# Coulombs Law Electric Field Questions-UCF .

Homework 1 Please mark your answers to the multiple-choice questions and write down the final results of your calculations; correct units are important (10% of weight). Multiple-choice questions are critical (60% of weight for each problem). Do calculations on your scratch paper, but don’t submit it. Submit only this file with your answers. Name Farah Qaqa 1. (20 points) At what distance would the repulsive force between two electrons have a magnitude of 2.00 μN? To solve this problem I used the following: A. B. C. D. E. The Newton’s second law; The Coulomb’s law; Superposition principle A and B; B and C. Answer: 1.073*10^-11 N 2. (20 points) In the figure, if Q = 30 μC, q = 5.0 μC, and d = 30 cm, what is the magnitude and direction (left/right/up/down) of the electrostatic force acting on q? To solve this problem I used the following: A. B. C. D. E. The Gauss’ law; The Coulomb’s law; Superposition principle A and B; B and C. Answer: 7.49 N pointing toward the right 3. (20 points) What should the charge (sign and magnitude) of a 1.5 g particle be to remain balanced against gravity when placed in a downward-directed electric field of magnitude 650 N/C? To solve this problem I used the following: A. The equation E = V/d; B. The equation E = q /2Aε0 C. The equation F = qE D. The Coulomb’s law; E. Superposition principle; F. D and E. Answer: 2.26 * 10^-5 C in the negative direction 4. (20 points) A point charge of -4.00 nC is at the origin, and a second point charge of +6.00 nC is on the x axis at x 0.800 m. Find the magnitude and direction (left/right) of the electric field at each of the following points on the x axis: (a) x = 20.0 cm. (b) x = 1.20 m, (c) x = -20.0 cm. a. 1050 N/C pointing towards the left. b. 312.5 N/C pointing towards the right. c. 846 N/C pointing towards the right. To solve this problem I used the following: A. The equation E = V/d; B. The equation E = (kq)/r C. The equation E = (kq)/r2 D. The equation E = (kq1q2)/r2 E. Superposition principle; F. C and E; G. D and E. 5. (20 points) Four point charges are located at the corners of a square of side a = 10 cm, as shown below. Find the electric potential at the center of the square if Q = 15 μC. a +Q To solve this problem I used the following: +3Q A. +Q -2Q B. C. D. E. F. The equation V = Ed The equation V = kq/r The equation V = kq/r2 Superposition principle; A and C; B and D. Answer: 5.7276*10^6 V Homework 2 Please mark your answers to the multiple-choice questions and write down the final results of your calculations; correct units are important (10% of weight). Multiple-choice questions are critical (60% of weight for each problem). Each problem score is 20 points. Do calculations on your scratch paper, but don’t submit it. Submit only this file with your answers. Name: Farah Qaqa 1. Two parallel squared metal sheets (of 1 m length side) are separated by 1 mm of vacuum. The sheets are connected to a dc source with potential difference of 500V. What is the magnitude of charges accumulated on the sheets upon connection to the source? To solve this problem I needed to: A. Use the equation E=V/d; B. Calculate the Coulomb’s forces acting on the charges; C. Figure out what a known device is represented by these sheets; D. Calculate the capacitance of the voltage source; E. Calculate the capacitance of the sheets; F. Find the formula linking voltage and charge in capacitor; G. A, B, and D; H. C, E, and F. Answer: C= 4.425*10^-6 C 2. Determine the equivalent capacitance of the combination shown when C = 45 F. To solve this problem I used the formula for: A. B. C. D. Equivalent capacitance of capacitors connected in series; Equivalent capacitance of capacitors connected in parallel; A and B; None of the above. Answer: 36 MF 3. Determine the total energy stored by all capacitors if C1 = 20 F, C4 = 30 F, and V0 = 45 V. F, C2 = 10 F, C3 = 14 Energy stored in capacitor is: A. B. C. D. E. F. Proportional to C2 Proportional to 1/C2 Proportional to V2 Proportional to 1/V2 Proportional to I2 Proportional to 1/I2 Answer: 2.2 MJ 4. A rod of 2.0-m length and a square (2.0 mm 2.0 mm) cross section is made of a material with a resistivity of 6.0 10 8 m. If a potential difference of 0.50 V is placed across the ends of the rod, what power is dissipated in the rod? To solve this problem I needed first to calculate: A. B. C. D. E. Total charge stored in the rod; Electric field inside the rod; Capacitance of the rod; Resistance of the rod; None of the above. Answer: 8.33 W 5. Find the total power dissipated in the resistors. The following statement is correct: A. If the resistor of 5.0 Ω is replaced by the resistor of 6.0 Ω the total dissipated power increases; B. If the resistor of 5.0 Ω is replaced by the resistor of 6.0 Ω the total dissipated power decreases; C. If the voltage of the source is doubled, the total P is also doubled; D. If the voltage of the source is doubled, the total P is increased by factor 4; E. A and C; F. B and D. Answer: 7.12W, 6.72W, 28.48W Study guide – Exam 1 This one-hour exam will start at 2:00 pm on Tuesday July 15. I will use our lecture Zoom meeting to answer your questions at the beginning, then I will end the meeting before you start working on the exam. A Word file with questions will be posted to Webcourses (link “Files”) at the time of the beginning of the exam. You will need to put your answer to the same file just below the corresponding questions. If the answer implies calculations, first, you will need to answer the provided multiple-chose question to explain what equation/formula/approach you use, and next, put the final number. The completed exams must be submitted to Webcourses at the due time. The students who need accommodation service will be granted with extra time (extra 100% of the exam time) and will submit their exams to me via email by the corresponding time. I provide here the topics, notions, and concepts, which you need to know and understand to succeed in the Exam 1. No topic beyond those listed here will be covered in the exam. If, in top of that, you were able to solve all homework problems, you will get “A” for sure. Chapter 18 You need to understand the concepts related to electric charge. There are two stable elementary particles: negatively charged electrons and positively charged protons. Charge is conserved. All materials are composed of these particles (also of neutrons which are not charged). What is a difference between conductors and dielectrics in terms of charge propagation? Be able to use Coulomb’s law for calculating forces exerted by several point charges on a test charge. Don’t forget that forces are vectors and that the superposition principle applies to the electrostatic forces. Expect the problems for one-dimensional systems, similar to those you solved in the class and home works. You can also expect conceptual questions in which you are asked to evaluate qualitatively changes in forces upon changes in distances between point charges and charge magnitudes, as we did it in the class or quiz 1. Electric field. Since electric field is equal to the force acting on the unit positive charge, the approaches to solutions are similar to those applied to forces. Again look at the problems and conceptual questions in home works and quiz 1. What is the electric field inside conductors? If a conductor is charged, are charges located inside or on the surface? What is the direction of electric field close to the surface of a charged conductor? Chapter 19 How the electric potential is related to the potential energy stored in electric field? Important: the value of the potential energy at a given point doesn’t make much sense, the difference in potential energy between points is important. It tells us how much kinetic energy of charge can be gained or lost, if the charge is moving between these points. The same is true for the electric potential: potential difference is important. Understand the relation between uniform electric field and potential difference. Potential due to a point charge at some point is the potential difference between that point and in

finity. Be able to calculate the electric potential due to several point charges. Look at the related homework problems, as well as at conceptual questions and problems solved in the class. Capacitance C is the coefficient of proportionality between Q and V. Be able to use relation between area of plates, separation between them, and capacitance of a parallel plate capacitor. Understand the charge and voltage distribution between capacitors connected in parallel and in series (in which case the charges are the same and voltages are different and vice versa). Be able to calculate equivalent capacitance of a combination of capacitors connected in parallel and in series (as we did in class and in homework). Be able to use formulas for the energy stored in capacitor expressed through Q or V. Chapter 20 What is current? Why does resistance exist? What is the difference between resistivity and resistance? Be able to use the formula R = L / A. Understand the Ohm’s law. Be able to properly operate with formulas expressing electric power through V, I, R. Look at the problems solved in the class and homework. Chapter 21 Understand the physics behind connections of resistors in parallel and in series. What is different in voltage drop and current distribution for these connections? Be able to calculate the equivalent resistance of combinations of the resistors connected in series and parallel, as well as power dissipated in these combinations. Look at the problems we solved in the class and homework. Understand the difference between emf and terminal voltage of the battery – consider that not all chemical energy is converted in the battery into electric energy, some energy is lost, converted to thermal energy. What is the total voltage of the batteries connected in series? In parallel? Understand basic ideas of the Kirchhoff’s rules. Understand how voltmeter and ammeter work. Are they connected in parallel or in series to do correct measurements? Question 1 40 pts Consider two systems: 1) two positive charges of the same magnitude 91 separated by distance r1 repel each other with the force F1; 2) two other positive charges, each of the magnitude 92 = 391 separated by distance r2 = 3r1, repel with the force F2. What is a relation between forces F1 and F2? A. F2 = F1/9; B. F2=F1/3; C.F, = F : D. F2 = 3F1; E. F2 = 9F1. с A B D E Question 2 30 pts Electric field inside of a conductor: A. Is always equal to the electric field outside; B. Is proportional to 1/r^2; C. Is always greater than the field outside; D. Equals to zero; E. Impossible to determine. B C Question 3 30 pts A solid conducting sphere is charged with the charge Q. What is the charge distribution over the sphere? A. Charge is distributed uniformly over the sphere volume; B. All charge is located at the sphere surface; C. Charge is located at the center of the sphere; D. Conductors cannot be charged. B A o o