Electrostatics

1. Electrostatics Physics

2. Electric Charge • Term “Charge” has new meaning from when it was first defined. • Charge Carriers are… • Electrons, protons, ions • Charge is quantized • Indivisible discreet amounts of charge carried by electrons / protons. • Units • Coulomb; 1 Coulomb = 6.25 x 1018 electrons • Charge of electron is 1.6 x 10-19 C • Law of Conservation of Charge: • Charge cannot be destroyed nor created, only transferred from one object to another.

3. Transfer of Charge • Conductors: Atoms / Molecules with several loosely bound electrons in outer energy levels • Ex’s: Gold, Silver, Copper, Aluminum, … • Insulators: Atoms / Molecules whose electrons are more “tightly” bound to the nucleus. • Ex’s: Hydrocarbons (plastic, acrylic, rubber, ..), Air, Wood • Semiconductors: Atoms / Molecules that have one loosely bound electron for every few atoms • Transistors: Act as switches; by applying a small electric field – more or less electrons enter the material making it either a conductor (on) or insulator (off).

4. Semiconductor doping

5. 3 ways objects get “Charged” • Charging by Contact / Friction: • Electrons are “scraped” off one material and adhere to another (happens with insulators) • Poor John Travolta • Tape repel • Charging by Induction: • A charged object is used to separate the charges in a nearby conductor. Then the opposite side of the conductor is “grounded” • Charge Polarization • A charged object is held near another object and “polarizes” the atoms / molecules in the other object.

6. GROSS. • How do cockroaches climb walls?

7. What is the purpose of hair conditioner?

8. Electric Force: Coulomb’s Law • Fe = k (q1 q2 ) / r2 • k = 9 x 109 N m2/C2 • Ex: • Magnitude of electric force compared to gravity. • Calculate the electric force between an electron and proton in a Hydrogen atom, r = 5 x 10-12 m. Electron mass = 9.11 x 10-31 kg, Proton mass = 1.67 x 10-27 kg. • Fe = • Fg = Reminder: G=6.67e-11

9. Tinsel Levitation • Determine the gravitational force pulling down on a strand of tinsel. • By using the induction method, charge a pie tin positively and DROP a loop of tinsel onto the pie tin and observe what happens. • Why does the tinsel attract to the pie tin and then repel? • Approximate the height above the pie tine that the tinsel levitates. • What is the net force on the tinsel? • What is the Electric Force supporting the tinsel? • By assuming the charge on the tinsel and the charge on the pie tine to be the same, what the amount of charge on each?

10. Electric Field • Definition: an area of “space” in which if a charged object were placed in, it would experience a force. • Magnitude: • E = Fe / qtest • (Honors) E = k q1 / r2 • Direction: the direction of the field is taken to be the direction of the force felt by a positive test charge. • Shielding: Inside of a conductor, the electric field is always zero! • Ex’s:

11. “Anatomy of a Lightning Bolt” • Rising Air, Water Droplets, and / or Ice crystals gain or loose electrons leaving the bottom of the cloud typically (-). • The bottom of the cloud induces a charge on the surface of the ground below. • “Leaders” branch out from the top of the cloud, the base of the cloud and from the ground. • When the “leaders” connect, the air ionizes and becomes a good conductor. • The Ionized air then allows for an avalanche of electrons to flow from (-) to (+). • Tall, pointed objects on the ground prevent charge from building up • If a lightning is produced it typically strikes tall, pointed objects (trees, poles, buildings,..) • To be safe, stay away from tall objects, crouch down into a “ball”

12. Electric Potential Energy • PEe = q E d • Similar to gravitational PE (mgh) • Assumes Electric field is constant through the distance, d. • (Honors) If distance d is considerable as compared to the size of the object that is establishing the field, then use other PE equation! • (Honors) PEe = k (q1 q2) / r

13. Electric Potential • Electric Potential = Electric Potential Energy per Coulomb of charge • V = PEe / q • Honors: V = k q / r • NOTE: PE and V are scalars not vectors! • Ex: 120 Volts means that there is 120 Joules of energy for every Coulomb of charge present. • Ex’s

14. Capacitance • Capacitors store PEe by separating positive and negative charge by a small distance. • The smaller the separation distance the more energy they can store per charge. • The larger the surface area the more charge they are able to separate. • Capacitance is used as a description of how much charge a “capacitor” can separate per Volt applied to it. • C = Q / V (Honors) • Ex’s • Net Charge of a capacitor is always ZERO – They store SEPARATED charge (equal amounts of + and -)

15. Capacitors come in many sizes

16. Warm-up 1. Consider two objects with the following initial conditions.  The charge on each is +q, the distance between them is R, and the electric force acting between them is F. If the distance between the objects were halved, the force between them would be what? 2.If the charge on both objects were doubled and the distance between the objects were quartered, the force between them would be what? 3. What is a capacitor? 4. What is voltage? What does voltage measure?

17. Batteries • Act as an “electric pump” – They do not store charge!!! • Because of the different affinity for electrons between different metals, charge will move from one metal to another. • This movement is of electrons and ions – so there needs to be a medium in between the metals that allows for the movement on ions (electrolytes, acids, etc..)

18. https://www.khanacademy.org/partner-content/mit-k12/mit-k12-physics/v/the-invention-of-the-batteryhttps://www.khanacademy.org/partner-content/mit-k12/mit-k12-physics/v/the-invention-of-the-battery