Basics of Electric Charge

1. Preview Section 1 Electric Charge Section 2 Electric Force Section 3 The Electric Field

2. Electric Charge

3. What do you think? • In the top picture, the girl has rubbed the balloon on her hair, and now there is a force of attraction between them. Normally, a balloon and hair would not attract each other. • What happened to each to produce this force? • In the lower picture, the two balloons are repelling each other. • How was this force of repulsion produced?

4. What do you think? • Suppose that after this balloon is rubbed against the girl’s hair, it is held against the wall. It will be attracted to the wall and stick to it. • Explain why the balloon is attracted to the wall. • Why does it eventually fall?

5. Electric Charge • There are two types of charge, positive and negative. • Like charges repel. • Positive and positive • Negative and negative • The two balloons • Opposite charges attract. • Positive and negative • The balloon and the hair.

6. Transferring Electric Charge • Atoms have smaller particles called protons (+ charge), neutrons, and electrons (- charge). • Number of protons = number of electrons • Atoms are neutral (no net charge). • Electrons are easily transferred from one atom to another. • Protons and neutrons remain in nearly fixed positions. • When rubbing a balloon on your hair, electrons are attracted to the balloon and transfer. • The balloon is left with excess electrons (- charge). • The hair is left with an equal excess of protons (+ charge).

7. Atoms are neutral because they have equal numbers of protons (+) and electrons (-). • Only electrons are free to move. • Positive charge occurs because electrons are lost and the positive charges remain behind. • The amount of + charge always equals the amount of - charge because protons and electrons have equal charges (just opposite signs). • Some materials attract electrons more strongly than others, so when contact occurs, the electrons are transferred. In this case, the balloon attracts electrons more strongly than the hair.

8. Millikan Oil Drop Experiment • Millikan sprayed oil drops between charged metal plates. • The oil drops were negatively charged by friction. • By adjusting the voltage on the plates, he could make the drops rise and fall.

9. Millikan’s Results • Millikan found that the amount of charge on objects was always a multiple of some fundamental charge (e). • In other words, charge is quantized. • e turned out to be the amount of charge on an electron. • e = 1.602176  10-19 coulombs • Coulomb is the SI unit of charge.

10. Millikan's Oil Drop Experiment Click below to watch the Visual Concept. Visual Concept

11. -1.60  10-19 C/electron 6.25  1018 electrons/C 5.69  10-12 kg about 5 billionths of a gram What amount of charge does a single electron carry? How many electrons are needed to produce an amount of charge equal to -1.00 C? What is the mass of this number of electrons? Coulombs and Electrons

12. What is meant by the term electrical conductor? Provide a few examples. What is meant by the term electrical insulator? Provide a few examples. Why do conductors and insulators behave differently? Conductors allow electrons to flow freely through them. Silver, copper, aluminum, and other metals Electrons do not flow freely though insulators. Plastic, rubber, glass Outer electrons in metals are loosely bound to the nucleus and relatively free to move. Conductors and Insulators

13. Charging by Contact • Both insulators and conductors can be charged by contact. • Rubbing two materials together results in a transfer of electrons. • When charging metal, the charge may move through your body into the ground. • The metal and your body are conductors, so the charge moves through them. • You must hold the conductor with an insulating material, such as rubber gloves, to keep the charge on the metal.

14. Charging by Induction • A charged rod is held near a metal sphere. Why do the charges in the metal arrange themselves as shown? • The metal sphere is connected to the ground with a conductor. Why did some of the electrons move off the sphere?

15. Charging by Induction • The conductor connecting the sphere to ground is removed. What type of net charge does the sphere now possess? • The negatively charged rod is removed. Why do the charges move into the positions shown? • Only occurs with conductors.

16. Would the sphere be charged if the rubber rod was removed before the ground was removed? Why or why not? • Sketch the four diagrams with just one change: make the rod a positively-charged glass rod. • Which way do the electrons move? What is the charge of the sphere?

17. Surface Charges • Why does a charged balloon stick to the wall? • A positive surface charge is induced on the wall by the negatively-charged balloon. • Electrons shift within atoms due to attraction or repulsion. • The insulator does not have a net charge. • Why are they attracted?. • Why can a charged comb pick up little pieces of paper?

18. Now what do you think? • In the top picture, the girl has rubbed the balloon on her hair, and now there is a force of attraction between them. Normally, a balloon and hair would not attract each other. • What happened to each to produce this force? • In the lower picture, the two balloons are repelling each other. • How was this force of repulsion produced?

19. Now what do you think? • Suppose that after this balloon is rubbed against the girl’s hair, it is held against the wall. It will be attracted to the wall and stick to it. • Explain why the balloon is attracted to the wall. • Why does it eventually fall?

20. Electric Force

21. What do you think? • Electric forces and gravitational forces are both field forces. Two charged particles would feel the effects of both fields. Imagine two electrons attracting each other due to the gravitational force and repelling each other due to the electrostatic force. • Which force is greater? • Is one slightly greater or much greater than the other, or are they about the same? • What evidence exists to support your answer?

22. Coulomb’s Law • The force between two charged particles depends on the amount of charge and on the distance between them. • Force has a direct relationship with both charges. • Force has an inverse square relationship with distance.

23. Coulomb’s Law • Use the known units for q, r, and F to determine the units of kc. • kc = 8.99  109 N•m2/C2 • The distance (r) is measured from center to center for spherical charge distributions. • Sign of attractive force? Sign of repulsive force?

24. Classroom Practice Problem • The electron and proton in a hydrogen atom are separated, on the average, a distance of about 5.3  10-11 m. Find the magnitude of both the gravitational force and the electric force acting between them. • Answer: Fe = 8.2  10-8 N, Fg = 3.6  10-47 N • The electric force is more than 1039 times greater than the gravitational force. • Atoms and molecules are held together by electric forces. Gravity has little effect.

25. Classroom Practice Problem • A balloon is rubbed against a small piece of wool and receives a charge of -0.60 C while the wool receives an equal positive charge. Assume the charges are located at a single point on each object and they are 3.0 cm apart. What is the force between the balloon and wool? • Answer: 3.6 N attractive

26. Classroom Practice Problem • Two charges, q1 and q2, lie on the x-axis. The first charge is at the origin and the second charge is at x = 1.0 m. Determine the force on a third charge, q3, placed at x = 0.75 m. The charges are as follows: q1 = +10.0C , q2 = +7.5C, q3 = -5.0C • Answer: Fleft = 0.80 N and Fright= 5.4 N, so Fnet = 4.6 N to the right

27. Electric Force • Like gravity, the electric force is a field force. • Similarities • Both forces are related to distance in the same way. • Differences • Two types of charge and only one type of mass • Electric forces can attract or repel while gravity only attracts. • Electric forces are far stronger than gravitational forces.

28. Coulomb’s Apparatus • Coulomb developed his law using a torsion balance like that shown. • He measured the force between the two charged spheres by the amount of twisting in the wire.

29. Now what do you think? • Electric forces and gravitational forces are both field forces. Two charged particles would feel the effects of both fields. Imagine two electrons attracting each other due to the gravitational force and repelling each other due to the electrostatic force. • Which force is greater? • Is one slightly greater or much greater than the other, or are they about the same? • What evidence exists to support your answer?

30. Electric Fields

31. What do you think? • In the chapter “Circular Motion and Gravitation,” you learned about the gravitational field (g). The diagram shows the “g” field around Earth. • In this section, we will study the electric field (E) around charged particles. On the next slide are three different diagrams. Make a sketch of the “E” field for each charge or combination of charges.

32. What do you think? • Make a sketch of the “E” field for each charge or combination of charges. • How are your sketches similar? • How are they different? • Explain.

33. Electric Field Strength • Electric fields (E) have magnitude and direction. • The direction is defined as the direction of the force on a small, positive test charge (q0) placed in the field caused by Q. • The magnitude of the field is defined as the force per unit charge on q0.

34. Electric Fields and Test Charges Click below to watch the Visual Concept. Visual Concept

35. Test Charges • If the test charge (q0) is large, it will affect the way the charges are distributed on the charged conductor. • A small test charge will not significantly affect the field. • This would change the field around the conductor. • Test charges will always be considered small enough to have no effect on the field.

36. Electric Field Strength • Combine Coulomb’s law with the definition of electric field to derive an equation for E due to a point charge. • SI unit: N/C • The field strength does not depend on the test charge.

37. Sample Electric Field Strengths

38. Classroom Practice Problems • An electric field around a charged object is 5.95  106 N/C at a distance of 0.100 m. Find the charge on the object. • Answer: 6.62  10-6 C or 6.62 C • Suppose a small test charge of 0.200 C was placed at the point that is 0.100 m from the charged object. What force would be exerted on the test charge and on the object? • Answer: 1.19 N for both test charge and object

39. Electric Field Lines - Rules • Apply the above rules and sketch the E field around the charge shown.

40. Electric Field Lines - Rules • Apply the above rules and sketch the E field around the charge shown.

41. Electric Field Lines - Rules • Apply the above rules and sketch the E field around the charge combination shown.

42. Electric Field Lines - Rules

43. Electric Field Lines - Rules • Apply the above rules and sketch the E field around the charge combination shown.

44. Electric Field Lines - Rules

45. Rules for Drawing Electric Field Lines Click below to watch the Visual Concept. Visual Concept

46. Rules for Sketching Fields Created by Several Charges Click below to watch the Visual Concept. Visual Concept

47. Electrostatic Equilibrium • Electrostatic equilibrium occurs in conductors when no net motion of charges exists within the conductor. • Charges in a conductor are free to move, but are not moving when equilibrium exists. • The rules below result from this fact.

48. Now what do you think? • What is an electric field? • When sketching electric fields, what information is conveyed by the direction of the field lines? • When sketching electric fields, what information is conveyed by the density of the field lines? • Why must electric field lines just outside a conductor be perpendicular to the conductor?