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Electric Charges and Coulomb's Law in Physics

Learn about electric charges, Coulomb's Law, and forces between charges in this physics lecture. Discover the principles of attraction and repulsion, atomic structure, and the concept of Coulombs. Explore experiments and calculations using superposition and symmetry to simplify complex problems.

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Electric Charges and Coulomb's Law in Physics

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  1. Physics 2113 Jonathan Dowling Physics 2102 Lecture 04: FRI 23 JAN Benjamin Franklin (1705–1790) Electric Charge I Charles-Augustin de Coulomb (1736–1806) Version: 1/1/2020

  2. 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

  3. Rules of Electric Attraction and Repulsion Discovered by Benjamin Franklin: Electrical Insulators Benjamin Franklin (1705–1790)

  4. Rules of Electric Attraction and Repulsion Discovered by Benjamin Franklin: Electric Conductors Benjamin Franklin (1705–1790)

  5. Rules of Electric Attraction and Repulsion: ICPP Benjamin Franklin (1705–1790) C and D attract B and D attract

  6. 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|!!

  7. Force Between Pairs of Point Charges: ICPP (a) (b) (c)

  8. Coulomb’s Law The “k” is the electric constant of proportionality. Usually, we write: Units: F = [N] = [Newton]; r = [m] = [meter]; q = [C] = [Coulomb]

  9. Coulomb’s Law: ICCP • a > c > b • less

  10. 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. http://www.dnatube.com/video/11874/Application-Of-Coulombs-Torsion-Balance

  11. 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

  12. 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!

  13. q1= q2= q3= 20 mC d = 1.0 cm q1 d 1 d d q3 d 3 d q2 d y 2 x Feel the Force! Example • 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? Fnet θ Solution: Set up a Coordinate System, Compute Vector Sum of F12 and F13

  14. 1 d d 3 d y 2 x q1= q2= q3= 20 mC d = 1.0 cm Feel the Force! Example θ Fnet ICPP: What are the magnitudes and directions of the forces on 2 and 3?

  15. r F All Forces Cancel Except From +2q! Another Example With Symmetry Charge +q Placed at Center +q What is the Force on Central Particle?

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