1 W05D2 Dielectrics and Conductors as Shields Today’s Reading Assignment: Course Notes Sections 5.4, 5.6, 5.8-5.9Announcements Math Review Week 06 Tuesday 9-11 pm in 26-152 PS 5 due W05 Tuesday at 9 pm in boxes outside 32-082 or 26-152 Add Date Week 05 Friday W05D3 Reading Assignment: Friday Problem Solving Capacitance and Dielectrics Course Notes Sections 5.6, 5.8-5.9 23 Outline Dielectrics Electric Fields in Matter Conductors as Shields4 Review Capacitors and Stored Energy5 Parallel Plate Capacitor C depends only on geometric factors A and d ΔV = −E ⋅ dSbottomtop∫= Ed =QAε0d C =QΔV=ε0Ad6 Capacitors and Dielectrics7 Dielectrics A dielectric is a non-conductor or insulator Examples: rubber, glass, waxed paper When placed in a charged capacitor, the dielectric reduces the potential difference between the two plates HOW???8 Molecular View of Dielectrics Polar Dielectrics : Dielectrics with permanent electric dipole moments Example: Water9 Molecular View of Dielectrics Non-Polar Dielectrics Dielectrics with induced electric dipole moments Example: CH410 Dielectric in Capacitor Potential difference decreases because dielectric polarization decreases Electric Field!11 Demonstration: Parallel Plate Capacitor with Dielectric E5 http://tsgphysics.mit.edu/front/?page=demo.php&letnum=E%205&show=012 Dielectric in a Capacitor Q0 = constant after battery is disconnected V =V0κ⇒ C =QV=Q0V0/κ=κQ0V0=κC0Upon inserting a dielectric free charge on plates does not change, potential decreases, capacitance increases13 Dielectric Constant!Dielectric weakens original field by a factor Dielectric constants Vacuum 1.0 Paper 3.7 Pyrex Glass 5.6 Water 80 Dielectric Constant κ V =V0κ⇒ Ed =E0dκ⇒ E =E0κ14 Group Problem: Induced Surface Charge Density A dielectric material with constant completely fills the space between two conducting plates that have a surface charge densities as shown in the figure. Induced surface charge densities appear on the surfaces of the dielectric. Find an expression for in terms of and . ±σκ ±σind σindσκ15 Demonstration: Dissectible Capacitor E2 http://tsgphysics.mit.edu/front/?page=demo.php&letnum=E%202&show=016 Concept Questions: Dielectric in a Capacitor17 Concept Question: Dielectric A parallel plate capacitor is charged to a total charge Q and the battery removed. A slab of material with dielectric constant is inserted between the plates. The charge stored in the capacitor + + + + + + + + - - - - - - - - 1. Increases 2. Decreases 3. Stays the Same κκ18 Concept Question: Dielectric A parallel plate capacitor is charged to a total charge Q and the battery removed. A slab of material with dielectric constant in inserted between the plates. The energy stored in the capacitor + + + + + + + + - - - - - - - - 1. Increases 2. Decreases 3. Stays the Same κκ19 Concept Question: Dielectric A parallel plate capacitor is charged to a total charge Q and the battery removed. A slab of material with dielectric constant in inserted between the plates. The force on the dielectric + + + + + + + + - - - - - - - - 1. pulls in the dielectric 2. pushes out the dielectric 3. is zero κκ20 Dielectric in a Capacitor V0 = constant when battery remains connected Upon inserting a dielectric free charge on plates increase Q =κQ0 Q = CV =κC0V021 Gauss’s Law with Dielectrics! κE ⋅ dAS∫∫=qfree,encε0In both cases:22 Conductors as Shields23 Experiment 1 Faraday Ice Cage24 Concept Question: Point Charge Inside Conductor25 Concept Question: Point Charge in Conductor A point charge +Q is placed inside a neutral, hollow, spherical conductor. As the charge is moved around inside, the surface charge density on the outside 1. is initially uniform and does not change when the charge is moved. 2. is initially uniform but does become non-uniform when the charge is moved. 3. is initially non-uniform but does not change when the charge is moved. 4. is initially non-uniform but does change when the charge is moved.26 Concept Question: Point Charge in Conductor A point charge +Q is placed inside a neutral, hollow, spherical conductor. As the charge is moved around inside, the electric field outside +Q 1. is zero and does not change 2. is non-zero but does not change 3. is zero when centered but changes 4. is non-zero and changes27 Shielding By Conducting Shell: Applet Charge placed INSIDE induces balancing charge ON INSIDE. Electric field outside is field of point charge. http://web.mit.edu/viz/EM/visualizations/electrostatics/ChargingByInduction/shielding/shielding.htm28 Shielding by Conducting Shell: Applet Charge placed OUTSIDE induces charge separation ON OUTSIDE. Electric field is zero inside. http://web.mit.edu/viz/EM/visualizations/electrostatics/ChargingByInduction/shielding/shielding.htmDemonstration: Faraday Cage D33 29
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