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Unformatted text preview: 5/23/2011 1 If the areas are A 1 and A-A 1 . [ ] 2 1 1 2 ( - + = A A d C o C 123 2.4 F, q = 28.8 C C 2 b C 24 = 12 F C 1234 = 3 F q =36 C ( 29 2 1 2 + = d A C o The force on a filling dielectric as it is inserted between the parallel plates of a capacitor. ( 29 ( 29 L C dx dC L x A d C C C o 1 1 1 2 1- = - + = + = x L With the battery connected, U 1 = CV 2 With the battery disconnected, U 2 = Q 2 /2C ( 29 L U dx dU F 1 1 1-- =- = ( 29 L U dx dU F 1 2 2- =- = With the battery connected, since x is increasing downwards, a negative force is upwards, pushing the dielectric away. With the battery disconnected, the force is positive and pointed downwards, pulling in the dielectric. The force is proportional to ( -1) and inversely to L. Chapter 26 Current and Resistance In this chapter we will introduce the following new concepts:-Electric current ( symbol i )-Electric current density vector (symbol )-Drift speed (symbol v d )- Resistance (symbol R ) and resistivity (symbol ) of a conductor -Ohmic and non-Ohmic conductors We will also cover the following topics:-Ohms law -Power in electric circuits J r (26 - 1) HITT The plate areas and plate separations of five parallel plate capacitors are capacitor 1: area A , separation d capacitor 2: area 2A , separation 2d capacitor 3: area 2A , separation d /2 capacitor 4: area A /2, separation 2d capacitor 5: area A , separation d /2 Rank these according to their capacitances, least to greatest. a. 1,2,3,4,5 b. 5,4,3,2,1 c. (524) ,(13) d. 4, (12), 5,3 e. None of these Consider the conductor shown in fig.a. All the points inside the conductor and on its surface are at the same potential. The free electrons inside the conductor move at random directi Electric current ons and thus there is not net charge transport. We now make a break in the conductor and insert a battery as shown in fig.b. Points A and B are now at potentials and , respectively. ( the A B A B V V V V V- = voltage of the battery) The situation is not static any more but charges move inside the conductor so that there is a net charge flow in a particular direction. This net flow of electric charge we define as "electric currrent" A B (26 - 2) i + q conductor v r i- q conductor v r Consider the conductor shown in the figure It is connected to a battery (not shown) and thus charges move through the conductor Consider one of the cross sections through the conductor ( aa or bb or c Current = rate at which charge flow c ) The electric current is defined Current SI Unit:...
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