Understanding Electrostatics: Charges, Forces, and Charging Methods

1. Chapter 22 Electrostatics Herriman High Physics

2. Electricity • Electricity at rest is static electricity or electrostatics Herriman High Physics

3. Electric Charge and Electric Field • Static Electricity – Unmoving charge • Two types • Positive – lack of electrons • Negative – excess electrons • Like charges - Repel • Opposite Charges – Attract • Charged measured in coulombs (c) Herriman High Physics

4. 32.2Conservation of Charge Principle of Conservation of Charge Electrons are neither created nor destroyed but are simply transferred from one material to another.

5. Conductors and Insulators • A good conductor transfers charge easily. • A good insulator inhibits the transfer of charge. • A good conductor is a poor insulator and a good insulator is a poor conductor. Herriman High Physics

6. 32.3Coulomb’s Law For charged objects, the force between the charges varies directly as the product of the charges and inversely as the square of the distance between them. Where: d is the distance between the charged particles. q1 represents the quantity of charge of one particle. q2 is the quantity of charge of the other particle. k is the proportionality constant.

7. Electric Forces • Coulomb’s Law • F = kQ1Q2/r2 • k = 9 x 109 N•m2/C2 • Positive solution – repulsion • Negative solution - attraction Herriman High Physics

8. Sample Problem Two charges, Q1 = +10 µC, and Q2 = -15 µC, are separated by 1.5 meters. What is the electrostatic force acting between them? Solution F = kQ1Q2/r2 = (9 x 109 N•m2/C2)(+10 x 10-6 C)(-15 x 10-6 C)/(1.5 m)2 = -0.6 N Herriman High Physics

9. Review Question Pg 400 #1, 2, 4, 5, 7,9,10, 11, 12, 17, 18 Herriman High Physics

10. Starter 2/1 and 2/2 • Electrons and protons have equal but opposite charges. The magnitude of this charge is known as the Elementary Charge = 1.6 x 10-19 C. A hydrogen atom contains one proton and one electron, if the electrostatic force of attraction is 8.2 x 108N, how far apart are they? Herriman High Physics

11. Starter 2/3 • An object has a charge of 3 x 10-6 C. A second object has a charge of -1.5 x 10-6 C. The two objects are 0.12m from each other. • Determine the electrostatic force between them. Herriman High Physics

12. 2/6 • What is the electric field strength at a distance of 10 cm from a charge of 2 μC? • 1. Write the equation needed. • 2. Solve the problem Herriman High Physics

13. 32.5Charging by Friction and Contact Two ways electric charge can be transferred are by friction and by contact.

14. 32.5Charging by Friction and Contact We can stroke a cat’s fur and hear the crackle of sparks that are produced. We can comb our hair in front of a mirror in a dark room and see as well as hear the sparks of electricity. We can scuff our shoes across a rug and feel the tingle as we reach for the doorknob. Electrons are being transferred by friction when one material rubs against another.

15. 32.5Charging by Friction and Contact Electrons can also be transferred from one material to another by simply touching. When a charged rod is placed in contact with a neutral object, some charge will transfer to the neutral object. This method of charging is called charging by contact. If the object is a good conductor, the charge will spread to all parts of its surface because the like charges repel each other.

16. 32.6Charging by Induction If a charged object is brought near a conducting surface, even without physical contact, electrons will move in the conducting surface.

17. 32.6Charging by Induction Charging by induction can be illustrated using two insulated metal spheres. Uncharged insulated metal spheres touching each other, in effect, form a single noncharged conductor.

18. 32.6Charging by Induction • When a negatively charged rod is held near one sphere, electrons in the metal are repelled by the rod. • Excess negative charge has moved to the other sphere, leaving the first sphere with an excess positive charge. • The charge on the spheres has been redistributed, or induced.

19. 32.6Charging by Induction • When the spheres are separated and the rod removed, the spheres are charged equally and oppositely. • They have been charged by induction, which is the charging of an object without direct contact.

20. 32.6Charging by Induction Charge induction by grounding can be illustrated using a metal sphere hanging from a nonconducting string.

21. 32.6Charging by Induction Charge induction by grounding can be illustrated using a metal sphere hanging from a nonconducting string. • A charge redistribution is induced by the presence of the charged rod. The net charge on the sphere is still zero.

22. 32.6Charging by Induction Charge induction by grounding can be illustrated using a metal sphere hanging from a nonconducting string. • A charge redistribution is induced by the presence of the charged rod. The net charge on the sphere is still zero. • Touching the sphere removes electrons by contact and the sphere is left positively charged.

23. 32.6Charging by Induction Charge induction by grounding can be illustrated using a metal sphere hanging from a nonconducting string. • A charge redistribution is induced by the presence of the charged rod. The net charge on the sphere is still zero. • Touching the sphere removes electrons by contact and the sphere is left positively charged. • The positively charged sphere is attracted to a negative rod.

24. 32.6Charging by Induction Charge induction by grounding can be illustrated using a metal sphere hanging from a nonconducting string. • A charge redistribution is induced by the presence of the charged rod. The net charge on the sphere is still zero. • Touching the sphere removes electrons by contact and the sphere is left positively charged. • The positively charged sphere is attracted to a negative rod. • When electrons move onto the sphere from the rod, it becomes negatively charged by contact.

25. 32.6Charging by Induction When we touch the metal surface with a finger, charges that repel each other have a conducting path to a practically infinite reservoir for electric charge—the ground. When we allow charges to move off (or onto) a conductor by touching it, we are grounding it.

26. 32.7Charge Polarization Charge polarization can occur in insulators that are near a charged object. When a charged rod is brought near an insulator, there are no free electrons to migrate throughout the insulating material. Instead, there is a rearrangement of the positions of charges within the atoms and molecules themselves.

27. 32.7Charge Polarization One side of the atom or molecule is induced to be slightly more positive (or negative) than the opposite side. The atom or molecule is said to be electrically polarized.

28. 32.7Charge Polarization • When an external negative charge is brought closer from the left, the charges within a neutral atom or molecule rearrange.

29. 32.7Charge Polarization • When an external negative charge is brought closer from the left, the charges within a neutral atom or molecule rearrange. • All the atoms or molecules near the surface of the insulator become electrically polarized.

30. Electric Field • Field – Affect that acts at a distance, without contact • Examples • Electric Field • Gravitational Field • Electric Field Strength – • E = F/q = kQ/r2 Herriman High Physics

31. Sample Problem Calculate the strength of an electric field at a point 30 cm from a point charge Q = +3 µC Solution E = kQ/r2 = (9 x 109 N•m2/C2)(+3 x 10-6 C)/(0.3 m)2 = 300000 N/C Herriman High Physics

32. Electric Potential • Electric potential = Electric potential energy Herriman High Physics

33. Electrical Energy Storage • Electrical Energy can be stored in two ways: • Batteries • Long term storage, even flow of charge • Storage ability measured in Volts • Capacitors • Short term storage, releases charge all at once (boost in charge) • Storage capacity measured in Farads (F) • 1 Farad = 1 Coloumb/Volt • Mathematically Charge = Capacitance * Voltage = q = CV Herriman High Physics

34. Chapter 23 Electrical Current Herriman High Physics

35. Electrical Energy • Electrical Energy is generated from other forms of energy and transmitted over power lines and/or stored in batteries • Vocabulary • Voltage (V) • Force in an electrical system; Volt = Work/Charge = W/q = Joule/Coloumb • Current (I) • Rate in an electrical system = Charge/time = q/t =Coloumb/sec = 1 Ampere Herriman High Physics

36. Energy in Electrical System • Volts =Work/charge = V =W/q • Work is measured in joules (the same as energy) • Charge is measured in Coloumbs (C) • The charge on an electron is 1.6 x 10-19 C • 1 V = 1 Joule/1 Coloumb • Work = Volts * Charge = Vq Herriman High Physics

37. Sample Problem • How much work is needed to move a 10 μC charge to a point where the potential is 70 V? • W = Vq = (70 V)(10 x 10-6 C) = 7 x 10-4 J Herriman High Physics

38. Flow of Charge Herriman High Physics

39. Electric Current • Circuit – A continuous path connected between the terminals of a power source. • Current – Flow of Charge • I = ΔQ/Δt • Current is measured in Coloumbs/Sec which is called an Ampere. Herriman High Physics

40. Electric Current • Electron Flow is from – terminal to + terminal. • Conventional Current is from + terminal to – terminal. Herriman High Physics

41. Sample Problem A steady current of 2.5 Amps passes through a wire for 4 minutes. How much charge passed through any point in the circuit? Solution Q = IΔt (2.5 C/s)(240 s) = 600 C Herriman High Physics

42. Electrical Resistance • Resistance – how much the conductor slows down the flow of electrons through it. • Resistance is measured in Ohms (Ω) Herriman High Physics

43. Ohm’s Law • Ohm’s law -In any Circuit: V = IR or R = V/I Herriman High Physics

44. Sample Problem A small flashlight bulb draws a current of 300 mA from a 1.5 V battery. What is the resistance of the bulb? Solution R = V/I = (1.5 V)/(0.3 A) = 5 Ω Herriman High Physics

45. Resistor Color Code • Resistors are banded in order to describe the amount of resistance they provide. Each resistor is banded with 4 stripes. Herriman High Physics

46. Resistor Color Code • Betty Brown Runs Over Your Garden But Violet Grey Walks. Herriman High Physics

47. Sample Problem Calculate the resistance of a resistor which is banded with the following colors: Red, Green, Blue, Silver. Solution Red = 2, Green = 5, Blue = 6 and Silver = 10% R = 25000000 ± 10% Or R = 25 MΩ ± 10% Herriman High Physics

48. 34.7Direct Current and Alternating Current By DC, we mean direct current, which refers to a flow of charge that always flows in one direction. • A battery produces direct current in a circuit because the terminals of the battery always have the same sign of charge. • Electrons always move through the circuit from the negative terminal toward the positive terminal. • Even if the current moves in unsteady pulses, so long as it moves in one direction only, it is DC.

49. 34.7Direct Current and Alternating Current Alternating current (AC), as the name implies, is electric current that repeatedly reverses direction. • Electrons in the circuit move first in one direction and then in the opposite direction. • They alternate back and forth about relatively fixed positions. • This is accomplished by alternating the polarity of voltage at the generator or other voltage source.

50. 34.8Converting AC to DC A converter uses a transformer to lower the voltage and a diode, an electronic device that allows electron flow in only one direction. Since alternating current vibrates in two directions, only half of each cycle will pass through a diode. The output is a rough DC, off half the time. To maintain continuous current while smoothing the bumps, a capacitor is used.