Chap 18 Lec1 Pithbal Jan08

7/31/2019 Chap 18 Lec1 Pithbal Jan08

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Electricity Lecture Series

email: [email protected]; [email protected]

http://www3.uitm.edu.my/staff/drjj/0193551621

Applied Sciences Education Research Group(ASERG)

Faculty of Applied SciencesUniversiti Teknologi MARA

Charges & Charging


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Electric Charges

1. Explain the gravitational forces acting on anyobject.

2. Mathematically represent the gravitational forceand describe its impact on physical events.

3. Describe existence of electrical charges in matterits magnitude, mass and its quantization

property.4. Sketch and explain the charging by friction,

contact and induction diagrammatically andapply charge conservation in the charging

process.

At the end of this unit, you will be able to:


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GRAVITATIONAL FORCESGalileo Science: All

objects regardless ofsize, shape or mass willfall at the same rate

Newton extended theprinciple: UniversalGravitational Law: Allobject will attract eachother with force inverselyproportional to square ofdistance

2

12

r

mmGF

212

12

r

mmF

21
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ATOMIC STRUCTURE
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Electric Charges

Matter: made up of atoms

and molecules

Charged object:

imbalance number ofelectrons & protons

Positively charged

Negatively charged

Atom: made up of nucleus,protons and electrons

Conductors: charges canmove freely

Insulators: charges cannotmove freely


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Electric Charges

Matter: made up of atoms

and molecules

Charged object: imbalance

number of electrons &protons

Positively charged:ve+ve

Atom: made up of nucleus,protons and electrons

Conductors: charges canmove freely

Insulators: charges cannotmove freely


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18.1 The Origin of ElectricityCutnell & Johnson 7E

The electrical nature ofmatter is inherent

in atomic structure.

kg10673.1 27pm

kg10675.1

27

nm

kg1011.931

em

C1060.1 19e

coulombs
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Electric Charges

Charge quantization:charges exist in multiples ofan elementary charge, thecharge of an electron

,..2, eeNeq

where N are the number ofelectrons & the elementarycharge e is

e =1.6 x 10-19 CNumber of charges in 1C??N=q/e =1 C/1.6 x 10-19 C

N = 6.25 x 1018

N Q (x10-19 C)

1

25

10

1.6=e

3.2=2e80=5e

16=10e
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Charges, charging, electrical force & discharging

MatterConductor

Insulator

Atom

Charges

Electron

Proton

Positive

Negative

Attract

Repel

Neutral

Charged

Discharged

Conduction

Induction

Friction

Contact

Ground

Lightning

Force

distance

Highestelectronaffinity

Rubbing

wool torubbercausedrubber tohaveexcess

electronswhich weretransferredfrom rubber


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Charging by Friction


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Charges, charging, electrical force & discharging


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Charging by contact

Bringing the rod near thepithball causes polarization(separation of charges)


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Charging by contact

Bringing the rod near thepithball causes polarization(separation of charges).


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Charging by contact

Bringing the rod near the pithball causespolarization (separation of charges). Touching therod will allow electrons to flow to the rod. The

rod remains positively charged since the numberof electrons transferred is far too small toneutralize the positive charges


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Charging by contact

The pithball is now repelled since itis positively charged after losingelectrons to the rod via contact


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Charging by contact

When the rod is pulled further away, thecharges on the pithball redistributes evenly.The repulsion between the rod and ball issmaller because the rod is far away.


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Charging by contact

The pithball is now neutralized bygrounding (pathway to transfer electronsto the positively charged pithball) it withmy finger.


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Charging by contact

Pithball is polarized (separation of charges) whenthe rod is brought nearer. The electron on thepithball is being repelled by the negativelycharged rod.


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Charging by contact

Pithball is polarized (separation of charges) even morewhen the rod is brought nearer. The electrons on thepithball are being repelled by the negatively charged rod.


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Charging by contact

Pithball is polarized (separation of charges) even morewhen the rod is brought nearer. The electrons on thepithball are being repelled by the negatively charged rod.


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Charging by contact

Electrons move from the rod to the side of the pithballwhich is being touched making the pithball has excesselectrons. The rod remains negatively charged becauseit only lost a small number of electrons


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Charging by contact

Since the rod and the pithball are both negativelycharged, the pithball is being repelled strongly.


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Charging by contact

The repulsion is getting smaller when the rod I pulledfarther away. At the same time, the electrons on thepithball begin to distribute evenly throughout the ball.


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Charging by contact

The ball is being grounded (leaking off the electrons toearth ie finger) to neutralize the pithball.

18 4 Charging by Contact


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18.4Charging by ContactCutnell & Johnson 7E

Charging by contact.

Electrons aretransferred to

the neutralconductingsphere whenthe sphere istouched bythe negativelycharged rod.


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18.4Charging by InductionCutnell & Johnson 7E

Charging by induction.

Charging by induction is a 3-stage process:

1. Bring a charged rod near the sphere to cause polarization of thecharges

2. Ground the side of the sphere which is furthest from the chargingsource.

3. Remove the charging source


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Charging by Induction:

1. Bring negativelycharged rod near thesphere

2. Ground the sphereto remove theelectrons

3. Sphere is positivelycharged

Animation sourcefrom: The Multimedia

Physics Studio

website and ThePhET website


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Charging by Induction: Two Neutral conductingspheres

1. Bring negatively

charged balloonsnear the sphere

2. Pull the secondsphere afterelectrons havemigrated to thesecond sphere.

3. Sphere 1 ispositively chargedand sphere 2 isnegatively charged


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18.2Charged Objects and the Electric ForceCutnell & Johnson 7E

LAW OF CONSERVATION OF ELECTRIC CHARGE

During any process, the net electric charge of an isolated systemremains constant (is conserved). Total number of negative charges

(electrons) and positive charges (protons) must be equal

Consider the fur and rod together as a system. Since the systemis uncharged initially, then the total charge must be zero beforeand after rubbing. Hence if rod acquires 6e due to rubbing(friction), then the fur must have lost 6e, the total charge for the

fur-rod is zero.


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During any process, the net electric charge of an isolated systemremains constant (is conserved). Total number of negative charges(electrons) and positive charges (protons) must be equal


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During any process, the net electric charge of an isolated systemremains constant (is conserved). Total number of negative charges(electrons) and positive charges (protons) must be equal


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Charge Conservation

A

5q

B

-3q

A

2q

B

0q

Aq

Bq

A5q

B-3q

C3q

Shown are conducting sphereseach of charges 5q, -3q and 5q

What is the total charge on the

spheres?Sphere A touches sphere B andthen separated.

What is the total charge after theprocess above, the charge on each

individual sphere?


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Charge Conservation

B

q

C

3q

B

2q

C

2q

Aq

Bq

C3q

Shown are conducting sphereseach of charges 5q, -3q and 5q

Sphere B touches sphere C andthen separated.

What is the total charge after the

process above, the charge on eachindividual sphere?

What is the total charge on thespheres?

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Chap 18 Lec1 Pithbal Jan08