7 Static Dynamic Electric

8/12/2019 7 Static Dynamic Electric

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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?

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7 Static Dynamic Electric

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