1. Electric Charge
Chapter 17-1 page 628

2. 17-1 Objectives Understand basic properties of electric charge
Differentiate between conductors and insulators

3. Properties of electric charge
What happens when you run a comb through your hair on a dry day?
What about when you rub your hair with a balloon?
What if we were to take that charged balloon and hold it against the wall?

4. When materials behave this way …
We call it electric charge
Why does it work best on dry days?
Because excessive moisture can provide a pathway for charge to leak off a charged object

5. There are two kinds of electric charge
Like charges repel
Unlike charges attract
Opposites attract
Benjamin Franklin ( ) named the two different kinds of charge positive and negative

6. So,
When you run a balloon against your hair, the charge on your hair is positive and the charge on the balloon is negative
And, opposites attract
When there are equal amounts of positive and negative charge, then there is no net charge

7. What about electrostatic painting?
Electrostatic painting uses the principles of attraction
Paint is given a negative charge
The object to be painted is positive
So, opposites attract and more paint hits the object

8. How about cosmetics? Some cosmetic products contain chitin
Organic; found in crab, lobster, and butterflies
Chitin is positively charged
Human skin is naturally slightly negatively charged
So, it helps makeup stick

9. Electric charge can be transferred
How does all this happen?
Think back to biology and chemistry …

10. Subatomic Particles Protons Neutrons Electrons
Positively charged; located in the nucleus
Neutrons
Neutrally charged; located in the nucleus
Electrons
Negatively charged; located outside nucleus & move around it
Easily transferred
When # protons = # electrons, then there is no net charge
When electrons are transferred, the atom gaining the electron becomes negatively charged and the atom loosing the electron becomes positively charged
Charged atoms are called ions

11. Transferring charge
Charge has a natural tendency to be transferred between unlike materials
Like a balloon and hair
Human hair and wool (like when you where a hat)

12. Electric charge is conserved
Charge is not created nor destroyed – the charge is just transferred from one object to another

13. Electric charge is quantized
1909 – Robert Millikan
Experiment at University of Chicago
Oil droplets between metal plates
Discovered that the charge of a charged object is always some multiple of a unit of charge
This unit: e
Electrons have a – e; protons have a + e
e = x C
C = coulomb (unit for electric charge)

14. Transfer of electric charge
We can classify substances in terms of their ability to transfer electric charge
Conductors:
Materials in which electric charge moves freely
Copper, aluminum
Insulators:
Materials in which electric charge does not move freely
Glass, rubber, silk, and plastic

15. Bring your book tomorrow