Unit 10 Electrostatics

Unit Starter Case 1: It is cold and dry in your house. You walk across the carpeted floor with thick wool socks while dragging your feet. You touch a metal door knob. What might you experience and why would it occur? (John Travoltage animation) 2

Unit Starter Case 2: How did this happen? 3

I. Origin of Electricity PROTONS (p) NEUTRONS (n) ____________ and ______________ are found in the nucleus. ________________ are found swarming around the outside of the nucleus. Electrons hold a _____________ charge and protons hold a ___________ charge.  How does the magnitude of the charge on an electron compare to the magnitude of the charge on a proton? ELECTRONS (e-) NEGATIVE POSITIVE EQUAL 4

I. Origin of Electricity Which subatomic particle is responsible for a neutral object becoming “charged?”    Units for charge: 1 e = OR 1 C = 6.25 x 10 18 e ELECTRONS (MOBILE) COULOMB (C) ELEMENTARY CHARGE 1.60 x 10 -19 C (see ref. tabs.) 5

I. Origin of Electricity Calculate the number of electrons on an object that has a charge of - 3 C.   How do the number of electrons compare to the number of protons in an object that has a neutral charge? EQUAL IN # 6

II. Conductors and Insulators Conductors: Materials that allow ________________________ to move easily   Examples:  Insulators: Materials that _________________________ to move easily CHARGES (ELECTRONS) copper, gold, aluminum DO NOT ALLOW CHARGES Glass, wood, most plastics 7

III. Rules of Electrostatics 1. Only electrons (e-) move 2. Opposite charges (+ & - ) attract, like charges (+ & + or - & -) repel 3. Charged objects (+ or -) are attracted to neutral objects Pith Ball animation 8

IV. Law of Conservation of Charge Transfer of Charge  ____________ may be transferred from one object to another  If object loses electrons (negative charge), it becomes _____________ charged  If object gains electrons (negative charge), it becomes _____________ charged ELECTRONS POSITIVELY NEGATIVELY 9

I. Origin of Electricity On pg. 3 of your work packet, complete the following problems: 6 7 (A, C, D ONLY) 8 9 10

IV. Law of Conservation of Charge The _______ charge of a ________ remains ____________ and charge is _________ distributed between objects the objects in contact. TOTAL SYSTEM CONSTANT EVENLY 11

WHY YOU SHOULD NOT PUT METALS (INORGANIC ELEMENTS) IN MICROWAVES WHY YOU SHOULD NOT PUT METALS (INORGANIC ELEMENTS) IN MICROWAVES? VIDEO Video 2 (DO NOT TRY AT HOME) 12

IV. Law of Conservation of Charge From your Unit 10 Work Packet, complete the following problems. Pg. 4: problems 2 – 5 13

V. Electrostatic Charging Charging by Conduction: (electroscope animation)  - Charging a neutral object by _____________ with a charged object - Neutral object gains the ___________ charge compared to the charged object CONTACT SAME 14

V. Electrostatic Charging Charging by Induction: (electroscope animation)  - Charging an object _______________ by allowing __________ charges to ______ on or off of neutral object - Neutral object gains the _______________charge compared to the charged object W/OUT CONTACT NEGATIVE MOVE OPPOSITE 15

VI. Coulomb's Law Nature of Electrostatic Forces: 1. Like charges repel, unlike charges attract 2. Charged objects apply equal (magnitude) and opposite (direction) forces on each other B. Factors that Affect the Electrostatic Force applied by charge object on other charged object 1. Amount of __________ on each object CHARGE 2. __________ between charged objects DISTANCE Animation – Coulomb’s Law 16

VI. Coulomb's Law Graphical Relationship 17

VI. Coulomb's Law D. Coulomb’s Law Fe = electrostatic force (N) q = charge (C) k = electrostatic constant (8.99x109 Nm2/C2) r = distance between charges (m) 18

F1 F2 - + r VI. Coulomb's Law Vector Nature: Two unlike charges (positive and negative) F1 = - F2 (opposite in direction) F1 F2 - + r 19

+ + F2 F1 r VI. Coulomb's Law Vector Nature: Two like charges (two positive charges) F1 = - F2 (opposite in direction) + + F2 F1 r 20

VI. Coulomb's Law Practice Problems: Example: What is the electrostatic force between two small spheres possessing net charges of +2 µC and -3µC, if the distance between them is 10 m? 21

VI. Coulomb's Law Practice Problems: Example: What is the magnitude of the electrostatic force between a proton and an electron in a hydrogen atom? ( r = 1 X 10 -10 m) 22

VI. Coulomb's Law Practice Problems: Example: A negative charge of - 6 X10 -6 C exerts an attractive force of 65 N on a second charge 0.050 m away. What is the magnitude of the second charge? 23

VI. Coulomb's Law 2.00 x 10 - 4 N, repelled 1.40 m Whiteboard: A charge of +5 x 10 -7 C and a charge of +4 x 10 -7 C are 3 m apart. Calculate the magnitude of the electrostatic force. Are they attracted or repelled away from each other? A charge of -4 x 10 -3 C exerts an attractive force of 55 N on a second charge of +3 x 10 -6 C. What is the distance between the charges? 2.00 x 10 - 4 N, repelled 1.40 m 24

VII. Understanding of Fields What is a field in physics? AN AREA OF SPACE WHERE AN OBJECT EXPERIENCES NON-CONTACT FORCES VIII. Electric Fields _________ quantity that relates ________ exerted on a charge to the size of the charge Direction of electric field: - If source charge is __________, the electric field (E) vector points ______________ - If source charge is ___________, the electric field (E) vector points ________________ VECTOR FORCE POSITIVE AWAY FROM IT NEGATIVE TOWARDS IT 25

VIII. Electric Fields Electric Field Lines – Imaginary lines which a positive test charge would experience a force and move in an electric field 26

VIII. Electric Fields Equation for Electric Field Strength: Units: 27

VIII. Electric Fields Example: At a distance of 2.0 m, the electric field surrounding a charged sphere is 2.4x10 5 N/C directed in towards the sphere. Is the charge on the sphere positive or negative? 28

VIII. Electric Fields Example: A negative charge of 2 x 10 -8 C experiences a force of 0.060 N to the right in an electric field. What are the field magnitude and direction? 29

VIII. Electric Fields Example: A positive test charge of 5 x 10 -4 C is in an electric field that exerts a force of 2.5 x 10 -4 N on it. What is the magnitude of the electric field at the location of the test charge? 30

VIII. Electric Fields Whiteboard Problems 1 - 6 on pg. 20 of Unit 10 Work Packet Answers: 1) 5) 2) 6) 3) 4) 31

IX. Gravitational vs. Electric Potential Energy It takes work to move a charged object from place to place in an E-field. This work changes the energy of the charge just like lifting an object in a gravitational field changes the energy of a mass. Reminder: W = Fd = Δ E

IX. Gravitational vs. Electric Potential Energy B ↑ GPE = mgh ↑ direction of field A If a mass is moved from Point A to Point B, there is a(n) ___________ in the potential energy of mass. If a mass is moved from Point B to Point A, there is a(n) ___________ in the potential energy of mass. INCREASE DECREASE 33

IX. Gravitational vs. Electric Potential Energy Electrical Potential Energy If a charge is moved from Point A to Point B, there is a(n) ___________ in the potential energy of charge. If a charge is moved from Point B to Point A, there is a(n) ___________ in the potential energy of charge. INCREASE DECREASE

IX. Gravitational vs. Electric Potential Energy Electrical Potential Energy If a charge is moved from Point A to Point B, there is a(n) ___________ in the potential energy of charge. If a charge is moved from Point B to Point A, there is a(n) ___________ in the potential energy of charge. DECREASE INCREASE 35

IX. Gravitational vs. Electric Potential Energy In General: Anytime a charge is moved up/down in an electric field there is a change in electrical potential energy 36

X. Electric Potential Difference (V) The work done per unit charge in moving a charge between two points in an electric field is known as electric potential difference (V)

X. Electric Potential Difference (V) Equation: Unit:

X. Electric Potential Difference (V) Example: Moving a point charge of 3.2 x 10 -19 C between points A and B in an electric field requires 4.8 x 10 -18 J of energy. What is the potential difference between these points? 39

X. Electric Potential Difference (V) Example: How much electrical energy is required to move 6 µC charge through a potential difference of 36 V? 40

X. Electric Potential Difference (V) Example: Determine how many electrons are moved if it takes 1.5 x 10 -5 J of energy to move them through a potential difference of 15 V. 41

XI. Electric Potential Difference (V) Whiteboard Problems 1, 2, 3, and 6 from the ELECTRIC POTENTIAL problems on pg. 8 of Unit 10 Work Packet Answers: 1) 2) 3) 6) 42

eV is a unit of _______________ ENERGY XI. Electron-Volt The amount of work it takes to move one elementary charge through a potential difference of 1 V is called an ____________________ ELECTRON VOLT (eV) (see ref. tabs.) 1 eV = 1.6 x 10 -19 J eV is a unit of _______________ ENERGY When is it used? When energies involved are VERY _____________ SMALL 43

XI. Electron-Volt Examples: Convert the following energy to electron volts: 1.4 x 10 -19 J 2. How much work, in eV, is done moving 6 electrons through a potential difference of 2 V? 44

XII. Electric Field Between Oppositely Charged Plates CONSTANT NOTE: Electric field is __________________ every between the plates (EA = EB = EC). Therefore, a charge would experience a constant _______________ anywhere between the plates. FORCE 45

XII. Electric Field Between Oppositely Charged Plates 1. At which location would an electron experience the greatest force? A, B, C, or force would be the same at all location 2. An electron and proton are both placed at position b, which one would experience a greater acceleration? 46

XIII. Millikan Oil Drop Experiment (1909) What did he discover? The charge of one electron (1 e- = 1.6 x 10 -19 C) How did he do it? (1 min) 47

XIII. Millikan Oil Drop Experiment Free Body Diagram of Oil Drop: 48

XIII. Millikan Oil Drop Experiment If the weight of a single oil drop has found to have a weight of 1.9 x 10 -14 N and it is suspended in an electric field of 4.0 x 10 4 N/C, what is the charge of the oil drop? How many electrons are on it? 49

XIV. Capacitor and Electric Field Between 2 Parallel Plates What a capacitor does (animation): Stores a charge using two oppositely charged plates 50

XIV. Capacitor and Electric Field Between 2 Parallel Plates Charging Capacitor Discharging Capacitor___ (+) and (–) charges are separated on each side 51