Lectures_Lect22

# Lectures_Lect22 - Physics 212 Lecture 21 Slide Physics 212...

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Unformatted text preview: Physics 212 Lecture 21, Slide Physics 212 Lecture 21, Slide 1 Physics 212 Physics 212 Lecture 22 Lecture 22 Physics 212 Lecture 21, Slide Physics 212 Lecture 21, Slide 2 What We Knew Before Prelecture 22 06 06 Phys ic s 212 Le c ture 21, S lide Phys ic s 212 Le c ture 21, S lide 3 Q E A σ ε ε = = After Prelecture 22: Modify Ampere’s Law After Prelecture 22: Modify Ampere’s Law 10 10 Q EA ε Φ = = Q ε = Φ D I dt d dt dQ ≡ Φ = ε Physics 212 Lecture 21, Slide Physics 212 Lecture 21, Slide 4 Displacement Current: Displacement Current: Generalization Generalization Real Current: Charge Q passes through area A in time t: Displacement Current: Electric flux through area A changes in time dQ I dt = E D d I dt ε Φ = 08 Free space Physics 212 Lecture 21, Slide Physics 212 Lecture 21, Slide 5 Calculation Calculation Switch S has been open a long time when at t = 0, it is closed. Capacitor C has circular plates of radius R . At time t = t 1 , a current I 1 flows in the circuit and the capacitor carries charge Q 1 . At time t 1 , what is the magnetic field B 1 at a radius r ( point d ) in between the plates of the capacitor? S S R R a V V C C R R r r ● I 1 Q 1 d d • Conceptual and Strategic Analysis • Charge Q 1 creates electric field between the plates of C • Charge Q 1 changing in time gives rise to a changing electric flux between the plates • Changing electric flux gives rise to a displacement current I D in between the plates • Apply (modified) Ampere’s law using I D (what does Amperian loop look like) to determine B 10 Phys ic s 212 Le c ture 21, S lide Phys ic s 212 Le c ture 21, S lide 6 Calculation Calculation Switch S has been open a long time when at t = 0, it is closed. Capacitor C has circular plates of radius R . At time t = t 1 , a current I 1 flows in the circuit and the capacitor carries charge Q 1 . S S R R a V V C C ● R R r r ● r r I 1 Q 1 c c d d Compare the magnitudes of the B fields at points c and d. (A) B c < B d (B) B c = B d (C ) B c > B d What is the difference?...
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Lectures_Lect22 - Physics 212 Lecture 21 Slide Physics 212...

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