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Unformatted text preview: yun (bsy72) Homework 5 Hoffmann (56745) 1 This print-out should have 11 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 10.0 points A 64 m length of coaxial cable has a solid cylindrical wire inner conductor with a di- ameter of 2 . 483 mm and carries a charge of 4 . 2 C. The surrounding conductor is a cylindrical shell and has an inner diameter of 11 . 591 mm and a charge of- 4 . 2 C. Assume the region between the conductors is air. The Coulomb constant is 8 . 98755 10 9 N m 2 / C 2 . What is the capacitance of this cable? Correct answer: 2 . 31086 nF. Explanation: Let : k e = 8 . 98755 10 9 N m 2 / C 2 , Q = 4 . 2 C , = 64 m , a = 2 . 483 mm , and b = 11 . 591 mm . The charge per unit length is Q . V =- integraldisplay b a vector E dvectors =- 2 k e integraldisplay b a dr r =- 2 k e Q ln parenleftbigg b a parenrightbigg . The capacitance of a cylindrical capacitor is given by C Q V = 2 k e 1 ln parenleftbigg b a parenrightbigg = 64 m 2 (8 . 98755 10 9 N m 2 / C 2 ) 1 ln parenleftbigg 11 . 591 mm 2 . 483 mm parenrightbigg parenleftbigg 1 10 9 nF 1 F parenrightbigg = 2 . 31086 nF 002 10.0 points A parallel-plate capacitor is charged by con- necting it to a battery. If the battery is disconnected and the sep- aration between the plates is increased, what will happen to the charge on the capacitor and the electric potential across it? 1. The charge and the electric potential in- crease. 2. The charge and the electric potential re- main fixed. 3. The charge remains fixed and the electric potential increases. correct 4. The charge increases and the electric po- tential remains fixed. 5. The charge decreases and the electric po- tential remains fixed. 6. The charge increases and the electric po- tential decreases. 7. The charge remains fixed and the electric potential decreases. 8. The charge decreases and the electric po- tential increases. 9. The charge and the electric potential de- crease. Explanation: Charge is conserved, so it must remain con- stant since it is stuck on the plates. With the battery disconnected, Q is fixed. C = A d A larger d makes the fraction smaller, so C is smaller. Thus the new potential V = Q C is larger. yun (bsy72) Homework 5 Hoffmann (56745) 2 003 10.0 points A capacitor is constructed of interlocking plates as shown in the figure (a cross-sectional view). The separation between adjacent plates is 0 . 68 mm, and the effective area of one of the adjacent plates is 4 . 2 cm 2 ....
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This note was uploaded on 10/28/2010 for the course PHY 12343 taught by Professor Turner during the Spring '10 term at University of Texas at Austin.
- Spring '10