Unformatted text preview: Exam 1 – white version Physics 2760 SP 2008 Last Name______________________________ First Name_____________________________ ID # ___________________ This is a closed book exam. I understand, pursuant to University Regulations on academic honesty, that I am not to use any notes or information other than what is in the official, non‐annotated formula sheet. Signature_______________________________________ For multiple choice questions, please make sure that you circle the letter for the answer which you believe to be correct and only that answer. If more than one answer is circled for the same problem, you will not receive credit for it. Don’t get hung up on questions. They should take only one or two minutes each. If you find yourself spending more than a few minutes on a multiple choice question you are probably looking at it the wrong way. You should skip it for now and come back to it later. For full credit show your work for solutions to questions that require calculations. Explain from where you start to solve the problem and show your math flowing from it for full credit. No shown work, no credit! Relax, read carefully, think – and then read everything again. During the exam, if you have questions please raise your hand and the TA or the instructor will come to you and provide help. 1. Two oppositely charged particles are inside a Gaussian sphere. Consider the statements below: Andrew: “Since the charges inside the sphere cancel or neutralize each other, the net electric flux through the sphere will be zero, so there will be no electric field.” Sam: “Since the net charge inside the sphere is zero, the net electric flux through the sphere will be zero. However, the electric field must have a non‐zero value inside and outside of the sphere because charged particles produce electric fields.” Sandy: “Since there are charges, there must be an electric field present. Since electric flux depends on the electric field and the area of the surface, neither of which is zero, there must be a non‐ zero net electric flux through the sphere.” Which of these students is correct? Circle one: Andrew Sam Sandy None of them All of them 2. A cube of length L = 4 m has one corner at the origin, as shown in the figure. The sides of the cube are parallel to the coordinate planes. The electric field in and around the cube is given by E = ( 2 + 3 x ) i − 4 j (in N/C). a. Find the electric flux through the left surface of the cube. b. Find the electric flux through the top surface of the cube. c. Find the electric flux through the front surface of the cube. 3. Shown below are six situations where three point charges are placed in a row. The magnitudes and signs of the charges are given in the figures. In which situation is the magnitude of the electric field that exerts a force on the right hand charge the greatest? Circle the letter on the drawing. 4. The dashed lines in the figure below represent equipotential surfaces in a region in which there is an electric field. Six points on the drawn circles are labeled A to F. A positive point charge is moved in a straight line from P to each of the six labeled points in turn. On the drawing provided, circle the letter corresponding to the point for which the work done by an external agent to move the positive charge from P to it (the letter) is the greatest. 5. In each case shown below, charged particles are arranged in identical equilateral triangles. The particles are all the same size and mass but they have different charges. Circle the arrangement that has the smallest electric potential energy. 6. In the figure below, a small, non‐conducting ball of mass m, and charge q hangs from an insulating thread that makes an angle θ with the vertical, uniformly charged non‐conducting sheet that extends far vertically and into and out of the page. The gravitational force on the ball must be taken into consideration. + a) On the figure provided, draw all the forces acting on the non‐conducting ball, label them, and show the direction of + the electric field. +θ + b) What type of charge is q, positive or negative? + m, q + c) Calculate the surface charge density σ of the non‐conducting sheet. Express your answer in terms of the given variables: m, q, θ, ε0, and gravitational constant g. 7. The figures below show cross‐sections of six spherical Gaussian surfaces labeled A‐F. The surfaces on the left enclose a uniformly charged insulator, and the surfaces on the right enclose a charged conductor. The insulator and the conductor have the same total charge +Q. Circle the statement that is correct: A. B. C. D. E. The flux through surface A is bigger than the flux through surface C. The flux through surface B is less than the flux through surface C. The flux through surface F is bigger than the flux through surface D. All the surfaces have the same flux. All surfaces have zero flux through them. 8. An uncharged conducting spherical shell has inner radius R1 = 2 cm and outer radius R2 = 4 cm. A positive point charge Q1 = 2 x 10‐6 C is placed at its center and a charge Q2 = ‐ 4 x 10‐6 C is placed on the spherical shell. Take 1/4πε0 = k = 9 x 109 Nm2/C2. a) What is the net charge on the surface of the spherical shell? b) Calculate the electric field and the electric potential at a distance r = 5 cm from the center of the sphere. c) Calculate the electric field and the electric potential at a distance r = 3 cm from the center of the sphere. MC 25/ Q6 25/ Q7 25/ Q8 25/ Total 100/ ...
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- Spring '08
- Physics, net electric flux, Andrew Sam Sandy