Elementary Physics II

8/14/2019 Elementary Physics II

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Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics

ELEMENTARY PHYSICS II

CHAPTER 21

Electric ChargeandElectric Field


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Fundamental Forces of Nature

Gravitational interaction

Electromagnetic interaction

Strong interaction

Weak interaction


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Electromagnetic Interactions

Electromagnetic force holds protons and

electrons together in an atom.


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Electromagnetic Interactions


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Overview

Electric charge (21-1, 21-2)

Properties Quantization Dichotomy

Conservation

Charging

Electrostatic interaction (21-3 to 21-7)

Coulumbs law Electric field


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21-1 Electric Charge

History 600 B.C.

reported by Thales of Miletus Greeks rubbed amber with wool; then it started to attractobjects

1600 William Gilbert in de Magnetecoined the latin word

electricusfrom (elektron), the Greek word for

"amber 1733

C. F. du Fays two-fluid theory vitreous electricity: charge of glass when rubbed with silk

resinous electricity: charge of amber when rubbed with fur

18th century Benjamin Franklin

identified the term "positive" with vitreous electricity and"negative" with resinous electricity


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Properties of charge Quantization property

Charge is a scalar quantity SI unit is Coulomb (C)

Magnitude of charge is a natural unitcharge

Integer multiple of this basic unit e =1.60217733 x 10-19 C

Charge of proton: +e = 1.60217733 x 10-19 C

Charge of electron: -e = - 1.60217733 x 10-19 C

Neutron = zero net charge


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Properties of charge

Dichotomy property

Electric charge is either positive ornegative

Like charges repel, opposite charges

attract


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Properties of charge

Conservation property

Algebraic sum of all the electric charge in anyclosed system is constant.

In charging, charge is neither created nordestroyed; it is only transferred from one bodyto another.


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21-2 Conductors, Insulators andInduced Charges

Types of materials

Conductors

Permits easy movement of electrons e.g. most metals, water, earth

Insulators

Do NOT permit easy movement of electrons

e.g. most non-metals, air, ceramics, wood

Charges can shift slightly: polarization

Semiconductors

Superconductors


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Ways of Charging

Rubbing

Polarization

Contact Induction


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Charging by rubbing There is transfer of electrons

Charge of charger changes


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Charging by polarization Charge of charger does NOT change

Charged objects can still attract neutral object


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Charging by contact There is transfer of electrons

Charge of charger changes

+

-

+ ++

-

+

-

+ + +-

2Q 0

+

-

+ + +

-

1Q 1Q


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Charging by induction (no grounding) Charge of charger does NOT change


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Charging by induction (with grounding) Charge of charger does NOT change

Negative charge induces a positive charge

(Positive charger induces a negative charge)


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Quiz

1. (Easy) Consider two conductors with charges +5 mCand 1 mC. They are placed in contact and theninsulated from each other. One of the conductors has a

final charge of 1.5 mC. What is the final charge onthe other conductor?

2. (Easy) A positively charged glass rod attracts anunknown object X. A negatively charged plastic rod

could also attract object X. What can we concludeabout X (in terms of its charge and type ofmaterial)?


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Quiz

3. (Average) Two uncharged conducting spheres with theirconducting surfaces in contact are supported on a largewooden table by insulated stands. A positively charged rod isbrought up close to the surface of one of the sphere.

a. Describe the induced charges on the two conductingspheres, and sketch the charge distributions on them.

b. The two spheres are separated far apart and the chargedrod is removed. Sketch the charge distributions on theseparated spheres.

c. The rod is removed and the two spheres are separated farapart. Sketch the charge distributions on the separatedspheres.


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Answers

1. (Easy) Consider two conductors with charges +5 mCand 1 mC. They are placed in contact and theninsulated from each other. One of the conductors has a

final charge of 1.5 mC. What is the final charge onthe other conductor? 2.5 mC

2. (Easy) A positively charged glass rod attracts an

unknown object X. A negatively charged plastic rodcould also attract object X. What can we concludeabout X (in terms of its charge and type ofmaterial)? X has zero net charge and can either bean insulator or conductor.


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Answers

3. (Average) Two uncharged conducting spheres with theirconducting surfaces in contact are supported on a largewooden table by insulated stands. A positively charged rod isbrought up close to the surface of one of the sphere.

a. Describe the induced charges on the two conductingspheres, and sketch the charge distributions on them.

b. The two spheres are separated far apart and the chargedrod is removed. Sketch the charge distributions on theseparated spheres.

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Elementary Physics II