
battery? q = I t = (6 C/s ) (5.5 3600 s) = 118800 C
[Voltage]
An electron in the picture tube of a TV set is acceleratedfrom rest through a potential difference Vba= 5000 volts.
What is the change in potential energy of the electron?
PE= q V = (1.6 x 1019)(5000) = 8.0 x 1016Jminus sign, show that PE decreases
[Resistance]Suppose you want to connect your stereo set to a remotespeaker. If each wire must be 10 m long, what diameter
copper wire (= 1.7 x 108) should you use to keep theresistance less than 0.10 per wire?
At 27oC a wire has resistance of 5.00 and when heatedto 107oC its resistance become 5.08 . Determine itsresistance when the temperature is on 67oC.
[Ohms Law]
What is the resistance of a 14 inch monitor if 220 volts
produces a current of 1.2 A.
A bird stands on an electric transmission line carrying
1200 A. The line has 1.0 x 105resistance per meterand the birds feet are 3.0 cm apart. What voltage does
the bird feel?