Lecture 15-R mar4

# Lecture 15-R mar4 - Lecture 15-R Review Chapters 24-27 Exam...

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Lecture 15-R Review Chapters 24-27 Exam 2 Thursday March 6 at 8:00 pm, same location No books, no electronic communications Pencils, 2 formula cards, calculator, Student ID

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Thee Fundamental Capacitor (parallel plate capacitor) ch24 Question 1 The electric field between the plates is (neglect edge effects): 1. Q/(2Aε 0 ) 2. Q/(Aε 0 ) 3. 0 Answer: By superposition or realizing E = σ c 0 = Q/(Aε 0 ) and is ~uniform. Then ΔV = V = Ed = Qd/(Aε ) ►►►► C = Q/V = ε 0 A/d = C parallel plate
Question 3 A 4 μF cap is charged with a 10V battery ►► Q 4 = 40 μC A 1 μF cap is charged with a 20V battery ►► Q 1 = 20 μC Caps are connected in parallel with + to - and – to + terminals together Considering C eq and the total charge separated Q total WHAT IS THE FINAL V on the “parallel” combination? 1. 20 V 2. 16 V 3. 12 V 4. 8 V 5. 4 V What are Q 4 and Q 1 after connection? 16 μC and 4 μC respectively, the charge has redistributed itself to produce 4 V on each capacitor + to + + to -

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Energy “stored” in a capacitor (Demo: The “Killer Capacitor”) Separating + from - charge requires work by an external force: Moving charge Q through ΔV =V requires ΔU = W ext = QV (NOT HERE!) (This is indeed the work done by the battery!) But V of capacitor builds up as q builds up: V = q/C Thus, dU = Vdq = dW ext = (q/C)dq 2 2 0 2 0 2 1 2 1 2 1 2 1 CV QV C Q C q C qdq U Q Q q = = = = = = So why the factor of ½ here??? Question #1>>>>>
Question #1 The work done by the battery to transfer Q from one conductor of the capacitor to the other is QV. The energy stored in the capacitor is only (½)QV. So what happened to the other (1/2)QV of energy? 1. Light generated while charging 2. Heating of the connecting wires 3. Energy is not conserved for moving charges 4. It went into dark energy of the universe 5. Kinetic energy of the capacitor

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Dielectrics (insulators)…..so what? Separate and insulate conductors Higher dielectric strength—higher E before breakdown Increases C !! How??
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