1. (Neutral objects are attracted to charged objects)
The “Franklin” Era
Kite & Lightning Experiment: Franklin was able to prove that lightning was a discharge of static electricity
Most of Franklin’s research focused on glass and amber rods:
Rubbed amber + rubbed amber = repulsion
Rubbed glass + rubbed glass = repulsion
Rubbed glass + rubbed amber = attraction
(Neutral objects are attracted to charged objects)
From this he determined that each object must have different charges – he chose to distinguish them as POSITIVE & NEGATIVE charges.

2. *Why is there more static electricity in the winter?
Most of the charge “leaks off” to water molecules in the air
Remember, water is a polar molecule, which means one end is more negative and the other is more positive.
The positive end can temporarily pick up electrons.
This is why there is more static electricity in the winter.
The air is drier, so the electrons aren’t picked up as often.

3. Electrical Interactions
Unit B: Forces & Fields
Electrical Interactions

4. Outcomes:
You will explain electrical interactions in terms of the law of conservation of charge
You will explain electrical interactions in terms of the repulsion and attraction of charges
You will compare the methods of transferring charge (conduction and induction)
You will explain, qualitatively, the distribution of charge on the surfaces of conductors and insulators

5. Electrical Interactions
Electrostatics: is the study of electrical charges that can be collected and held in one place (“static electricity” – electricity at rest)

6. Who “Discovered” Electricity?
The ancient Greeks realized that sometimes objects would attract one another and sometimes they would repel one another
This later developed into the idea that there were two types of charges:
Like charges, which repel
Opposite charges, which attract
This phenomena is known as: THE LAW OF CHARGES

7. Modern Theories
LAW OF CONSERVATION OF CHARGE: The net amount of charge produced in any process is zero.
*Even though charges can be moved, they are neither created nor destroyed!!!*

8. Conductivity
Conductivity: is a way of describing how easily charges can move through a material
There are four types:
In conductors, the electrons furthest away from the nucleus in the outer levels (valence electrons) are not attracted as strongly by the nucleus of the atom.
In insulators the electrons are tightly bound to the nucleus and don’t move as freely.

9. Separation of Charge
This explains why neutral objects are attracted to charged objects:
When a charged object is brought close to a conductor, the free electrons in the conductor will move either away or towards the object depending on the charge of the other object.
NOTE: There are still equal numbers of negative and positive charges that cancel each other out, but they are separate from each other.

10. Separation of Charge

11. Charging Objects There are three ways to charge objects:
Friction (a.k.a. Abduction)
Induction
Conduction

12. Charging through Friction
This is the simplest way to charge an object.
If two objects are made of different materials, their atoms will hold onto their electrons with different strengths.
As they pass over each other the electrons with weaker bonds are “ripped” off one material and collect on the other material.

13. Charging through Friction
Example: Determine the type of charge on a piece of copper if you rub it with a piece of silk.

14. Charging by Conduction
Conduction means that two objects come into actual physical contact with each other and an exchange of electrons from one object to the other takes place

15. Law of Conservation of Charges & Conduction
Example 1: You have two similar objects; one of them has a charge of +7, and the other has a charge of -3. They touch, share their charges, and then are moved apart. Determine the final charge of each of the objects.
= +4 (This is the total net charge between the two objects.)
+4 / 2 = +2 (Since the charge is shared between the two objects, each is +2)

16. Charging by Induction
It is possible to charge a conductor without coming into direct contact with it. You do have to follow some special procedures.
The most important one is the use of a grounding wire.
A grounding wire is simply a conductor that connects the object to the ground.
Think of the earth as a huge reservoir of charge… it can either take or donate electrons as needed. Depending on what the situation is, electrons will either travel up the grounding wire to the object being charged, or travel down to the ground.

17. Charging by Induction

18. Electric Charge Distribution
When electrons are added to an insulator they behave differently than if they are added to a conductor.
Insulators: If electrons transfer to an insulator, the electrons will remain at the point where they were added. This is because insulators do not allow electrons to move about.
Conductors: When electrons are added to a conductor, the electrons will repel each other and will move as far away from each other as possible. This means they will end up on the surface of the conductor. If this conductor is spherical, the charge distribution will be uniform over the entire conductor surface.

19. What if the conductor is not spherical?
Then, the greater the curvature the greater the concentration of charge.

20. And finally…
If the electrons are removed from a conductor, there will be excess positive charges. These excess positive charges are distributed on the surface of the conductor in the same way as the excess electrons.

21. Homework
Concept Q’s: p. 523 #1 – 6, 8 – 12