1. Electrostatics Notes (670)
Charge!

2. Have you ever walked across the carpet and gotten “shocked” when you touched the doorknob?

3. What about static cling
What about static cling? Have you ever gotten to school only to be embarrassed when someone points out the sock sticking to your back?

4. What’s going on in these cases? Why did they occur?

5. Review: Atomic Structure
3 Basic Particles make up Atoms:
Protons
Neutrons
Electrons

6. Charge Protons & Electrons have a property called electric charge
Protons: positive electric charge (+)
Electrons: negative electric charge (-)
The strength of the positive charge on a proton is the same as the strength of the negative charge on the electron

7. Charge cont. Neutrons do not have charge
Neutrons: neutral
Neutrons must be from Switzerland

8. Particle Charge Summary:
Particles may be charged (positive or negative) or neutral (not charged)
Particle
Charge
Protons
positive (+)
Neutrons
neutral
Electrons
negative (-)

9. How do charged particles behave?
Opposites Attract!

10. In the real world, we rarely deal with individual charged particles
Everyday objects are made of lots of atoms
Most atoms have an equal number of protons and electrons
Therefore, atoms are neutral
Remember, even though atoms are neutral, they are still made of charges

11. What does it mean to say that an object is neutral?
A neutral object has no net (overall) charge
A neutral object has equal amounts of positive and negative charge

12. What does it mean to say that an object is charged?
A charged object has a net charge
A positively charged object has a greater quantity of positive charge than negative charge
A negatively charged object has a greater quantity of negative charge than positive charge

13. Electrons move, Protons don’t!
Protons don’t move!
Protons are very massive. They have too much inertia.
They are in the center of the atom.
Electrons are outside the nucleus.
It is easier to move particles on the perimeter.

14. What do you have to do to make an object positively charged?
You need to take electrons away from the object

15. What do you have to do to make an object negatively charged?
You need to transfer electrons to the object.

16. Basically… Oppositely charged particles attract each other
Ex: Protons (+) and Electrons (-) attract
Particles with the same charge repel each other
Ex: 2 Electrons (-) would repel each other
Ex: 2 Protons (+) would repel each other

17. Continued….. Particles with neutral charge do not interact
Neutrons do not attract or repel each other
Neutrons do not attract or repel electrons or protons

18. How large is the force?
The force between charged objects is calculated using Coulomb’s Law:
k = constant
q = quantity of charge
d = distance (between the centers)

19. Which pair will have the largest force of repulsion?
Pair C has the largest force of repulsion since they have the largest charge B C

20. Which pair will have the largest force of repulsion?
Pair A has the largest force of repulsion since the distance is the smallest B C

21. Factors which affect electrostatic forces:
Quantity of Charge
The larger the quantity of charge, the larger the force
The smaller the charge the charge, the smaller the force
Distance
The larger the distance, the smaller the force
The smaller the distance, the larger the force

22. So, a charged sock can stick to my shirt… Does that mean that my shirt is charged?
Not necessarily
Remember that a neutral object is made up of innumerable positively and negatively charged particles.
A charged object (positive or negative) will be attracted to a neutral object.
We’ll discuss exactly why later…

23. Conductors vs. Insulators
Loosely bound electrons
Allow the flow of electrons
Examples: metals
Insulators:
Tightly bound electrons
Slow the flow of electric charge
Examples: rubber, plastic, and styrofoam

24. So what is happening when you rub a balloon on your head and it becomes charged? Is friction creating charge?
No! Charge cannot be created or destroyed.
Conservation of Charge
Charges are being exchanged…

25. Charging Methods Objects can be charged by Conduction Induction
Friction (triboelectricity)
Polarization
Grounding

26. Conduction
Requires the objects to be in contact (or close enough for a spark)
Electrons are exchanged
Works best from conductor to conductor

27. Induction A charged object charges a neutral conductor without contact
The conductor does make contact with a neutral object (often the ground)
The conductor ends up with a charge opposite that of the charged object brought near

28. Triboelectric (friction)
Two dissimilar materials are brought into contact and charge is exchanged
The contact often involves friction
Works best with insulators

29. More on Triboelectricity
Charge separation occurs when two insulators are rubbed together
One of the insulators is more likely to grab electrons and the other insulator is more likely to donate electrons
CHARGE IS NOT CREATED! Electrons are simply being transferred.

30. The Triboelectric Series
 Electron Donors
(objects that give electrons become positive)
(objects that take electrons become negative)
 Electron Grabbers

31. Triboelectric Charging
You rub a balloon against your hair, and the hair becomes positively charged. This means that
Electrons moved from the balloon to your hair.
Protons moved from the balloon to your hair.
Protons moved from your hair to the balloon.
Electrons moved from your hair to the balloon.
The rubbing destroyed electrons in your hair, leaving it positive.

32. A Triboelectric Sequence
Triboelectric Series
A Triboelectric Sequence
ELECTRON GRABBERS ELECTRON DONORS
Rubber Amber Cotton Silk Cat fur Wool Glass Rabbit fur
If you rub cotton with amber, which becomes positive? A. Amber B. Neither C. Cotton
Which of the following can make glass negative? A. Amber B. Cat fur C. Rabbit fur

33. Polarization
As we said earlier, charged objects can be attracted to neutral objects
Charges can rearrange on neutral objects

34. Neutralizing/Grounding Objects
When a charged object comes in contact with a very large, neutral conductor, the object becomes neutralized.
Earth itself is a large, neutral conductor, so it neutralizes charged objects quite well.

35. Review
Charged objects exert forces: Like repels like Opposites attract Charged objects and neutral objects attract
How objects get charged or neutralized:
Conduction
Induction
Friction/Triboelectric
Polarization
Grounding

36. The End!
HOORAY!!!