Electric Forces and Fields

1. Electric Forces and Fields

2. It all began with a kite! • 1752, Ben Franklin set off a flurry of research in the field of electricity when his famous kite experiment showed that lightening is similar to the sparks caused by friction. As a thunderstorm approached, the loose threads of the kite string began to stand up and repel one another. When Franklin brought his knuckle close to the key, he experienced a spark.

3. Electrical Charges Properties of Electrical Charge • Two types of charge (+) and (-). • Benjamin Franklin was the first to call them positive and negative. • Like charges repel and unlike charges attract. • Charges are conserved. • Charge is quantized.

4. Two Kinds of electric charge(named by Ben Franklin) • Positive • Negative Why + and -? It is a totally arbitrary Designation. Ex. Rub two balloons together! What do you see? The two balloons repel one another! Rub a balloon and your hair! What do you see? The hair and the balloon are attracted to one another!

5. Why does this happen? • We can conclude that “like charges” repel, and “unlike charges” attract! • This is THE fundamental rule of electricity. • These two different kinds of charges are called positive and negative (by arbitrary convention) • Two balloons – “like charges” – so they repel! • Balloon and hair – “unlike charges” – so they attract! • An object that is electrically neutral has equal amounts of + and – and thus a net charge of zero!

6. Fundamental Rule Like charges repel and unlike charges attract

7. Electric Charge is Conserved How does the balloon and you hair become electrically charged? Both the balloon and your hair contain a very large number of neutral atoms (same # of protons and electrons). When you rub the two together, electrons are stripped from your hair to the balloon. Thus, the your hair becomes positively charged (cations formed) and the balloon becomes negatively charged (anions formed). (Charging by Friction) The positive charge on your hair is equal in magnitude to the negative charge on the balloon. Electric charge is conserved: The amount of charge in the universe is constant! (Law of Conservation of Charge)

8. Electric charge is quantized Quantized: This means that something occurs in discrete amounts. (Example: Charge, energy levels on atoms, photons) -e (electron) and +e (proton) have an equal but opposite charge. The value of e has been experimentally determined to be 1.60 x10-19 C, where the coulomb (C) is the SI unit for electric charge. KNOW THIS NUMBER Robert Millikan discovered the fact that charge is quantized (Oil Drop Experiment).

9. Transfer of Electric Charge You classify substances in terms of their ability to transfer electric charges! • Electrical conductors: Stuff in which electrical charges move freely. • Examples: silver, gold, copper and aluminum (most metals are conductors of electricity and heat). • Electrical insulators: Stuff in which electric charges do not move freely. • Examples: glass, rubber, silk, plastic, pure water

10. Semiconductors: Stuff that are somewhere between insulators and conductors. • Examples: silicon and germanium Silicon semiconductor • Superconductors: Stuff that has zero electrical resistance when they are at or below a certain temperature, and can conduct electricity indefinitely without heating. Superconductors are SUPERCOOL... No really. Super cooled superconductor

11. Getting All Charged Up? • Polarization: Charge separation within an object. • An electrically neutral object may have regions of positive and negative charge within it, separated from one another. [We say it is polarized] • A polarized object can experience an electrical force even though its net charge is zero. • Bits of paper are neutral, but an electrically charged rod polarizes the paper (attracts the unlike charges and pushes away the like charges). • We say the paper is polarized by induction.

12. Charging by Friction • When objects are charged by rubbing them against one another ,both electrons and ions (charged atoms) can be transferred from one object to the other. (This works best in dry air). • When humidity is high, moisture condenses on the surfaces of objects; charge can leak off. • Insulators are best charged by friction. In metals its too easy for the charge to move around and avoid getting scraped off. • Once an insulator is charged the charge remains where it is.

13. Charging by Friction (Rubbing) Rub two insulators together to separate the charge

14. Grounding • How can a conductor be discharged? • The Earth is a conductor because of the presence of ions and moisture and is large enough that it can be considered a limitless reservoir of charge. • To ground a conductor means to provide a conducting path between it and the Earth (or to another charge reservoir). • The round opening of modern electrical outlets is called ground. It is literally connected by a conducting wire to the ground.

15. Charging by Induction • Charging by induction is essentially charging an object by bringing another charged object nearby. • Charge a rod. • Hold near a conductor. • Ground the conductor. • Excess electrons leave. • Remove ground.

16. Gravity vs. Electrostatic Force • Electrostatic force wins by A LONG SHOT! • Gravitation is the weakest force, Electroweak force is like 1020 times stronger than gravity. • THAT IS 10 QUINTILLION TIMES STRONGER!!!! • That is A BILLION TRILLION times…WHOA! • On the other hand, gravity and electrostatic forces are two completely different things, so its like comparing apples to oranges. So the gravitational force is what it is and the electroweak force is what it is.

17. Gravity vs. Electrostatic Force • Gravitational forces only attract. Electrostatic forces can attract AND repel. • Gravitational forces cannot be shielded but Electrostatic forces can be shielded. • Gravity depends on masses where electric forces depend on charges. Enough already….

18. Let’s compare Law of Universal Gravitation Coulomb’s Law Fg = gravitational force G = gravity constant = 6.673x10-11 N·m2/C2 ma = mass (kg) mb = mass (kg) r = distance (m) F electric = electric force (N) K = Coulomb constant = 8.99x109 N·m2/C2 q1= charge 1 (C) q2= charge 2 (C) r = distance (m)

19. Ex. The electron (mass 9.109x10-31kg) and proton (mass 1.673x10-27kg) of a hydrogen atom are separated, on average, by a distance of about 5.3x10-11 m. Find the magnitude of the electric force and the gravitational force that each particle exerts on the other. Let’s solve gravitational force first! Fg= 3.6x10-47 N

20. Now solve for electric force F electric = electric force (N) K = Coulomb constant = 8.99x109 N·m2/C2 q1= charge 1 (C) = -1.60x10-19C q2= charge 2 (C) = +1.60x10-19C r = distance (m)