Unit 9 Part 1: Electric Force & Coulomb’s Law

1. Unit 9 Part 1: Electric Force & Coulomb’s Law

2. Review of Forces… • Vector quantity • Magnitude (numerical quantity) • Direction • Net Force • unbalanced forces cause motion • balanced forces keep objects at rest • Types of Forces • Contact forces like normal or tension • Field forces, like gravity (field = “acts at a distance”)

3. Review of Atoms… • All matter is made of atoms • Atoms have 3 basic parts: • positively charged protons (+) • negatively charged electron (-) • neutral (no charge) neutrons (0) • Electric charge is a fundamentalproperty of matter! • Electrons have the smallest mass, thus they are the only subatomic particle that can move from one atom/object to the next (if the right conditions are present)

4. Electric Force • Caused by positive and negative charges

5. Electric Force • Electric Force is the attraction or repulsion between objects • can cause a change in motion • Is a fieldforce because it “acts at a distance” FORCE DIAGRAMS

6. Electric Fields • Exist in the region surrounding charged particles or objects • Represented by lines of force • Start at positive charges • End on negative charges • Strongest near the object

7. Consider the electric field lines drawn at the right for a configuration of two charges. Several locations are labeled on the diagram. Rank these locations in order of the electric field strength - from smallest to largest.

8. Coulomb’s Law • Quantifies the electric force between 2 charged objects that are at some distance apart. • a Coulomb (C) is the SI unit of charge • 1 C = 6.24 x 1018 Newtons (N) • The charge on one electron or proton = (+/-)1.6 x 10-19 C • Called the “elementary charge” • Symbolized by q0 • Formula: • k is coulomb’s law constant=8.99x 109 N-m2/C2 • Q1 and Q2 are the charges of each object • d is the distance between the objects

9. Practice Using Coulomb’s Law Determine the magnitude of electrical force of attraction between two balloons with separate charges of +3.5 x 10-8 C and 2.9 x 10-8 C when separated a distance of 0.65 m.

10. Practice Using Coulomb’s Law A young woman (m=55 kg) accumulates a charge of +2x105 C while sliding out of the front seat of her car. Her boy friend (m=80 kg) has been standing in the wind and has accumulated a charged of -8 x10-5 C. Calculate the magnitude of electrical force the woman exerts on her boyfriend when they are 0.5 m apart. F= kQ1Q2/d2 F

11. Practice Using Coulomb’s Law F= kQ1Q2/d2 F= (8.99x109)(+2x10-5)(-8x10-5)/(0.5)2 F= 57.54 N F

12. Practice Using Coulomb’s Law How does the magnitude of the force the boyfriend exerts on the woman compare to the magnitude of the force the woman exerts on her boyfriend. F

13. Practice Using Coulomb’s Law Determine the magnitude of electrical force between two protons in nucleus of helium atom when separate by 2.0x10-15 m.. F= kQ1Q2/d2 F

14. Practice Using Coulomb’s Law F= kQ1Q2/d2 F= (8.99x109)(+1.6x10-19)(+1.6x10-19)/(2x10-15)2 F= 57.5 N F

15. Practice Using Coulomb’s Law A cell membrane has a positive charge of 1.6 x10-19 C on the outside wall and negative charged of -1.6x10-19 C on the inside wall. Calculate the magnitude of electrons of electrical force between these two ions if the membrane thickness is 8x10-10 m. F= kQ1Q2/d2 F

16. Practice Using Coulomb’s Law F= kQ1Q2/d2 F= (8.99x109)(1.6x10-19)(-1.6x10-19)/(8.0x10-10)2 F= 3.59x10-10 N F

17. Proportional Reasoning & Coulomb’s Law • Force between the objects is: • directly proportional to the product of their charges (QxQ) • inversely proportional to the distance squared between them F ∽ C

18. Coulomb’s Law & Proportional Reasoning ∽ ∽ • Follows the Inverse Square Law • If distance between 2 objects is doubled, what happens to the force between them? • If distance between 2 objects is halved, what happens to the force between them? The magnitude of electrical force between two particles is 10 N.

19. ∽ ∽ ∽ ∽ Coulomb’s Law & Proportional Reasoning • If charge of one object is halved, what happens to the force between the objects? • If the charge of one object is doubled, what happens to the force? F ∽ C The magnitude of electrical force between two particles is 10 N.

20. Comparing Coulomb’s Law to the Law of Universal Gravitation • How are these Laws the same? • Both concern field forces between objects • Both have a constant value • Both follow the inverse square law • How are these Laws different? • Gravitational force can only be attractive • Electrical force can be attractive or repulsive

21. Unit 9 Part 2: Electrostatics Study of electric charge “at rest” within objects

22. Positive electric charges a. attract both positive and negative charges. b. repel both positive and negative charges. c. attract positive and repel negative charges. d. repel positive and attract negative charges.

23. A beam of electrons is directed into the electric field between two oppositely charged parallel plates, as shown in the diagram below. The electrostatic force exerted on the electrons by the electric field is directed a. into the screen. b. out of the screen. c. toward the bottom of the screen. d. toward the top of the screen.

24. A beam of electrons is directed into the electric field between two oppositely charged parallel plates, as shown in the diagram below. What is the direction of the electric field in the above situation? a. into the screen. b. out of the screen. c. toward the bottom of the screen. d. toward the top of the screen. Different Question

25. The diagram below shows two identical metal spheres, A and B, separated by distance d. Each sphere has mass m and possesses charge q. Which diagram best represents the electrostatic force Fe and the gravitational force Fg acting on sphere B due to sphere A?

26. V. Electrostatics Basics Electrostaticsis defined as the study of electric charge at rest within objects. (static electricity) • It is the accumulation of charge, followed by a rapid discharge How do objects become charged? • Charges within objects can become separated, due to the movement of electrons • Neutral atoms/objects have equal protons and electrons • Charged atoms/objects have more or less electrons compared to it’s number of protons John Travoltage Gas Station Flash Fire

27. V. Electrostatics Basics Remember, the only subatomic particle that can move from one atom/object to another is the electron. • An object that: • Gaining electrons, will result in a negative charge • Loses electrons, will result in a positive charge

28. Practice Neutral sample Charged sample Neutral Object Charged Object -8 +8 -6 +8 Net Charge: Net Charge: 0 +2

29. The most common way for a negative pith ball to become neutral is for it to a. gain some electrons. b. lose some electrons. c. gain some protons. d. lose all its electrons.

30. V. Electrostatics Basics Conductors: metals, aqueous solutions of salts (i.e., ionic compounds dissolved in water), graphite, water, and the human body. Insulators: plastics, styrofoam, paper, rubber, glass, and dry air. • Electrons will move differently, depending on the kind of material they are moving within • In conductors: • electrons are “free” & can move throughout the material • in good conductors, can move from atom to atom & object to object • In Insulators: • do not allow free movement of electrons throughout the material, or from the object to other things

31. Semiconductors are found in microprocessor chips, transistors, and anything that's computerized or uses radio waves depends on semiconductors

32. A pure silicon crystal is nearly an insulator Doping Silicon Changing the behavior of silicon mixing a small amount of an impurity into the silicon crystal. N-type - phosphorus or arsenic is added to the silicon. Both have five outer electrons, so they're out of place when they get into the silicon lattice. P-type - boron or gallium are added. Each have only three outer electrons. They form "holes" in the lattice where a silicon electron has nothing to bond to. The absence of an electron creates the effect of a positive charge, hence the name P-type. Holes can conduct current. A hole happily accepts an electron from a neighbor, moving the hole over a space. In a silicon lattice, all silicon atoms bond perfectly to four neighbors, leaving no free electrons to conduct electric current.

33. Law Of Conservation Of Electric Charge *net charges must equal zero* • Within objects: • If part of an object becomes positive (or negative), the other part of an object must become equally but oppositely charged • Example: water molecules Negative Pole Positive Pole

34. Law Of Conservation Of Electric Charge *net charges must equal zero*

35. Law Of Conservation Of Electric Charge • Between interacting objects • If an object becomes charged from interacting with another, the other object must become equally, but oppositely charged. Static Electricity Balloon Phet

36. Law Of Conservation Of Electric Charge Net charge: 0 Net charge: 0 Balloon net charge: – 10 Sweater net charge: +10 Net overall charge: 0

37. VI. 4 Ways to Charge objects #1: Friction • Electrons transferred between two neutral objects by rubbing 2 them together

38. Charging by Friction • Friction creates oppositely charged objects. The net charge of both together is zero.

39. Friction, cont. Triboelectric Series Different materials have different affinities for electrons. • Rubbing amber with silk causes electrons from the silk to move to the amber • Rubbing glass with silk causes electrons to move from the glass to the silk More likely to lose electrons (+) Human hands (when dry) Leather Rabbit fur Glass Human hair Wool Silk Paper Amber Brass Silver Polyester Sytrofoam Scotch tape PVC More likely to gain electrons (-)

40. Which has the greatest affinity for electrons? Teflon

41. When amber is rubbed on a piece of silk, what happens? a. Electrons are transferred from the amber to the silk. b. Electrons are transferred from the silk to the amber. c. Protons are transferred from the amber to the silk. d. Protons are transferred from the silk to the amber.

42. VI. 4 Ways to Charge objects #2: Conduction • Occurs when a charged object transfers it’s electron to a neutral object, by coming into direct contact with it. (touching) John Travoltage pith ball simulation

43. Conduction, cont. • Step 1: Objects are brought close together • Step 2: They touch

44. Conduction, cont. • Step 3: Electrons are transferred from the charged to the neutral object • Step 4: Both objects are now charged

45. Sharing Charge via Conduction B A A A B B - + - - - - + + + + + + + - + + + + + - + - - + - - - - - - + - • A metal sphere (A) with a charge of +4 units is brought in contact with a metal sphere (B) with -2 units of charge, and the spheres are then separated. What is the charge on each sphere after separation? + - - + + + +4 -2 Electrons evenly distribute. + + + + Charges will be equal! +1 +1

46. VI. 4 Ways to Charge objects #3: Polarization • Surface charge (object remains neutral, overall) • Charges within object shift • in response to being near a charged object • Only exists while near the charged object

47. Lightning

48. Lightning A LARGE Discharge Of Static Electricity • Caused by the buildup of electrical charge in clouds (due to the friction caused by the movement of water droplets) • Bottom is usually negative, and the top positive • This accumulation repels negative charges in the ground, leaving the surface of the ground positive • As the charge increases, a bolt of many billions of electrons are transferred at the same time to the Earth. • This is followed by a streamer of positively charged air particles that is repelled by the ground that travels back upwards • Lightning bolts ionize the atoms in the air producing great amounts of heat, causing compressional waves (sound) called thunder.

50. Lightning, Cont.