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Physics 2102 Lecture 04: WED 03 SEP
Jonathan Dowling Benjamin Franklin (1705–1790) Physics Lecture 04: WED 03 SEP Electric Charge I Charles-Augustin de Coulomb (1736–1806) Version: 5/6/2018
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Let’s Get Started! Electric Charges…
Two Types of Charges: Positive/Negative Like Charges Repel Opposite Charges Attract Atomic Structure: Negative Electron Cloud Nucleus of Positive Protons, Uncharged Neutrons The Unit of Electric Charge is the “Coulomb” which is “C”. Proton Charge: e = 1.60 × 10–19 C
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Electrical Insulators
Rules of Electric Attraction and Repulsion Discovered by Benjamin Franklin: Electrical Insulators Benjamin Franklin (1705–1790)
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Rules of Electric Attraction and Repulsion Discovered by Benjamin Franklin:
Electric Conductors Benjamin Franklin (1705–1790)
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Rules of Electric Attraction and Repulsion: ICPP
Benjamin Franklin (1705–1790) C and D attract B and D attract
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Force Between Pairs of Point Charges: Coulomb’s Law
Charles-Augustin De Coulomb (1736–1806) Coulomb’s Law — the Force Between Point Charges: Lies Along the Line Connecting the Charges. Is Proportional to the Product of the Magnitudes. Is Inversely Proportional to the Distance Squared. Note That Newton’s Third Law Says |F12| = |F21|!!
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Force Between Pairs of Point Charges: ICPP
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Coulomb’s Law The “k” is the electric constant of proportionality.
Usually, we write: Units: F = [N] = [Newton]; r = [m] = [meter]; q = [C] = [Coulomb]
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Coulomb’s Law: ICCP a > c > b less
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Coulomb’s Torsion Balance Experiment For Electric Force Identical to Cavendish’s Experiment For Gravitational Force! The experiment measures “k” the electric constant of proportionality and confirms inverse square law.
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Two Inverse Square Laws
Newton’s Law of Gravitational Force Coulomb’s Law of Electrical Force Area of Sphere = 4πr2 Number of Lines of Force is Constant. Hence #Force Lines Per-Unit-Area is Proportional to 1/r2
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Superposition Question: How Do We Figure Out the Force on a Point Charge Due to Many Other Point Charges? Answer: Consider One Pair at a Time, Calculate the Force (a Vector!) In Each Case Using Coulomb’s Law and Finally Add All the Vectors! (“Superposition”) Useful To Look Out for SYMMETRY to Simplify Calculations!
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Feel the Force! Example Fnet
d q1 q2 q3 q1= q2= q3= 20 mC d = 1.0 cm Three Equal Charges Form an Equilateral Triangle of Side 1.5 m as Shown Compute the Force on q1 ICPP: What are the Forces on the Other Charges? d 1 2 3 y x Fnet θ Solution: Set up a Coordinate System, Compute Vector Sum of F12 and F13
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Fnet Feel the Force! Example q1= q2= q3= 20 mC d = 1.0 cm θ
y x Fnet θ ICPP: What are the magnitudes and directions of the forces on 2 and 3?
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Another Example With Symmetry
All Forces Cancel Except From +2q! F Charge +q Placed at Center +q What is the Force on Central Particle?
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