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iClicker Quiz

iClicker Quiz. (1) I have completed at least 50% of the reading and study-guide assignments associated with the lecture, as indicated on the course schedule. a) True b) False. -. +. http://physics.kenyon.edu/EarlyApparatus/Electricity/Condenser/Condenser.html.

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iClicker Quiz

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  1. iClicker Quiz (1) I have completed at least 50% of the reading and study-guide assignments associated with the lecture, as indicated on the course schedule. a) True b) False

  2. - + http://physics.kenyon.edu/EarlyApparatus/Electricity/Condenser/Condenser.html http://s.bourdreux.free.fr/cabinet_Sigaud/chronologie/musschenbroek.htm Pieter van Musschenbroek (1692-1761) – University of Leyden (Holland) “I would like to tell you about a new but terrible experiment, which I advise you never to try yourself, nor would I, who experienced it and survived by the grace of God, do it again for all the kingdom of France.”

  3. William Watson (1715-1787) www.wikipedia.com http://www.howstuffworks.com

  4. http://www.codecheck.com/cc/BenAndTheKite.html

  5. E Quasi-uniform field between two parallel-plates

  6. Stretch a spring Charge a capacitor Capacitance is like flexibility: How much charge does one Volt buy you? If the capacitance is high, one Volt goes a long ways. The SI unit of capacitance is the Farad = Coulomb/Volt.

  7. Potential difference: V E The parallel-plate capacitor • Given A and d, if we now double the voltage, the capacitance will • increase by a factor of two, • (2) decrease by a factor of two, • (3) stay the same. If V doubles, then I also doubles. C doesn’t depend on V or I.

  8. iClicker Quiz (1) I have completed at least 50% of the reading and study-guide assignments associated with the lecture, as indicated on the course schedule. a) True b) False

  9. Ceq is larger than either C1 or C2.

  10. Ceq is smaller than smallest of C1 and C2.

  11. C C C C C C  C C C C  C/2 2C C C/2  Reduction of a capacitive network

  12. a Easy: C1 C1 C2 C2 C5 C5 a b b C4 C4 C3 C3 b Network capacitance depends on which two points you choose to measure between. Much harder (needs matrix methods)

  13. Energy storage in Capacitors Dielectrics Dipoles

  14. Potential difference: V E Electric potential energy stored in a capacitor Charging: initial Q and V are zero. The first dQ is easy. But as V increases, each dQ is a little more expensive.

  15. Potential difference: V E Energy density in an electric field

  16. V E x Force between the plates of a parallel-plate capacitor? • Quiz: The two plates of a capacitor should experience a force that is: • attractive (2) repulsive (3) zero. • Remember that F = dU/dx, which means that the force will act to lower the electrical potential energy. Need to hold either Q or V constant! constant Q attractive constant V repulsive

  17. d Rotation aligns the moment vector (p) with the field? Which way does torque vector point in this figure? Use the RHR.

  18. water Dielectric materials contain polar molecules that can align with an external field. The field produced by these dipoles gets added to the external field.

  19. Electrolytic: common polarizeed large Q at small V Tubular: most common low Q and low V Oil: high V Many types of capacitors

  20. Ultracapacitors (a.k.a. supercapacitors) Huge internal A. Very small d between + and  Extraordinarily-high Q and u, but low V http://en.wikipedia.org/wiki/electric_double-layer_capacitor

  21. Adding a dielectric to any situation

  22. Parallel-plate capacitor with dielectric: Constant Q: How do (A,d,) affect V, E, U and u?    C   V  E  U  u  A   C   V  E  U  u  d   C   V  E  U  u  Constant V: How do (A,d,) affect Q, E, U and u?    C   Q  E  U  u  A   C   Q  E  U  u  d   C   Q  E  U  u 

  23. Parallel-plate capacitor with dielectric: Quiz: Double  at constant V. What happens to E? (1) doubled (2) quadrupled (3) unchanged (4) halved (5) quartered Quiz: Double A at constant Q. What happens to U? (1) doubled (2) quadrupled (3) unchanged (4) halved (5) quartered

  24. + +   + +   + +   + +   V2 V1 C1 C2 C1 C2 C1 C2 + +   + +   C1 C2 Crossed capacitor problem

  25. + +   + +   + +   + +   V2 V1 C1 C2 C1 C2 C1 C2 C1 C2 + +   + +   Suppose you do the same thing without crossing them.

  26. + +   + +   + +   + +   V C1 C2 C1 C2 C1 C2 + +   + +   C1 C2 Crossed capacitor problem: special case where V1 = V2

  27. + + + + + + + + + + + ΔV - - - - - - - - - - - Two Capacitors in Parallel 6V

  28. + + + + + + ΔV - - - - - - + + + + + - - - - - Two Capacitors in Series 6V

  29. Discussion question: Two identical capacitors are connected first in parallel and then in series. Which combination has the greater capacitance? • The pair in parallel • The pair in series • The two combinations have the same capacitance

  30. C5 C5 C2 C3 C2 C1 C3 C2 C4

  31. The electrical potential energy stored in a capacitor is given by U=(1/2)CV^2.  It is also given by U=(1/2C)Q^2.  Is the energy really proportional to C or to 1/C, or is neither really true? • i don't totally understand the question but i do know that both formulas are correct and therefore it would seem that U is proportional to C • neither is really true. imagine a small capacitor and also imagine an infinitely large capacitor the is no charge on either of them there is no potential energy stored on either though one has an infinitely large capacitance. • Really, both are true depending on which equation you use and which variables you set as constants. The reason this occurs is because these equations are equivalent, by the relationship C=Q/V. Therefore, depending on whether you set voltage or charge as a constant, you can really look at energy as proportional to either C or 1/C. • Energy is actually proportional to the charge (Q) and potential difference (V) of the capacitor. The capacitance, C, comes into it because Q is also proportional to V, where the constant C for a particular configuration is equal to Q/V. The two expressions U=(1/2)CV^2 and U=(1/2C)Q^2, when Q/V is substituted for C, both reduce to the same expression, U= QV/2. Capacitance is itself a proportionality constant, so in a sense energy is proportional to C and 1/C, but really it is just a different way of saying it is proportional to charge and potential difference.

  32. d

  33. Dipole Quiz Let the i unit vector point to the right. Let j point towards the board. Let k point up. Let an upward-pointing E-field be present in the room. A dipole moment directed along the j – i direction will experience a torque along what direction. a) i b) j c) k d) –i e) –j f) –k g) none of these k i

  34. In an electrically polarized material, one obtains extra surface charge per volt, and therefore, an increase in capacitance.

  35. Energy density in an electric field

  36. Shorted Capacitor Problem

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