Properties of Electric Charges, Electric Force,

8/3/2019 Properties of Electric Charges, Electric Force,

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Robertson A. LabanRobertson A. Laban

PhysicsPhysics DepartmentDepartmentAdamson UniversityAdamson University


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ELECTROMAGNETISMELECTROMAGNETISM

The study of the forces acting between charges inmotion.

Physics that deals with the relationship between

electricity and magnetism. One of the fundamental forces of nature along with

thegravitational force,strong nuclear force, andweak nuclear force.


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Electricity Phenomenon resulting

from the presence andflow of electric charge.

Magnetism

Phenomenonexhibited by magnets.


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ELECTRICITYELECTRICITY

Study of charges nearly at rest.

Electrostatics

Electrodynamics Study of charges in motion.


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ELECTRIC CHARGEELECTRIC CHARGE

Fundamental quantity of electrostatics.

Carried by certain subatomic particles, such as protons andelectrons.

First observed by Ancient Greeks,Th

ales of Miletus (600 B.C.),after polishing amber with a piece of wool or fur. After rubbing

the amber, which created a static electric charge, other lightobjects such as straw or feathers stuck to the amber.

The SI unit of charge is Coulomb (C) in honor ofCharles

Augusti

n de Coulomb.


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PROPERTIES OF ELECTRIC CHARGEPROPERTIES OF ELECTRIC CHARGE

1. Charge comes in two and only two types.

Positive charge (+) happens when an atom has greaternumber of proton than electron (cation).

Negative charge (-)

happens when an atom has greaternumber of electron than proton (anion).

2. Charge is conserved

Charge can be separated and can be transferred from onematter to another, but they can neither be created nordestroyed.


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3. Charge is quantized Charge comes in integer multiples with elementary charge, e = 1.6

x10-19

C. Charge of proton (e= 1.6 x10 -19 C)

Charge of electron (e= -1.6 x10 -19 C)

Where,

q= amount of charge (C)

n= number of particles

e = elementary charge (e= 1.6 x10 -19 C)

Unit: Coulomb (C)


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Given:n= 12

e = 1.6 x10 -19 C

Required: q

Solution:

Ex. A carbon nucleus has 12 protons. Calculatethe amount of charge.


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4. Like charges repel, unlike charges attract.


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Atom - basic unit of matterthat consists of a dense, centralnucleus surrounded by a cloudof negatively charged electrons

Mostly empty space At normal condition, the

number of protons equals thenumber of electrons.Therefore, the net charge ofan atom is zero.

Mass of neutron = Mass ofproton = 1.67x10-27 Kg

Mass of electron = 9.1x10-31

Kg

ELECTRIC CHARGE AND STRUCTUREELECTRIC CHARGE AND STRUCTURE

OF MATTEROF MATTER


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CHARGING (IONIZATION OF CHARGE)CHARGING (IONIZATION OF CHARGE)

Process that involves transfer of electron from onebody to another.

The object that loses electrons is said to bepositively chargedwhile the object that gained isnegatively charged.


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1. Charging by Friction(rubbing)

Example: rubbing a glass rod witha silk, the loosely held particle ofglass rod will likely to transfer to

silk. Silk will become negativelycharge since it gains electron,while the glass rod will becomepositively charge due to electronlosses.

2. Charging byConduction

neutrally charged object willgain a charge if it is in directcontact with a charged object.


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Ex. If sphere A has a charge of +5 C andsphere B has a charge of +2C, what is thecharge on each sphere after A is touched to Band then removed?


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1. Conductors

Allows the flow of charges.

Some of the electrons are free electrons that can move freely

from one atom to another. Has fewer than four valence electron (electron on the

outermost shell of an atom)

Example: Copper ( 1 valence electron)

CLASSIFICATION OF MATTER BASED ONCLASSIFICATION OF MATTER BASED ON

ELECTRICAL CONDUCTIVITYELECTRICAL CONDUCTIVITY


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2. Insulators

Resists the flow of charges.

Electron are bound to atoms and cannot move freely throughanother atom.

Have more than four valence electrons.

Example: Phosporus (5 valence electrons)

3. Semiconductors

normally an insulator but may become conductor at certain

condition.

exactly has four valence electron.

Example: Germanium (4 valence electrons)


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ELECTRIC FORCE (FELECTRIC FORCE (Fee ) AND) AND

COULOMBS LAWCOULOMBS LAW The magnitude of the electric force between two point

charges is directly proportional to the product of thecharges and inversely proportional to the square of thedistance between them.


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Unit: Newton (N)

Where,

Fe = electric force

q1 = test charge; experiencing the effect of the other charge/s near it. (C)q2 = charge exerting the force on the test charge (C)

r = radial distance between two charges (m)

k = proportionality constant (k = 9x109 N.m2/C2)

0 = electric permittivity of free space (0 = 8.85x10-12

C2

/N.m2

)


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Ex. In the Bohr model of the hydrogen atom, the

electron is in orbit about the nuclear proton at a radiusof 5.29x10-11m. Determine the speed of the electron,assuming the orbit to be circular.


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Ex. Two charges are locatedon the positive x-axis of acoordinate system 4 cmapart as shown in the

figure where q1 = 3x10-9 Cand q2 = -5x10

-9 C.Calculate the electric forceexperienced by q2 due toq

1

.


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Given:

q1 = 3x10-9 C

q2 = -5x10-9 C

r = 4cm = 0.04 m

k=9x109 N.m2/C2

Required: Fe

Solution:

Fe = 8.4x10-5 N, to the left


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Ex. Two charges are located on the positive x-axis of acoordinate system as shown. Charge q2= 3x10

-9 C and q3=

-5x10-9 C. What is the total force exerted by these twocharges on a charge q1= 2x10

-9 C located at the origin?


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Ex. Given the figure, calculate the net electricalforce of charges q2 = -2x10

-5 C and q3= 4x10-5 C

on q1 =3x10-5 C.


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ELECTRIC FIELD (E)ELECTRIC FIELD (E)

Region or space that surrounds an electric charge.

The electrostatic force experienced by a small test charge placed at thatpoint divided by the charge itself.

Where, q0 = test charge

The electric field does not depend on the test charge.

Unit: Newton per Coulomb (N/C)


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Electric fields cannot be seen by our naked eyes, it canonly be manifested.

Electric Field Lines

used to show the magnitude and the directionof electric field.

emanate in all direction outward from the

positive charge and inward to a negative charge. always begin on a positive charge and end on a

negative charge and do not stop in mid space.

the number of lines leaving a positive charge or

entering a negative charge is proportional to themagnitude of the charge

no two field lines can cross.


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Ex. Consider the situation of the figure below. Wherein the region between the charges where the netelectric field is zero.


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Ex. A 2 C charge is 1x104 m to the right of a -4C charge. Calculate the electric field

(magnitude and direction) at a point 2x104 m tothe right of the positive charge and along linepassing the two charges.

-4 C 2 C

1x104 m 2x104 m

P


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Ex. Four equal magnitude charges (q= 4 C)are placed at the four corners of a square that is20 cm on each side. Find the electric fieldintensity at the center of the square if thecharges are all positive.


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The product of the

magnitude of electric field

Eand surface areaAperpendicular to the field.

Unit: N.m2/ C

ELECTRIC FLUX (ELECTRIC FLUX (EE))


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The electric flux through aGaussian surface is equal to

the net charge enclosed inthat surface divided by the

permittivity of free space.

Where,

qenc= enclosed charge (C)

0 = electric permittivity of freespace (0 = 8.85x0

-12C2/N.m2)

A = Gaussian surface area (m2)

GAUSSSGAUSSS LAWLAW


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Ex. What is the diameter of a sphericalgaussian surface that enclosed a 4.3x10-11

C charge with an electric field of 5 N/C?


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ELECTRIC POTENTIAL ENERGY (ELECTRIC POTENTIAL ENERGY (UUee))

Potential energy associated with coulomb forces.

Work done against the electric force in a movingelectric charge.


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But


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So,

substituting we get,

Unit: Joule (J)


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If the system consists ofmore than two chargedparticles, we can obtain the

total potential energy of thesystem by calculating U forevery pair of charges andsumming the termsalgebraically. For example,

the total potential energy ofthe system of three chargesshown in the figure is:


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Ex. Figure below shows three point charges held infixed positions by forces that are not shown. What is

the electric potential energyUof this system ofcharges? Assume that d= 12 cm and that q1=q,q2= +4q,and q3= -2q, in which q= +150 nC.


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ELECTRIC POTENTIAL/VOLTAGE (V)ELECTRIC POTENTIAL/VOLTAGE (V)

the electric potential energy of a small test charge divided by the chargeitself.

Where, q = test charge

Unit: Joule per Coulomb (J/C) or Volt (V)


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If several charges exist, their net potential difference is:

The work done by an external agent in moving a charge q through an electricfield at constant velocity is:


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ELECTRONELECTRON VOLTVOLT ((eeVV))

unit for energy commonly used in atomic andnuclear physics.

the energy a chargefield system gains or loseswhen a charge of magnitude e (that is, an electronor a proton) is moved through a potential differenceof 1 V.


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Pushes the electrons so theywill f low along the conductor.

Equipotential Surface

A surface in which theelectric potential is thesame everywhere (nopotential difference, no

current will be produced).

The net electric forcedoes no work on a chargeas it moves on anequipotential surface.

POTENTIAL DIFFERENCE /POTENTIAL DIFFERENCE /

ELECTROMOTIVE FORCE (ELECTROMOTIVE FORCE (EMFEMF))


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Ex. In the given figure, the charge at A is +200 pC,while the charge at B is - 100 pC. (a) Find the absolute

potentials at points C and D, (b) How much work mustbe done to transfer a charge of +500C from point C topoint D?


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Ex. As shown in the figure, a charged particle remainsstationary between the two horizontal charged plates.The plate separation is 2.0 cm, and m= 4x10-13 kg andq= 2.4x10-18C for the particle. Find the potentialdifference between the plates.


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END

Properties of Electric Charges, Electric Force,

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