Electricity and Magnetism

1. Electricity and Magnetism Electric Charge Coulomb’s Law Capacitors Electric Charges and Forces

2. Objectives • Describe and calculate the forces between like and unlike electric charges. • Identify the parts of the atom that carry electric charge. • Apply the concept of an electric field to describe how charges exert force on other charges. • Sketch the electric field around a positive or negative point charge. • Describe how a conductor shields electric fields from its interior. • Describe the voltage and current in a circuit with a battery, switch, resistor, and capacitor. • Calculate the charge stored in a capacitor.

3. Vocabulary Terms: homework • charge • electrically neutral • static electricity • positive charge • negative charge • electric forces • charge by friction • electroscope • protons • neutrons • electrons • gravitational field • charged • induction • Coulomb’s law • capacitor • parallel plate capacitor • microfarad • coulomb • electric field • capacitance • charge • polarization • shielding test • charge • farad • field inverse • square law • discharged field • lines

4. Electrostatics In Physics

5. Key Questions: Define electrically neutral Define net charge or excess charge What causes static electricity Electric Charge

6. Electric Charge • All ordinary matter contains both positive and negative charge. • You do not usually notice the charge because most matter contains the exact same number of positive and negative charges. • An object is electrically neutral when it has equal amounts of both types of charge.

7. Electric Charge • Objects can lose or gain electric charges. • The net charge is also sometimes called excess charge because a charged object has an excess of either positive or negative charges. • A tiny imbalance in either positive or negative charge on an object is the cause of static electricity.

8. Electric charge • Answer the following: • 4. What is the unit of electrical charge? • 5. What is the mass of an electron, proton, and neutron? • 6. What are the charges in coulombs of a protron, neutron, and electron? • 7. Describe the attracting and repelling characteristics of protons and electrons

9. Electric Charge • Electric charge is a property of tiny particles in atoms. • The unit of electric charge is the coulomb (C). • A quantity of charge should always be identified with a positive or a negative sign.

11. Electric forces • Electric forces are created between all electric charges. • Because there are two kinds of charge (positive and negative) the electrical force between charges can attract or repel.

12. 8. Draw a picture of an electroscope with a charge (positive or negative) • 9. Describe how the electroscope becomes positively charged through conduction • 10. Describe how the electroscope becomes negatively charged through conduction

13. Electric forces • The forces between the two kinds of charge can be observed with an electroscope.

14. Electric forces • Charge can be transferred by conduction.

15. 11. What moving particles create a current in metals? • 12. Define current • 13. Write the formula for current • 14. What is the unit for current?

16. In conductive liquids (salt water) both positive and negative charges carry current. • In solid metal conductors, only the electrons can move, so current is carried by the flow of negative electrons. • The direction of current was historically defined as the direction that positivecharges move. • Both positive and negative charges can carry current.

17. I = q t Electric current • Current is the movement of electric charge through a substance. Charge that flows (coulombs) Current (amps) Time (sec)

18. Calculate current • 15. Two coulombs of charge pass through a wire in five seconds. • Calculate the current in the wire.

19. 16. Compare the movement of an electron in a conductor and insulator • 17. Describe a semi-conductor • 18. Describe charging by friction • 19. What causes a negatively charged balloon to stick to a wall?

20. Conductors and insulators • All materials contain electrons. • The electrons are what carry the current in a conductor. • The electrons in insulators are not free to move—they are tightly bound inside atoms.

21. Conductors and insulators • A semiconductor has a few free electrons and atoms with bound electrons that act as insulators.

22. Conductors and insulators • When two neutral objects are rubbed together, charge is transferred from one to the other and the objects become oppositely charged. • This is called charging by friction. • Objects charged by this method will attract each other.

24. Define coulomb’s law and write the formula • What is coulomb’s constant? • Describe the force between two charges if the distance between them is tripled • Describe the force between two charges if both charges are increased by a factor of two.

25. F = K q1 q2 r2 Coulomb's Law • Coulomb’s law relates the force between two single charges separated by a distance. Constant 9 x109 N.m2/C2 Force (N) Charges (C) Distance (m)

27. Coulomb's Law • The force between two charges gets stronger as the charges move closer together. • The force also gets stronger if the amount of charge becomes larger.

28. Coulomb's Law • The force between two charges is directed along the line connecting their centers. • Electric forces always occur in pairs according to Newton’s third law, like all forces.

29. Coulomb's Law • The force between charges is directly proportional to the magnitude, or amount, of each charge. • Doubling one charge doubles the force. • Doubling both charges quadruples the force.

30. Coulomb's Law • The force between charges is inversely proportional to the square of the distance between them. • Doubling the distance reduces the force by a factor of 22 = (4), decreasing the force to one-fourth its original value (1/4). • This relationship is called an inverse square law because force and distance follow an inverse square relationship. • Now answer questions #22, 23

31. Calculating force • 24. Two balls are each given a static electric charge of one ten-thousandth (0.0001) of a coulomb. • Calculate the force between the charges when they are separated by one-tenth (0.1) of a meter.

32. 25. Describe an electric field • 26. What type of field does mass create? • 27. Compare the gravitational force and electrical force between two charges. • 28. Draw an electrical field between: • a. A proton and electron • b. A proton and proton • c. An electron and electron • 29. Draw the electric field coming from a single proton and a single electron

33. Fields and forces • The concept of a field is used to describe any quantity that has a value for all points in space. • You can think of the field as the way forces are transmitted between objects. • Charge creates an electric field that creates forces on other charges.

34. Fields and forces • Mass creates a gravitational field that exerts forces on other masses.

35. Fields and forces • Gravitational forces are far weaker than electric forces.

37. Drawing the electric field

38. 30. Describe the units and equations for both a gravity field and electric field • 31. Describe how electric fields accelerate particles causing changes in voltage • 32. Why is shielding necessary around electrical wires inside of sensitive electronic equipment?

39. Electric fields and electric force • On the Earth’s surface, the gravitational field creates 9.8 N of force on each kilogram of mass. • With gravity, the strength of the field is in newtons per kilogram (N/kg) because the field describes the amount of force per kilogram of mass.

40. Electric fields and electric force • With the electric field, the strength is in newtons per coulomb (N/C). • The electric field describes the amount of force per coulomb of charge.

42. Accelerators • An electric field can be produced by maintaining a voltage difference across any insulating space, such as air or a vacuum. • Electric fields are used to create beams of high-speed electrons by accelerating them. • Electron beams are used in x-ray machines, televisions, computer displays, and many other technologies.

43. Electric shielding • Electric fields are created all around us by electric appliances, lightning, and even static electricity. • These stray electric fields can interfere with the operation of computers and other sensitive electronics. • Many electrical devices and wires that connect them are enclosed in conducting metal shells to take advantage of the shielding effect.

44. Practice problems: 33. A particle in a microwave has a charge of -9 x 10-15 C and is deflected into popcorn with a force of 4.5 x 10-2 N. What is the strength of the electric field. USE F = Eq q = -9 x10-15 C F = 4.5 x 10-2 N E = F/q = 4.5 x 10-2 N/- 9 x10-15 C = 5 x 1012 N/C

45. 34. A positive and negative particle, each with a 5 x 10-8 charge, are separated by a distance of 6 mm. What is the Force acting on them? Use coulomb’s law F = k q1 q2/r2 F = (9x109) (-) (5 x 10-8 ) (5 x 10-8 ) / (6 x 10-3)2 = -6.25x10-1N

46. 35. What would the force be in number 34 if the distance were tripled F = k q1 q2/r2 If r is tripled the r2 = 32 = 9 so the force would be 1/9 as strong = -6.25x10-1N/9

47. 36. Find the size of the electrical field at the location of one charge due to the other charge? USE F = Eq, Use information from question #35 E = F/q = -6.25x10-1N/ (5 x 10-8 )

48. 37. Find the potential energy of the charged pair. Use UE = ke q1q2/r Use UE = ke q1q2/r = (9x109) (-) (5 x 10-8 ) (5 x 10-8 ) / (6 x 10-3)

49. 38. How many excess electrons does the microwave particle in #33 have? • Q = Ne where Q = the total charge N = the number of electrons e = electronic charge 1.6 x 10-19 C N = Q/e = 9 x10-15 C/ 1.6 x 10-19 C = 5.625 x 104

50. Capacitors • What is a capacitor? • What is the symbol for a capacitor in a circuit diagram? 41. How is a capacitor charged? 42. How is a capacitor discharged? 43. What determines flow in and out of a capacitor? • Describe a parallel plate capacitor • Describe how the parallel plate capacitor works