1. 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

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. Electrical Insulators
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

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.

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. Feel the Force! Example Fnetd 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

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

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