{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

Exam IV_solns - Phys 11120 page 1 Final Exam solutions...

Info icon This preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Phys 11120, page 1 Final Exam solutions - Physics 1120 - Fall, 2007 1. A balloon of mass M has been positively charged ( its charge is + Q ) and is held up to the ceiling. The ceiling polarizes, so the balloon sticks to it. The friction coefficient between the balloon and the ceiling is μ , and we will assume that the electric field, E, created by the ceiling is uniform and directed upwards. What is the (minimum) magnitude E of this electric field? A) E = μ Q/Mg B) E = Mg/ μ Q C) E = μ Mg/Q D) E = Q/Mg E) E = Mg/Q The upward force would be F=QE, the downward force is just Mg. Those are the only forces acting in the vertical direction. (Friction is horizontal, and thus totally irrelevant for this question!) QE = Mg means E = Mg/Q. The next two questions refer to this situation: Two positively charged particles, labeled 1 and 2, are placed a distance R apart in empty space and are released from rest. Particle 1 has mass m and charge + Q ; particle 2 has mass 2 m and charge + Q/2 . Each particle feels only the static Coulomb force due to the other particle. (There is no gravity, no friction, nor any other forces in this problem.) 2. How do the magnitudes of the initial accelerations of the two particles compare? [a 1 = magnitude of initial acceleration of particle 1, a 2 = magnitude of initial acceleration of particle 2] A) a 1 = 2 a 2 B) a 1 = (1/2)a 2 C) a 1 = 4 a 2 D) a 1 = a 2 E) None of these. The forces on each must be equal (and opposite) by Newton III. But F=ma, so the smaller mass has the larger acceleration, by the ratio of masses. 3. As the particles continue to move apart after their release, the speed of each particle... A) decreases as time goes by B) increases as time goes by C) stays the same. The force is repulsive, and although it gets smaller and smaller, it is still always repulsive. A positive force means a positive acceleration, forever. Acceleration means increasing speed in this case. SPEED just keeps on getting bigger and bigger!
Image of page 1

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Phys 11120, page 2 The following two questions refer to this situation: Two negative charges are each located a distance r from the origin, as shown. Note that the upper charge is twice as strong as the one to the right. (-2Q compared to -Q) 4. At the origin, the direction of electric field is ... There are two Field vectors at the origin , one pointing straight right, one pointing straight up with twice the magnitude. The sum of those two arrows is indeed "up and right", but it is NOT at a 45 degree angle! (The two forces would have to be equal in magnitude to get that angle)
Image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern