oldhomework 10-solutions

oldhomework 10-solutions - howard (cah3459) – oldhomework...

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Unformatted text preview: howard (cah3459) – oldhomework 10 – Turner – (56705) 1 This print-out should have 12 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 (part 1 of 4) 10.0 points An air-filled capacitor consists of two parallel plates, each with an area of 8 . 8 cm 2 , sepa- rated by a distance 1 . 3 mm . A 20 V potential difference is applied to these plates. The permittivity of a vacuum is 8 . 85419 × 10- 12 C 2 / N · m 2 . 1 pF is equal to 10- 12 F . The magnitude of the electric field between the plates is 1. E = V d 2 . 2. E = ( V d ) 2 . 3. E = 1 ( V d ) 2 . 4. None of these 5. E = d V . 6. E = 1 V d . 7. E = V d . correct 8. E = V d . 9. E = d V 2 . Explanation: Since E is constant between the plates, V = E · d l = E d E = V d . 002 (part 2 of 4) 10.0 points The magnitude of the surface charge density on each plate is 1. σ = d V 2 . 2. σ = ( V d ) 2 . 3. None of these 4. σ = d V . 5. σ = V d . 6. σ = V d 2 . 7. σ = ( V d ) 2 . 8. σ = V d 9. σ = V d . correct Explanation: Use Gauss’s Law. We find that a pillbox of cross section S which sticks through the sur- face on one of the plates encloses charge σ S. The flux through the pillbox is only through the top, so the total flux is E S. Gauss’ Law gives σ = E = V d Alternatively, we could just recall this result for an infinite conducting plate (meaning we neglect edge effects) and apply it. 003 (part 3 of 4) 10.0 points Calculate the capacitance. howard (cah3459) – oldhomework 10 – Turner – (56705) 2 Correct answer: 5 . 9936 pF. Explanation: Let : A = 0 . 00088 m 2 , d = 0 . 0013 m , V = 20 V , and = 8 . 85419 × 10- 12 C 2 / N · m 2 . The capacitance is given by C = A d = 8 . 85419 × 10- 12 C 2 / N · m 2 × . 00088 m 2 . 0013 m = 5 . 9936 × 10- 12 F = 5 . 9936 pF ....
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This note was uploaded on 10/11/2010 for the course PHY 56715 taught by Professor Turner during the Spring '10 term at University of Texas.

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oldhomework 10-solutions - howard (cah3459) – oldhomework...

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