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oldmidterm 01-solutions

# oldmidterm 01-solutions - howard(cah3459 – oldmidterm 01...

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Unformatted text preview: howard (cah3459) – oldmidterm 01 – Turner – (56705) 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 (part 1 of 2) 10.0 points The figure below shows two particles, each with a charge of + Q , that are located at the opposite corners of a square of side d . + + P Q Q d What is the direction of the net electric field at point P ? 1. correct 2. 3. 4. 5. Explanation: At point P , the electric fields due to the two positive charged particles have the same mag- nitude. One points downward, and the other horizontally to the left. Thus the direction of the net electric field points left and downward, forming a 45 ◦ angle with the horizontal. 002 (part 2 of 2) 10.0 points What is the potential energy of a particle of charge q that is held at point P ? 1. U = √ 2 2 π * q Q d 2. U = √ 2 4 π * q Q d 3. U = 1 4 π * q Q d 4. U = 0 5. U = 1 2 π * q Q d correct Explanation: The potential energy of the particle charged + q is the sum of the potential energies due to the two + Q charged particles. U = U 1 + U 2 = 1 4 π * q Q d + 1 4 π * q Q d = 1 2 π * q Q d . 003 10.0 points Two spheres, fastened to “pucks”, are rid- ing on a frictionless airtrack. Sphere 1 is charged with 1 nC, and sphere 2 is charged with 2 nC. Both objects have the same mass. 1 nC is equal to 1 × 10- 9 C. As they repel, 1. sphere 1 accelerates 4 times as fast as sphere 2. 2. they have the same magnitude of acceler- ation. correct 3. sphere 1 accelerates 2 times as fast as sphere 2. 4. sphere 2 accelerates 2 times as fast as sphere 1. 5. they do not accelerate at all, but rather separate at constant velocity. 6. sphere 2 accelerates 4 times as fast as sphere 1. Explanation: howard (cah3459) – oldmidterm 01 – Turner – (56705) 2 The force of repulsion exerted on each mass is determined by F = 1 4 π * Q 1 Q 2 r 2 = ma where r is the distance between the centers of the two spheres: + F 12 = + F 21 . Since both spheres have the same mass and are subject to the same force, they have the same acceleration. 004 10.0 points From the electric field vector at a point, one can determine which of the following? I. the electrostatic charge at that point; II. the direction of the electrostatic force on a test charge of known sign at that point; III. the magnitude of the electrostatic force exerted per unit charge on a test charge at that point. 1. I only 2. III only 3. All of these 4. II and III only correct 5. None of these 6. II only 7. I and II only 8. I and III only Explanation: The definition of the electrostatic force is + E = + F q , so + F = q + E and + F acts in the same or opposite direction to + 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 ....
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oldmidterm 01-solutions - howard(cah3459 – oldmidterm 01...

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