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6.2 Electric Force and Field (2 hr) Name _____________________ Activity 621 Investigating Electric Force Activity 622 Electric Field Hockey Read Tsokos pp 279-287, 289-291 Read Cutnell pp 533 to 552 Assignment A621 -- Tsokos pp 287, 288 #3-10, Holt SG pp91-96, Assignment A622 -- Cutnell p563 #8,9,12,13,17, Assignment A623 -- Ranking pp38,39(?), 125, 126, 134, 135, 136, 137137, 138, 140, 141,142, 145, 146, 148, 149 1 6.2 Electric Force and Field Electric Charge A. Properties of Charges (a) Two types of charge: Positive: proton, quarks (up, charmed, and top) Negative: electron, quarks (down, strange, and bottom) (b) Charges are measured in a unit called Coulomb (C). The charge on an electron e– = –1.602x10–19 C The charge on a proton p+ = + 1.602x10–19 C The charge on up, charmed, and top quarks = + The charge on down, strange, and bottom = - e e The symbol e = 1.6x10–19 C (c) An atom is neutral when Number of e- = number of p+ An object becomes positively charged when it loses electrons; an object becomes negatively charged when it gains electrons. (d) Electric charge, which can be positive or negative, occurs in discrete natural units (Charge always comes in an integral multiple of a basic unit - it is quantized.) (e) Electric charge is neither created nor destroyed. (f) The total electric charge of the universe is constant. No physical process can make this happen Summary 2 (g) Any kind of charge attracts neutral object because of polarization. (Polarization is the process of separating opposite charges within an object. The object still remains neutral while the electrons are pulled toward one of the ends by an external electrical attraction causing both ends with different excessive charges) (h) This basic unit of charge is conventionally denoted by e : e = 1.602 x 10- 19 Coulombs (C) (i) All natural charged objects must have a net charge that is an integral of e, which was confirmed by American Physicist Robert A. Millikan.(1868-1953) <Sample question 1> How much charge does a lithium ion, carries? There are 3 protons and 4 neutrons in the lithium nucleus. ______________________________________ ______________________________________ <Sample question 2> An electronically neutral object acquires a net electric charge. Which one of the following statements concerning the mass of the object is true? ______ (i) The mass does not change (ii) The mass decreases if the charge is negative and increases if the charge is positive. (iii) The mass increases if the charge is negative and decreases if the charge is positive. Summary 3 Electrostatic Phenomenon: a. Charged objects may bend water, attract dust, attract small pieces of paper, cause electric shock. b. Colliding crystals of ice in a rain cloud can cause charge separation – result in lightning bolts, to bring unlike charges together again. c. Voyager spacecraft observed electric discharges between the particles in the rings of Saturn. The particles are constantly undergoing collision and becoming charged. Application of Electrostatic d. Air filter e. metal coating f. CO2 fire extinguisher g. Photocopiers and printers h. Disordering tools (to apply strong suction to remove excess molten solder from a printed circuit board or connection terminal.) B. Law of Conservation of Electric Charges During any process, the net electric charge of an isolated system remains constant. <Sample process> When Kim uses a piece of nylon cloth to wipe a Teflon frying pan, the nylon becomes positively charged while the Teflon becomes negatively charge because the electrons on the nylon are transferred to the Teflon. <Sample process> 1 1 H 12H 23He (Fusion) 1 0 236 * 143 90 1 n 235 92 U 92 U 55 Cs 37 Rb 30 n (Fission) Summary 4 C. Conductors and Insulators Conductors: Materials, in which electric charges move about more or less freely. Copper and gold are called electrical conductors Insulators: Materials, in which electric charges do not move freely because of the strong bonding. Glass, rubbers, silk, and plastic, are called electrical insulators Both conductors and insulators may be charged by induction and contact. Most metals are good conductors and most non-metals are not. Most electric insulators are also good thermal insulators. Semiconductors: the materials that has properties between conductors and insulators; the materials that can be “finetuned” to display desired conductivity by controlling the concentration of “impurities” inside of the materials. http://superconductors.org/INdex.htm Photoconductive materials: the materials that conducts electricity when exposed to light (or due to the absorption of electromagnetic radiation/wave but are insulators when in the dark. Summary 5 Coulomb’s Law (Electric Force) Coulomb’s Law states that the electric force between two point charges is directly proportional to the charges and inversely proportional to the distance between two charges. The electric force between Q1 and Q2 is given by F k q1 q 2 r2 or Where Q1, Q2 are amount of charges, r is the separation between them, and k is Coulomb’s constant; = 8.99x109 N m2 C–2 k= is the permittivity of free space – N–1 m2 C–2 <Sample Question> Q2 Two charges, q 1 = 4μC and q2 = 6 μC, are placed along a straight line and separated by a distance of 2 cm. Find the force exerted on each other. ____________________________________________________________ ___________________________________________________________ ____________________________________________________________ 6 Q3 At what distance from q1 of example question Q2 would a third positive charge experienced no net electric force? ____________________________________________________________ ___________________________________________________________ ____________________________________________________________ ___________________________________________________________ ____________________________________________________________ _____________________________________________________________ ____________________________________________________________ ___________________________________________________________ Q4 Three charges, q 1 = 2μC and q2 = 2 μC and q3 = –3 μC, are at the vertices of an equilateral triangle, of side 3 cm. Find the force on q1. ___________________________________________________________ ____________________________________________________________ ___________________________________________________________ ____________________________________________________________ ____________________________________________________________ ___________________________________________________________ ___________________________________________________________ 7 Q5 Two equal charges q are suspended from string as shown. Show that tan = , _____________________________________________________________ ____________________________________________________________ ___________________________________________________________ ___________________________________________________________ ____________________________________________________________ ____________________________________________________________ ___________________________________________________________ ___________________________________________________________ ____________________________________________________________ Q6 Two identical conducting spheres are kept a certain distance “r” apart. One sphere has a positive charge Q on the surface and the other is neutral. The spheres are allowed to touch and then separated. Write down an expression for the electric force between the spheres. One sphere is discharged. The spheres are allowed to touch and then separated again. Write down the expression for the electric force between the spheres now. Summary 8 Q 7 Compare the net electric force acting on charge #1, 2, 3, and 4 as shown in the diagram ____ > ____ > _____ >______ ____________________________________________________________ ___________________________________________________________ ____________________________________________________________ ___________________________________________________________ ____________________________________________________________ ___________________________________________________________ ____________________________________________________________ ___________________________________________________________ 9 Electric Field, E Electric field is defined as the force per unit charge experienced by a small positive test charge. The strength of an electric field, E, at a certain point in the space is defined to be the magnitude of the electric force, F, acting on a small test charge divided by the magnitude of its charge, q0: E F q0 Where E is the electric field, F is the electric force acting on the text charge, qo is the amount of charge on the text charge SI unit of electric field is newton per coulomb (NC– -1 ) or volt per meter (voltm–1) Direction of E at a point is defined to be the direction of the electric force that would be exerted on a small positive charge placed at that point. The test charge should be small, both in physical size and charge, to accurately measure the electric field. Strong test charge may influence the source of the electric field to be determined. Acting through space – no physical contact. An electric field exists in the region of space around a charged object. Electric forces arise when another charged object enters this electric field. Consider the electric field due to a point charge, Q. The magnitude of the force exerted on a test charge, q, at a distance r away is F k Qq r2 The magnitude of the electrical field due to charge Q at the position of q is F/q Ek Summary 10 Q r2 Q7 To determine the direction and the strength of an uniform electric field, Tom places a very small particle charge carrying +2.0 μC in the field. The electric force acting on the test charge is detected as 26 pN, south. What is the direction and the magnitude of the electric field? ____________________________________________________________ ___________________________________________________________ ____________________________________________________________ ___________________________________________________________ Q8 A + 20 nC positively charge creates an electric field in space around it. What is the electric field 0.4 m from the center of the charge? ____________________________________________________________ ___________________________________________________________ ____________________________________________________________ ___________________________________________________________ Q9 A charge, q1 = 5.00 μC, is at the origin, and a second charge, q2 = –3.00 μC, is on the x-axis 0.800 m from the origin. Find the electric field at a point on x = 0.200m. Summary 11 Q10 An electric field of 2.0x104 NC –1 is directed along the positive x-axis. What is the electric force on an electron in this field? ____________________________________________________________ ___________________________________________________________ ____________________________________________________________ ___________________________________________________________ Q11 Describe the motion of a proton in an uniform electric field as shown. ____________________________________________________________ ___________________________________________________________ ____________________________________________________________ ____________________________________________________________ ___________________________________________________________ ____________________________________________________________ Summary 12 Q12 A small 2.0 g plastic ball is suspected by a 20.0 cm string in a uniform electric field of 1.0 x 104 NC–1, as shown below. (a) In the ball’s charge positive or negative? (b) If the ball is in equilibrium when the string makes a 15o angle with the vertical as indicated, what is the net charge on the ball? ____________________________________________________________ ___________________________________________________________ ____________________________________________________________ ____________________________________________________________ ___________________________________________________________ ____________________________________________________________ ___________________________________________________________ ____________________________________________________________ ____________________________________________________________ Summary 13 Electric Field Lines The following rules apply to electric field lines: 1. Lines begin and end only at charges (beginning at + charges, ending at - charges) or at Infinity. 2. Lines are closer together where the field is stronger. 3. Larger charges have more field lines beginning or ending on them. 4. Electric Field lines never cross (since E must point in a definite direction unless it is zero). 5. At any location, the direction of the electric field is tangent to the electric field line that passes through that location. 6. Electric file lines are perpendicular to surface of conductors. Examples: 1. Positive point charge 2. Negative point charge 3. Electric dipole 4. Electric dipole (Two equal and opposite point charges) (Two equal and same point charges) 5. Two parallel charged plates 6. Two unequal charges 14 15 16 17 Activity 621 Investigating Electric Force Coulomb's law, or Coulomb's inverse-square law, is a law of physics describing the electrostatic interaction between electrically charged particles. The magnitude of the electric force is directly proportional to the charges and inversely proportional to distance square. Essential Question: How do you determine the electric force between two charged objects? 1. Identify the chosen variables. 2. Decide the procedure and divide the work among partners 3. Carry it out collaboratively 4. Check your findings and analyze it. Activity 622 Electric Field Hockey Go to the following link http://phet.colorado.edu/en/simulation/electric-hockey, play all three levels of the electric field games, copy (control + C) and paste (control +V)the wining image for the proof. 18