# lect08 - Physics 227 Lecture 8 Dielectrics and Capacitors...

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Physics 227: Lecture 8 Dielectrics, and Capacitors Lecture 7 review: Capacitors come in many sizes and shapes Q=CV - charge ±Q on the two plates, voltage V across them Energy Stored = U = Q 2 /2C = CV 2 /2 u = U/Volume = ½ε 0 E 2 In a circuit, capacitors in parallel add, capacitors in series add inversely Thursday, September 29, 2011

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Capacitive Iclicker Puzzle Student A says: The plates of my parallel plate capacitor attract each other, so I have to hold them to keep them apart. Student B says: The plates of my capacitor repel each other, so I have to hold them to keep them together. Who is right? A. A. B. B. C. Both A and B. D. Neither. E. This does not make sense. Remember your answer to this brain teaser. We will Fnd out the right answer in the next few slides. Thursday, September 29, 2011
Capacitive Iclicker Puzzle Student A says: The plates of my parallel plate capacitor attract each other, so I have to hold them to keep them apart. Student B says: The plates of my capacitor repel each other, so I have to hold them to keep them together. Who is right? A. A. B. B. C. Both A and B. D. Neither. E. This does not make sense. Remember your answer to this brain teaser. We will Fnd out the right answer in the next few slides. Thursday, September 29, 2011

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Changing Capacitance A parallel plate capacitor has Q = CV with capacitance C = ε 0 A/d. How can we change the capacitance? The easy way is to move the plates closer together or farther apart. It is natural to think that, since the plates have opposite charge, the forces between them are attractive. Student A is right!? Let’s look in more detail at what happens when the plates come closer together . .. Since Q is the same, the electric Feld is constant, so the energy density u = ½ε 0 E 2 is constant. Constant u with decreasing volume less stored potential energy, so U is being converted into kinetic energy of the plates. It all makes sense, student A was right ! The force is attractive, the plates would naturally come together, they have to be held apart. Thursday, September 29, 2011
Changing Capacitance - Not so fast A parallel plate capacitor has Q = CV with capacitance C = ε 0 A/d. We made an assumption: ``

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lect08 - Physics 227 Lecture 8 Dielectrics and Capacitors...

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