elementary physics ii
TRANSCRIPT
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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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21-1 Electric Charge
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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21-1 Electric Charge
Properties of charge
Dichotomy property
Electric charge is either positive ornegative
Like charges repel, opposite charges
attract
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21-1 Electric Charge
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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21-2 Conductors, Insulators andInduced Charges
Ways of Charging
Rubbing
Polarization
Contact Induction
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21-2 Conductors, Insulators andInduced Charges
Charging by rubbing There is transfer of electrons
Charge of charger changes
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21-2 Conductors, Insulators andInduced Charges
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21-2 Conductors, Insulators andInduced Charges
Charging by polarization Charge of charger does NOT change
Charged objects can still attract neutral object
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21-2 Conductors, Insulators andInduced Charges
Charging by contact There is transfer of electrons
Charge of charger changes
+
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+ ++
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+
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+ + +-
2Q 0
+
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+ + +
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1Q 1Q
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21-2 Conductors, Insulators andInduced Charges
Charging by induction (no grounding) Charge of charger does NOT change
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21-2 Conductors, Insulators andInduced Charges
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.