1. Electric Force

2. Goal of the class
To calculate electric force using Coulomb’s law and it’s relationship to the gravitational force.
Question of the Day: Which is stronger, gravity or the electromagnetic force?
Previous answer: He discovered that electric charge was quantised. (e=1.6x10-19 C)
Previous question: What did Millikan's oil drop experiment discover?

3. Question
Electric forces and gravitational forces are both field forces. Two charged particles would feel the effects of both fields. Imagine two electrons attracting each other due to the gravitational force and repelling each other due to the electrostatic force.
Which force is greater?
Is one slightly greater or much greater than the other, or are they about the same?
What evidence exists to support your answer?

4. Coulomb’s Law
The force between two charged particles depends on the amount of charge and on the distance between them.
Force has a direct relationship with both charges.
Force has an inverse square relationship with distance.
k = 1/(4πε0)

5. Coulomb’s Law
Use the known units for q, r, and F to determine the units of k.
Coulomb’s constant, k = 8.99 x 109 N·m2/C2
The distance (r) is measured from center to center for spherical charge distributions.
k = 1/(4πε0)

6. Practice
The electron and proton in a hydrogen atom are separated, on the average, a distance of about 5.3  m.
Find the magnitude of both the gravitational force and the electric force acting between them.
Answer: Fe = 8.2  10-8 N, Fg = 3.6  N
The electric force is more than 1039 times greater than the gravitational force.
Atoms and molecules are held together by electric forces. Gravity has little effect.

7. Practice
A balloon is rubbed against a small piece of wool and receives a charge of C while the wool receives an equal positive charge. Assume the charges are located at a single point on each object and they are 3.0 cm apart. What is the force between the balloon and wool?
Answer: 3.6 N attractive

8. Superposition Principle
The net force on a charged object is the sum of all of the forces due to other charged objects.
Charge q3 shown has two forces acting on it.
q2 pulls to the left.
q1 pushes up and to the right.
The vector sum is shown in the lower diagram.

9. Practice
Two charges, q1 and q2, lie on the x-axis. The first charge is at the origin and the second charge is at x = 1.0 m. Determine the force on a third charge, q3, placed at x = 0.75 m. The charges are as follows:
q1 = +10.0C , q2 = +7.5C, q3 = -5.0C
Answer: Fleft = 0.80 N and Fright= 5.4 N, so Fnet = 4.6 N to the right

10. Electric Force Like gravity, the electric force is a field force.
Similarities
Both forces are related to distance in the same way.
Differences
Two types of charge and only one type of mass
Electric forces can attract or repel while gravity only attracts.
Electric forces are far stronger than gravitational forces.
Talk about electron microscopes

11. Coulomb’s Apparatus
Coulomb developed his law using a torsion balance like that shown.
He measured the force between the two charged spheres by the amount of twisting in the wire.
This is why people get electrocuted.
It’s also why wires are coated with plastic so they don’t conduct.

12. Answer
Electric forces and gravitational forces are both field forces. Two charged particles would feel the effects of both fields. Imagine two electrons attracting each other due to the gravitational force and repelling each other due to the electrostatic force.
Which force is greater?
Is one slightly greater or much greater than the other, or are they about the same?
What evidence exists to support your answer?
The electrical force of repulsion would completely overwhelm the gravitational force of attraction. It is significantly greater. Students may cite the Classroom Practice Problem (from slide 4) as evidence. The gravitational force is insignificant at the atomic level; atoms and molecules are held together by electric forces.

13. Homework
Please complete questions on page 571
Q 10, 15, 18,19, 21