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Unformatted text preview: pulat (fap294) – oldmidterm 01 – Turner – (56725) 1 This print-out should have 19 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points From the electric field vector at a point, one can determine which of the following? I. the direction of the electrostatic force on a test charge of known sign at that point; II. the electrostatic charge at that point; III. the magnitude of the electrostatic force exerted per unit charge on a test charge at that point. 1. II only 2. I and II only 3. III only 4. None of these 5. II and III only 6. All of these 7. I only 8. I and III only correct Explanation: The definition of the electrostatic force is vector E = vector F q , so vector F = q vector E and vector F acts in the same or opposite direction to vector E depending on the sign of the charge. If we only consider the magnitude F = q E for a unit charge, the electric field’s magnitude is E = F q . 002 (part 1 of 2) 10.0 points A dipole (electrically neutral) is placed in an external field. − + (a) − + (b) − + (c) − + (d) For which situation(s) shown above is the net force on the dipole equal to zero? 1. ( a ) and ( c ) 2. ( b ) and ( d ) 3. ( c ) only 4. Another combination 5. None of these 6. ( c ) and ( d ) correct 7. ( a ) only Explanation: Basic Concepts: Field patterns of point charge and parallel plates of infinite extent. The force on a charge in the electric field is given by vector F = q vector E and the torque is defined as vector T = vectorr × vector F Δ vector E = k Δ q r 2 ˆ r vector E = summationdisplay Δ vector E i . Symmetry of the configuration will cause some component of the electric field to be zero. Gauss’ law states Φ S = contintegraldisplay vector E · d vector A = Q ǫ . pulat (fap294) – oldmidterm 01 – Turner – (56725) 2 Solutions: The electric dipole consists of two equal and opposite charges separated by a distance. In either situation ( c ) or ( d ), the electric field is uniform everywhere between the parallel infinite plates. Thus, the electric force on one charge is equal but opposite to that on another so that the net force on the whole dipole is zero. By contrast, electric fields are nonuniform for situations both ( a ) and ( b ). 003 (part 2 of 2) 10.0 points For which situation(s) shown above is the net torque on the dipole equal to zero? 1. other combination 2. ( b ) and ( d ) 3. None of these 4. ( a ) only 5. (a) and (b) 6. ( c ) only 7. ( c ) and ( d ) 8. ( a ) and ( c ) correct Explanation: A electric dipole can be regarded as a pair of charges of opposite sign. Only in figures (a) and (c), the electric fields are along the direction of vectorr , where vectorr is the vector between the pair of charges. Therefore the force vector F is also along vectorr . This will lead to zero torque, since vector T = vectorr × vector F ∝ vectorr × vectorr = 0 ....
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This note was uploaded on 10/19/2010 for the course PHY 303L taught by Professor Turner during the Spring '08 term at University of Texas.
- Spring '08