Week 13 Day 2 Concept Question Capacitor Ampere s Law Capacitor Displacement Current Correct answer 1 The B eld is counterclockwise which means that ux is increasing in time So positive charge is increasing on the bottom plate Consider a charging capacitor Use Ampere s Law to calculate the magnetic eld just above the top plate B s I I J n a Surface 1 I 1 and Surface 2 I 0 We don t have current between the capacitor plates but we do have a changing E eld I t This is called the displacement current It is not a ow of change but proportional to changing electric ux If Surface 2 encloses all of the electric ux due to the charge plate the I I Maxwell Ampere s Law Group Problem Capacitor B s I I J n a t E n a A capacitor consisting of circular plates of radius R separated by a distance d is in a circuit carrying the steady current I as shown At time t 0 the plates are uncharged 1 Use Gauss s Law to nd a vector expression for the electric eld E t as a function of time at the point at P Clearly show your choice of Gaussian closed surface 2 Use the generalized Ampere Law to nd a vector expression for the magnetic eld B at the point P for both r R and r R For r R use the circle of radius r as the Amperian loop as shown in the gure 3 Find a vector expression for the poynting vector at r R 4 What is the ux of the Poynting vector power into out of the capacitor Poynting Vector Poynting Vector S 1 E x B Power through a surface P S n a Concept Question Capacitor Electromagnetism Review Correct answer 3 Energy stored in the electric eld is decreasing The direction of the Poynting ux inside the capacitor is radially outward Therefore electromagnetic energy is owing outward and the energy in the electric eld inside is decreasing Electric elds are associated with electric charges Gauss s Law and time changing magnetic elds Faraday s Law Magnetic elds are associated with moving changes and time changing electric elds Ampere maxwell s Law
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