1. Electricity & Magnetism
Lecture 4: Coulomb’s Law

2. Summary: Lecture 3 Newtons Laws of motion Vectors Units
1. if F = 0, a = 0
2. F = m a (N.B. F, a are vectors)
3. F12=-F21
Vectors
Vectors & Scalars
Vector addition Geometric
Unit vectors
Vector components & addition of components
Units

3. Today’s Topics
Coulomb’s Law
Vector form of Coulomb’s Law
Coulomb vs Newton
Superposition

4. Coulomb’s Law

5. Coulomb's Torsion Balance
This dial allows you to adjust and measure the torque in the fibre and thus the force restraining the charge
This scale allows you to read the separation of the charges

6. Coulomb’s ExperimentsF
Line Fr-2 r

7. Coulomb's Law Coulomb determined
Force is attractive if charges are opposite sign
Force proportional to the product of the charges q1 and q2 along the lines joing them
Force inversly proportional square of the distance
I.e.
|F12|  |Q1| |Q2| / r122 or
|F12|= k |Q1| |Q2| / r122

8. Coulomb's Law Units of constant can be determined from Coulomb's Law
Colomb (and others since) have determined this constant which (in a vacuum) in SI units is
k = x109 Nm2C-2
k is normally expressed as k = 1/40
where is the permittivity of free space

9. Coulomb's Law

10. Vector form of Coulomb’s Law+ - +
r12 Q2 + Q1

11. Quiz A: FAB=-3FBA B: FAB=-FBA C: 3FAB=-FBA D: FAB=12FBA
Object A has a charge of +2 C and Object B has a charge of +6 C. Which statement is true?
FBA?
A: FAB=-3FBA
B: FAB=-FBA
C: 3FAB=-FBA
D: FAB=12FBA A
+2 C
FAB? B
+6 C

12. Force from many charges
Superposition

13. Force from many charges- Q2 + Q1 - Q3
Force on charge is vector sum of forces from all charges
Principle of superposition + Q4

14. Coulomb vs Newton

15. Coulomb’s Law vs Newton’s Law of Gravity
Attractive or repulsive
1/r2
very strong
only relevant relatively local scales
Always attractive
1/r2
very weak
important on very large scales, planets, the Universe
Two spheres

16. Summary: Lecture 4 Coulomb's Law Coulomb’s Law (vector) form
Electrostatic Force between charges
Coulomb’s Law (vector) form
Coulomb force vs Gravity
Electrostatic force is in general much stronger
Superposition