1. Electric Forces and Fields

2. Coulomb’s Law
Electric force – the force of attraction or repulsion between objects due to charge
Electric force makes solids feel solid
Electric force holds atoms together
Like all forces it has units of Newtons (N)
Electric force depends on charge and distance
The closer two charges are the greater the force between them

3. Coulomb’s Law
Coulomb’s law relates electric force, charge, and distance
Named in honor of Charles Coulomb
Uses a constant called Coulomb’s constant (kC)
Electric force = coulomb’s constant * (charge 1) * (charge 2) / (distance)2
Felectric = kCq1q2/r2
kC has a value of *109Nm2/C2
We will use 8.99*109Nm2/C2 for our calculations

4. Coulomb’s Law Electric force is a field force like gravitational force
The equations are very similar
However while gravitational force is always attractive, electric force is either attractive or repulsive
Electric force is much stronger
A small electric force can overcome the gravitational force

5. Coulomb’s Law Often more than two charges are present
Find the individual charges using Coulomb’s law
Use the Superposition principle – use vector addition (right triangle functions and Pythagorean theorem) to find net electric force

6. Coulomb’s Law

7. Coulomb’s Law
Equilibrium position – the location at which the net electric force on a charge is zero
Find charges using Coulomb’s law and set as equal
Solve for the distance between one charge and the equilibrium position

8. Coulomb’s Law
Three charges lie along the x-axis. One positive charge, q1 = 15μC, is at x=2.0m, and another positive charge, q2=6.0μC, is at the origin. At what point on the x-axis must a negative charge, q3, be placed so that the resultant force on it is zero?
Given: q1 = 15μC (15*10-6C) r3,1 = 2.0-P
q2=6.0μC (6.0*10-6C) r3,2 = P

9. Coulomb’s Law q3 must be in the middle between q1 and q2
Set the Felectric between q1 and q3 and between q2 and q3 equal to one another
Use Coulomb’s law to solve for P
F3,1=F3,2
kC(q3q1/(r3,1)2) = kC(q3q2/(r3,2)2)
kC(q3(15*10-6)/(2.0-P)2) = kC(q36.0*10-6/(P)2)
15/(2.0-P)2) = 6.0/ /(P)2)
P2(15) = (2.0-P)2(6.0)
P=.80m from q2

10. Coulomb’s Law
Coulomb was the first person to quantify electric force and established the inverse square law for electric charges
Hence, Coulomb’s law, Coulomb’s constant, and the unit Coulomb
Performed his experiments with a torsion balance of small spheres fixed on the ends of a rod
One sphere was given a charge and brought near another charged object
This caused the rod to rotate
The faster the rotation, the greater the charge

11. Coulomb’s Law