1. Electricity We will learn about 2 aspects of electricity 1)Charged objects apply forces to each other (like when a balloon sticks to a wall) 2)Circuits (submicroscopic flowing charges produce effects that can be measured and observed)

2. Coulomb’s Law

3. Matter is made of particles which are positively or negatively charged. Most objects are neutral meaning they carry the same amount of positive charge as negative charge The unit of charge is the Coulomb (C) Charges are conserved meaning that they cannot be created or destroyed (just transferred from one item to another). It is thought that the total charge of the entire universe is constant and neutral.

4. Examples of electrostatic charges in everyday life Rubbing your feet on carpet then touching a doorknob Pulling your clothes out of the dryer and your sock sticks to your sweater Touching a car door handle in the winter

5. http://physics.wfu.edu/demolabs/demos/avimov /e_and_m/electrostatic_yardstick/staticyrdstick. MPG http://physics.wfu.edu/demolabs/demos/avimov /e_and_m/electrostatic_yardstick/staticyrdstick. MPG https://www.youtube.com/watch?v=4UnggxPVC Yw https://www.youtube.com/watch?v=4UnggxPVC Yw

6. The smallest unit of charge is called the elementary charge which is equal to the charge on one proton (+) or one electron (-).

7. Coulomb determined that the force between two charged objects is proportional to their charges and inversely proportional to the square of their distances or: Where:q 1 = 1 st charge (C) q 2 = 2 nd charge (C) r = distance between charges (m) k = Coulomb’s Constant = 9.0x10 9 Nm 2 /C 2 F E = kq 1 q 2 r 2

8. There are two important things to notice from this equation. First, this equation is quite similar to Newton’s Law of Universal Gravitation: F g = Gm 1 m 2 r 2 Second, electrostatic forces are much stronger than gravitational forces. Notice the huge difference between G and k.

9. There is a very important fundamental difference between gravitational and electrostatic forces: Gravity ALWAYS attracts objects, Electrostatic forces can attract OR repel. When solving for electrostatic force we will NOT use the + or – signs of the charges. Instead we will determine the direction of the force based on the charges.

10. Example: Two 85 kg students are 1.0 m apart. What is the gravitational force between them? If these two students each have a charge of 2.0x10 -3 C, what is the electrostatic force between them?

11. Example: Two point charges of 1.8x10 -6 C and 2.4x10 - 6 C produce a force of 2.2x10 -3 N on each other. How far apart are these two charges?

12. If r is doubled then F is : If q 1 is doubled then F is : If q 1 and q 2 are doubled and r is halved then F is : ¼ of F 2F 16F Two charges are separated by a distance r and have a force F on each other. q1q1 q2q2 r F F Example:

13. Two 40 gram masses each with a charge of 3μC are placed 50cm apart. Compare the gravitational force between the two masses to the electric force between the two masses. (Ignore the force of the earth on the two masses) 3μC 40g 50cm 3μC 40g

14. The electric force is much greater than the gravitational force

15. Example: A charge of 1.7x10 -6 C is placed 2.0x10 -2 m from a charge of 2.5x10 -6 C and 3.5x10 -2 m from a charge of -2.0x10 -6 as shown. What is the net electric force on the 1.7x10 -6 charge? 3.5x10 -2 m 1.7x10 -6 C2.5x10 -6 C-2.0x10 -6 C 2.0x10 -2 m