Electrostatics. Do Now: Describe the Concept of charge

Transcription

1 Electrostatics Do Now: Describe the Concept of charge

2 Electrostatics The study of electrical charges that can be collected and held in one place Also referred to as static electricity Types of Charge: Benjamin Franklin noticed that two different charges existed; he called them positive and negative

3 2 Types of Charge & Interactions

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6 How do objects become charged? Objects become charged by gaining or losing electrons.

7 J.J. Thompson In 1890 Thompson discovered that all materials contain light, negatively charged particles called electrons. Cathode Ray Tube

8 Separation of Charge by Friction

9 Conservation of Electric Charge 1. The total charge in any closed system never changes. (The combined total charge of two objects remains the same.) 2. Individual charges can neither be created nor destroyed; they can only be transferred from one object to another. Which charge can actually be transferred? ELECTRONS ( - )

10 Types of Materials Insulator Conductor Materials through which charges (electrons) will not move easily Ex: 1. Glass 2. Dry Wood 3. Most Plastics 4. Cloth 5. Distilled Water Materials that allow charges (electrons) to move about easily Ex: 1. Metals 2. Water Solution (because it has ions in it)

11 Types of Materials Insulator Conductor Not tightly bound to the atom (far from nucleus) Charge Stays Put Tightly bound to the nucleus and do not move very easily (although it is possible) Charge Spreads Out Look Back at Chemistry

12 Ex: Conducting sphere, charge is Uniformly distributed

13 Can air become a conductor? Normally, the air surrounding a cloud would be a good enough insulator to prevent a discharge of electrons to Earth. Yet, the strong electric fields surrounding a cloud are capable of ionizing the surrounding air and making it more conductive. The ionization involves the shredding of electrons from the outer shells of gas molecules. The gas molecules which compose air are thus turned into a soup of positive ions and free electrons. The insulating air is transformed into a conductive plasma. The ability of a storm cloud's electric fields to transform air into a conductor makes charge transfer (in the form of a lightning bolt) from the cloud to the ground (or even to other clouds) possible.

14 More types of Conductors Semiconductors Is a material that conducts charge better than an insulator but worse than a a conductor Ex: 1. Silicon 2. Germanium 3. Humans Superconductors Is a material that conducts charge with zero resistance below a certain (critical) temperature. Ex: 1. Ceramic Oxide 2. Aluminum 3. Tin 4. Lead 5. Zinc

15 Grounding and Leakage Grounding Removing excess charge from a charged body by connecting it to the earth Leakage The discharging of a charged object due to the acceptance of electrons by the air Ex: Earth Ground is the largest example: charge spreads out to larger body

16 By touching a charged object to Ground, the opposite type of charge will be supplied -- neutralizing the object s charge What s an example? 1. Socks on a carpet 2. Getting Gas (BE CAREFUL) What s being moved?

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18 Robert Millikan s Oil Drop Experiment In 1909, Robert Milikan performed an experiment at the University of Chicago in which he observed the motion of tiny oil droplets between two parallel metal plates. The oil droplets were charged by friction in an atomizer and allowed to pass through a hole in the top plate. Initially the droplets fell due to their weight. The top plate was given a positive charge as the droplets fell, and the droplets with a negative charge were attracted back upward toward the positively charged plate. By turning the charge on and off, Millikan was able to watch a single oil droplet for many hours as it alternately rose and fell.

19 Lightning Strikes The negative bottom part of the cloud induces a charge separation in the ground below. Air is normally a very good insulator, but if the charge separation is big enough, the air between the cloud and ground can become ionized (a plasma). This allows some of the electrons in the cloud to begin to migrate into the ionized air below. This is called a leader. Positive ions from the ground migrate up to meet the leader. This is called a streamer. As soon as the leader and streamer meet, a fully conductive path exists between the cloud and ground and a lightning strike occurs. Billions of trillions of electrons flow into the ground in less than a millisecond. The strike can be hotter than the surface of the sun. The heat expands the surrounding air; which then claps as thunder

20 Explain why electric charge is quantized. Electric charge is quantized because all electrons have one specific charge. Therefore, charge can only occur in discrete amounts of charge ± e, or ± 2 e, or ± 3 e, and so on. Elementary Unit of Charge The amount of negative charge that the electron (- e) has and positive charge that a proton has (+e). e = 1.60 x C

21 Quantized Charge e = 1.60 x C ± e, or ± 2 e, or ± 3 e, and so on. Has to be a whole number!

22 Electricity & Magnetism: Electrostatics Transfer of Charge It s all ELECTRONS Electrons transfer in fixed amounts (±1,2,3 e - ) Ex. Cannot get ½ of an electron charge (only whole value] We say that charge is QUANTIZED (fixed amount) Ex. Charge must be in integer multiples of 1.6 X C

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24 How many elementary charges are in one coulomb of charge? 1 C = the charge on 6.25 X10 18 electrons or protons Therefore 1 Coulomb of Charge is 6.25 X10 18 elementary charges

25 An object has acquired a charge of 3.2 x C. How many excess electrons are on the object? -17 æ 1 electrons ö -3.2 x 10 C = 200 electrons (2.0 x 10 2 ç e) è x C ø A glass rod loses 2500 electrons after being rubbed with silk. What is the charge on the rod? The silk? 2500 electrons (-1.60 x C) = 4.0x10-16 C

26 A balloon gains a charge of 6.4 μc after being rubbed on your hair. How many excess electrons is this? -6 æ 1 electrons ö -6.4 x 10 C = 4.0 x ç C è x C ø A helium nucleus is known as an alpha particle. It consists of two protons and two neutrons. What is the charge on an alpha particle? 2 Protons (+1.60 x C) = 3.20 x C

27 Which of the following charges are possible for an object to have? (Self Practice) A x C 2e B. 4.8 x C 3e C. 5.6 x C D. 1.6 x C E. 5.6 C 3.5 x e

28 Charging by Conduction: Charging a neutral object by touching it with a charged object. contact occurs charge transferred Charge may spread

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32 Separation of Charge on Neutral Objects Sketch the charge distribution on the soda can:

33 Separation of Charge on Neutral Objects Sketch the charge distribution as a negatively charged rod is brought nearby Sketch the charge distribution as a positively charged rod is brought nearby demo Negative charges in the neutral object are attracted to the positively charged object, and the positive charges in the neutral object will be repelled. The neutral object will remain neutral, but the positive and negative charges (inside neutral object) will be separated.

34 Polarization to cause one side of an object to become negative, and the other side to be positive.

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36 Why don t Long Distance Relationships Work?

37 Electricity & Magnetism: Electrostatics The Coulombic Wood & Water Inspections into the Atom

38 Electricity & Magnetism: Electrostatics The Jumping Crispies Inspections into the Atom Paper

39 Electricity & Magnetism: Electrostatics Wood

40 Charging by Induction Induction causing a neutral object to become charged without direct contact between the charged object and the neutral object.

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43 The Needle Electroscope 1. Conducting Plate 1. What is the position of the see-saw when the electroscope is uncharged? Needle is standing vertically (up and down) 2. Needle Pivot What is the position of the see-saw when the electroscope is charged? Needle is standing diagonally Leaves 4.

44 Induced Charged Separation Conclusions: Net charge on electroscope remains neutral.

45 Charging by Conduction

46 Conclusions: Net charge on electroscope is the same as the charge on the rod / strip.

47 Charging by Induction (Negative Rod)

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50 Principles of Induction The charged object is never touched to the object being charged by induction. The charged object does not transfer electrons to or receive electrons from the object being charged. The charged object only served to polarize the object being charged. The object being charged is touched by a ground; electrons are transferred between the ground and the object being charged (either into the object or out of it). The object being charged ultimately receives a charge that is opposite that of the charged object which is used to polarize it.

51 Testing a Charged Electroscope Conclusions: Electroscope is positively charged.

52 Conclusions: Electroscope is negatively charged.

53 Recap after Break What is a conductor? What is an insulator? What is grounding? What is the value of an elementary charge? Is it possible to have a charge of 8.60 x C? What is conduction? What is polarization? What is the reason peoples hair stands up when touching the Van de Graaf? What happens when something charged goes near/touches an electroscope?

54 The Electrostatic Force (Coulomb s Law) What causes the charged balloons to push or pull one another? An electrostatic force causes the balloons to push or pull on one another due to their charge. Factors that effect Electrostatic Force 1. size of charge 2. distance

55 Point Charge a charged conducting sphere interacts with other charged objects in a manner that it is as though its charge were located at its center.

56 Electricity & Magnetism: Electrostatics! F g =! F e = Gm Kq1 m q 2 r 2 The Gravitational Force is similar in nature to the Electrostatic Force do you know how?

57 Coulomb s Law the magnitude of the force between two charges separated by a distance (r), can be found using. qq 1 2 e 2 F=k r = 8.99 x 10 9 N m 2 /C 2 Ref. Table

58 Charles-Augustin de Coulomb June 14, 1736 August 23, 1806 was a French physicist. He was best known for developing Coulomb s Law, the definition of the electrostatic force of attraction and repulsion, but also did important work on friction.

59 Electric Force Electric Force Charge OR qq F=k r 1 2 e 2 Separation

60 F +1C +1C 1 meter F! Q q (1)(1) F = k 1 2 = k = 8.99x10 9 e r N Coulomb Force measured to be 9 Billion Newtons (2.25 Billion Pounds)!!! 100x the weight of the space shuttle on Earth

61 ! F = k e Q q 1 2 r 2 If the distance between Q 1 & q 2 is doubled, the force is. ¼ as great If the distance between Q 1 & q 2 is decreased from 4 m to 1 m, the force is 16 times greater If the distance is held constant, but Q 1 is tripled, the force is Tripled If Q 1 is doubled and q 2 is halved, the force is unchanged

62 Which object below experiences a greater force, A or B? Think back to Force of gravity between two objects Both experience the same amount of force.

63 Calculate the electric force between them if their centers are 7.5 x 10-1 meters apart. qq 1 2 e 2 F=k r (3.0x10-6 C)(6.0x10-6 C) x 9 Ngm 2 C 2 = N (7.5x10-1 m) /.29 If object A has its charge doubled, what is the new force it exerts on B? What is the force that B exerts on A?.58 N

64 Three charges exist in a closed system. If q 1 = +1.2 μc, q 2 = -3.4 μc, and q 3 = +6.8 μc, what is the net force on charge q 2 in each situation? q 1 q 2 q m 5.5 m q q (1.2x10 C)(3.4x10 C) F =k =8.99x10 Ngm /C =.0092N qonq 1 2 r 2 (2.0m) q q (6.8x10 C)(3.4x10 C) F =k =8.99x10 Ngm /C =.0069N qonq 3 2 r 2 (5.5m) 2 F net =.0023 N Left

65 Electric Fields How can a force be exerted across what seems to be empty space? In trying to understand electric force, Michael Faraday developed the concept of an electric field. According to Faraday, a charge creates an electric field about it in all directions. If a second charge is placed at some point in the field, the second charge interacts with the field at that point. The resulting force is the result of a local interaction that travels along the electric field lines.

66 Electric Field vector quantity that relates the force exerted on a test charge to the size of the test charge. All charged objects create an electric field which extends outward into the space which surrounds it. The charge alters that space, causing any other charged object that enters the space to be affected by this field.

67 Test Charge a small positive charge that is placed near another charge to produce a force (attractive or repulsive) in order to observe an electrical field. Why should the test charge be small? The test charge also exerts a force on q. It is important that the force exerted by the test charge doesn t move q to another location, and thus change the force on q and the electric field being measured.

68 Calculating the Strength of an Electric Field F onq E= q Units - N/C (Newton / Coulomb)

69 An electric field is to be measured using a positive test charge of 3.0 x 10-6 C. This test charge experiences a force of 0.12 N acting at an angle of 15º. What is the magnitude and direction of the electric field at the location of the test charge?

70 A charge of C experiences a force of 0.2 N when located a certain position in the electric field produced by a second charge. What is the electric field strength at that point? F 0.2N E= = =2x10 4 N/C q 1.0x10-5 C

71 Drawing Electric Fields

72 Drawing Electric Fields 1.The strength of the electric field is indicated by the spacing between lines. The field is strong where the lines are close together. It is weaker where the lines are spaced farther apart. 2.The electric force, and thus the electric field, is always directed perpendicular to the surface of an object. 3.Field lines always leave a positive charge and enter a negative charge. 4.The direction of the arrow shows the field direction 5.Electric field lines should never cross

73 Which point charge has the greatest field strength?

74 Draw the electric field lines surrounding the positive and negative charge below. + -

75 Change up the Picture (Extra don t need to write down)

76 Electric Fields for Equal Charges Equal and opposite charges

77 Electric Fields for Equal Charges Equal positive charges

78 Electric Fields for Equal Charges Draw the electric field for two equal negative charges

79 Electric Field for Unequal Charges Unequal opposite charges

80 Draw the electric field around two unequal negative charges

81 Draw the electric field between two oppositely charged parallel conducting plates.

82 Electric Potential Energy How could you increase the gravitational potential energy of a ball on Earth? By doing work on it to increase its height from Earth s surface. Electrical Potential Energy: potential energy associated with an object due to its position relative to a source of electric force.

83 In which scenario is the electric potential energy increasing? Explain. Scenario B F you on charge B + A Scenario A + F you on charge E d E In Scenario A, because work is being done by an external force to pull the positive test charge away from the negative charge. The farther the test charge is from the negative charge, the more potential it has to move.

84 Consider the following scenarios and the forces present. In which scenario does the test charge have the greatest potential energy? Scenario A Scenario B Is there always an electric potential difference between the two positions? Suppose you move the test charge in a circle around a negative charge. The force the electric field exerts on the test charge is always perpendicular to the direction you moved it, so you do no work.

85 Electric Potential Difference in a Uniform Field Potential Difference ( V ) difference in potential energy between two points. At which location will the test charge have more electric potential energy? Which spot in general, A or B, is at a higher electric potential? Electric potential energy is dependent on the charge. A

86 Units J/C = Volt (V)

87 If 21 joules of work is required to move 7.0 coulombs of charge between two plates, the potential difference between the two plates is W 21J V= = =3.0V q 7.0C

88 Two large, charged parallel plates are 4.0 cm apart. The potential difference between the two plates in 25 V. What work will you do to move a charge equal to that of one proton from the negative to the positive plate? W = qv = (1.60 x C)(25 J/C) = 4.0 x J

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91 If a proton is placed as shown between plates that are separated by a distance of 3.5 cm with an electric field of 2.9 x 10 4 N/C. a) what is the force on the proton? F = Eq = (2.9x10 4 N/C)(1.60x10-19 C) = 4.6 x N b) what is the acceleration of the proton? F 4.6x10-15 N a= = =2.8x10 12 m/s 2 m 1.67x10-27 kg c) how much work is done on the proton moving it to the bottom plate? W = qv = (1.60 x C)(1000.v) = 1.6 x J

92 electron neutron proton

93 electron neutron Which particle experiences a greater force? proton The electron and proton experience the greater force. A greater acceleration? The electron, because it has the smallest mass.

94 The Electronvolt Electronvolt the energy that an electron (or proton) gains when accelerated through a potential difference of 1 Volt. A unit of energy commonly used in atomic and nuclear physics because of its convenient small size.

95 A proton has 4.0 ev of work done on it as it moves in an electric field. How much energy in joules does the proton gain as the work is done? -19 æ1.60x10 J ö 4.0eV =6.4x10-19 ç J è 1eV ø

96 2. An electron falls through a potential difference of 30. Volts. How much kinetic energy does the electron gain? Express your answer in both joules and electronvolts. W = qv = (1.60 x C)(30. V) = 4.8 x J 1eV 4.8x10-18 æ ö J =30.eV ç è1.6x10-19 J ø