Correct answer A point charge q is placed inside a hollow cavity of a conductor that carries a net charge Q What is the total charge on the outer conductor The charge on the outer conductor is Week 4 Day 2 Concept Question Point Charge in a Conductor Conductors and Insulators Conductor Insulator Charges are free to move Electrons are weakly bound to atoms Example metals Charges are not free to move Electrons are strongly bound to atoms Example plastic paper wood Charged Distribution and Conductors The charged metal slab applet non zero charge placed in metal slab Charges move to the surface They move as far apart as possible Electric eld is perpendicular to the surface The electric eld inside is zero Induced Charge Distribution in External Electric Field Charging by induction exterior of a neutral metallic box Induced charges move to the surface Electric eld is perpendicular to the surface The electric eld inside is zero Conductors in Equilibrium Conductor placed in an external electric eld The electric eld inside is zero The electric eld is perpendicular to the surface Induced surface charge distribution Hollow Conductors Applet Charge placed outside induced charge separation on the outside Electric eld inside is zero Electric Field on the Surface of a Conductor Electric eld is perpendicular to the surface There is excess charge on the surface Apply Gauss s Law E A A E Conductors are Equipotential Surfaces Conductors are equipotential objects Electric eld is perpendicular to the surface Group Problem Metal Spheres Connected by a Wire Two conducting spheres 1 and 2 with radii r and r are connected by a thin wire What is the ratio of the charges q q on the surfaces of the spheres You may assume that the spheres are very far apart so that the charge distributions on the spheres are uniform Concept Question Point Charge in a Conductor Concept Question Point Charge in a Conductor Correct answer 1 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 is initially uniform and does not change when the charge is moved Correct answer 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 is Hollow Conductors Applet Charge placed INSIDE induces balancing charge on the inside The electric eld outside is a eld of point charge Capacitors Store Electric Charge Capacitor Two isolated conductors Equal and opposite charges Q Potential difference V between them Capacitance is always positive Parallel Plate Capacitor Applet Oppositely charged plates Charges move to inner surfaces Electric eld is perpendicular to the surface The electric eld inside the plates is zero Calculating Electric Field Gauss s Law Superposition Principle Between the plates Above the plates Below the plates Parallel Plate Capacitor V E S Ed Qd A Concept Question Changing Dimensions Correct answer A parallel plate capacitor is charged until the plates have equal and opposite charges Q separated by a distance d and then disconnected from the charging source battery The plates are pulled apart to a distance D d What happens to the magnitude of the charge on the plates Q Concept Question Changing Dimensions Correct answer A parallel plate capacitor is charged until the plates have equal and opposite charges Q separated by a distance d and then disconnected from the charging source battery The plates are pulled apart to a distance D d What happens to the magnitude of the potential difference V V A spherical conductor of radius a carries a charge Q A second thin conducting spherical shell of radius b carries a charge Q Calculate the capacitance Capacitance is 4 a Other equipotential surface second conducting surface is located at in nity In previous calculation set b Group Problem Spherical Shells Isolated Spherical Conductor Energy to Charge a Capacitor Capacitor starts uncharged Carry q from bottom to top Now the top has a charge q q and the bottom has a charge q q Repeat Finish when the top has charge q Q and the bottom has charge q Q Stored Energy in Charging Capacitor At some point top plate has q and bottom has q Potential difference is V q C Change in stored energy done lifting another q is U qV So change in store energy to q is U qV q Q C q C q Total energy to charge Q U U 1 C q q Q 2C Energy Stored in a Capacitor C Q V U Q 2C Q V C V Energy is stored in the electric eld Parallel plate capacitor C A d and V Ed Energy density u E Concept Question Changing Dimensions Correct answer A parallel plate capacitor disconnected from a battery has plates with equal and opposite charges separated by a distance d Suppose the plates are pulled apart until separated by a distance D d How does the nal electrostatic energy stored in the capacitor compare to the initial energy
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