Elementary Physics Ii
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ELEMENTARY PHYSICS II Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
CHAPTER 21 Electric Charge and
Electric Field
Fundamental Forces of Nature Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
• • • •
Gravitational interaction Electromagnetic interaction Strong interaction Weak interaction
Electromagnetic Interactions Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Electromagnetic force holds protons and electrons together in an atom.
Electromagnetic Interactions Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Overview Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
• Electric charge (21-1, 21-2) – Properties • Quantization • Dichotomy • Conservation
– Charging
• Electrostatic interaction (21-3 to 21-7) – Coulumb’s law – Electric field
21-1 Electric Charge Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
•
History… – 600 B.C. • reported by Thales of Miletus • Greeks rubbed amber with wool; then it started to attract objects
– 1600 • William Gilbert in “de Magnete”coined the latin word electricus from (elektron), the Greek word for "amber“
– 1733 • C. F. du Fay’s 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
21-1 Electric Charge Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Properties of charge • Quantization property – Charge is a scalar quantity • SI unit is Coulomb (C)
– Magnitude of charge is a natural unit charge – 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
21-1 Electric Charge Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Properties of charge • Dichotomy property – Electric charge is either positive or negative – Like charges repel, opposite charges attract
21-1 Electric Charge Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Properties of charge • Conservation property – Algebraic sum of all the electric charge in any closed system is constant. – In charging, charge is neither created nor destroyed; it is only transferred from one body to another.
21-2 Conductors, Insulators and Induced Charges
Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
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
21-2 Conductors, Insulators and Induced Charges
Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Ways of Charging – Rubbing – Polarization – Contact – Induction
21-2 Conductors, Insulators and Induced Charges Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Charging by rubbing – There is transfer of electrons – Charge of ‘charger’ changes
21-2 Conductors, Insulators and Induced Charges Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
21-2 Conductors, Insulators and Induced Charges Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Charging by polarization – Charge of ‘charger’ does NOT change – Charged objects can still attract neutral object
21-2 Conductors, Insulators and Induced Charges Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Charging by contact – There is transfer of electrons – Charge of ‘charger’ changes
+ +
+ + -
2Q
0
+ +
+ + 1Q
+ + -
+ + 1Q
21-2 Conductors, Insulators and Induced Charges Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Charging by induction (no grounding) – Charge of ‘charger’ does NOT change
21-2 Conductors, Insulators and Induced Charges
Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Charging by induction (with grounding) – Charge of ‘charger’ does NOT change – Negative charge induces a positive charge (Positive charger induces a negative charge)
Quiz Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
1.
(Easy) Consider two conductors with charges +5 mC and –1 mC. They are placed in contact and then insulated from each other. One of the conductors has a final charge of 1.5 mC. What is the final charge on the other conductor?
2.
(Easy) A positively charged glass rod attracts an unknown object X. A negatively charged plastic rod could also attract object X. What can we conclude about X (in terms of its charge and type of material)?
Quiz Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
3.
(Average) Two uncharged conducting spheres with their conducting surfaces in contact are supported on a large wooden table by insulated stands. A positively charged rod is brought up close to the surface of one of the sphere. a. Describe the induced charges on the two conducting spheres, and sketch the charge distributions on them. b. The two spheres are separated far apart and the charged rod is removed. Sketch the charge distributions on the separated spheres. c. The rod is removed and the two spheres are separated far apart. Sketch the charge distributions on the separated spheres.
Answers Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
1.
(Easy) Consider two conductors with charges +5 mC and –1 mC. They are placed in contact and then insulated from each other. One of the conductors has a final charge of 1.5 mC. What is the final charge on the other conductor? 2.5 mC
2.
(Easy) A positively charged glass rod attracts an unknown object X. A negatively charged plastic rod could also attract object X. What can we conclude about X (in terms of its charge and type of material)? X has zero net charge and can either be an insulator or conductor.
Answers Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
3.
(Average) Two uncharged conducting spheres with their conducting surfaces in contact are supported on a large wooden table by insulated stands. A positively charged rod is brought up close to the surface of one of the sphere. a. Describe the induced charges on the two conducting spheres, and sketch the charge distributions on them.
b. The two spheres are separated far apart and the charged rod is removed. Sketch the charge distributions on the separated spheres.
CHAPTER 21 Electric Charge and
Electric Field
Fundamental Forces of Nature Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
• • • •
Gravitational interaction Electromagnetic interaction Strong interaction Weak interaction
Electromagnetic Interactions Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Electromagnetic force holds protons and electrons together in an atom.
Electromagnetic Interactions Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Overview Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
• Electric charge (21-1, 21-2) – Properties • Quantization • Dichotomy • Conservation
– Charging
• Electrostatic interaction (21-3 to 21-7) – Coulumb’s law – Electric field
21-1 Electric Charge Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
•
History… – 600 B.C. • reported by Thales of Miletus • Greeks rubbed amber with wool; then it started to attract objects
– 1600 • William Gilbert in “de Magnete”coined the latin word electricus from (elektron), the Greek word for "amber“
– 1733 • C. F. du Fay’s 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
21-1 Electric Charge Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Properties of charge • Quantization property – Charge is a scalar quantity • SI unit is Coulomb (C)
– Magnitude of charge is a natural unit charge – 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
21-1 Electric Charge Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Properties of charge • Dichotomy property – Electric charge is either positive or negative – Like charges repel, opposite charges attract
21-1 Electric Charge Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Properties of charge • Conservation property – Algebraic sum of all the electric charge in any closed system is constant. – In charging, charge is neither created nor destroyed; it is only transferred from one body to another.
21-2 Conductors, Insulators and Induced Charges
Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
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
21-2 Conductors, Insulators and Induced Charges
Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Ways of Charging – Rubbing – Polarization – Contact – Induction
21-2 Conductors, Insulators and Induced Charges Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Charging by rubbing – There is transfer of electrons – Charge of ‘charger’ changes
21-2 Conductors, Insulators and Induced Charges Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
21-2 Conductors, Insulators and Induced Charges Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Charging by polarization – Charge of ‘charger’ does NOT change – Charged objects can still attract neutral object
21-2 Conductors, Insulators and Induced Charges Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Charging by contact – There is transfer of electrons – Charge of ‘charger’ changes
+ +
+ + -
2Q
0
+ +
+ + 1Q
+ + -
+ + 1Q
21-2 Conductors, Insulators and Induced Charges Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Charging by induction (no grounding) – Charge of ‘charger’ does NOT change
21-2 Conductors, Insulators and Induced Charges
Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
Charging by induction (with grounding) – Charge of ‘charger’ does NOT change – Negative charge induces a positive charge (Positive charger induces a negative charge)
Quiz Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
1.
(Easy) Consider two conductors with charges +5 mC and –1 mC. They are placed in contact and then insulated from each other. One of the conductors has a final charge of 1.5 mC. What is the final charge on the other conductor?
2.
(Easy) A positively charged glass rod attracts an unknown object X. A negatively charged plastic rod could also attract object X. What can we conclude about X (in terms of its charge and type of material)?
Quiz Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
3.
(Average) Two uncharged conducting spheres with their conducting surfaces in contact are supported on a large wooden table by insulated stands. A positively charged rod is brought up close to the surface of one of the sphere. a. Describe the induced charges on the two conducting spheres, and sketch the charge distributions on them. b. The two spheres are separated far apart and the charged rod is removed. Sketch the charge distributions on the separated spheres. c. The rod is removed and the two spheres are separated far apart. Sketch the charge distributions on the separated spheres.
Answers Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
1.
(Easy) Consider two conductors with charges +5 mC and –1 mC. They are placed in contact and then insulated from each other. One of the conductors has a final charge of 1.5 mC. What is the final charge on the other conductor? 2.5 mC
2.
(Easy) A positively charged glass rod attracts an unknown object X. A negatively charged plastic rod could also attract object X. What can we conclude about X (in terms of its charge and type of material)? X has zero net charge and can either be an insulator or conductor.
Answers Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics Electromagnetism Optics
3.
(Average) Two uncharged conducting spheres with their conducting surfaces in contact are supported on a large wooden table by insulated stands. A positively charged rod is brought up close to the surface of one of the sphere. a. Describe the induced charges on the two conducting spheres, and sketch the charge distributions on them.
b. The two spheres are separated far apart and the charged rod is removed. Sketch the charge distributions on the separated spheres.
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