Lecture 6 Chapter 28 Phyic II The Electric Potential Energy Coure webite: http://aculty.uml.edu/andriy_danylov/teaching/phyicii
New Idea So ar, we ued vector quantitie: 1. Electric Force (F) Depreed! 2. Electric Field (E) But, a you know, it i not eay to deal with vector Idea!!!! Let introduce calar quantitie intead o FORCE and FIELD
i Gravitation initial 0 + mgy i Electrotatic i Electric ield mg inal 0 + mgy 0 =0 (Reerence Level) Thi all can be decribed uing a gravitational orce (vector quantity), which decribe an interaction between the Earth and a cat. Since the gravitational orce i a conervative orce, a gravitational potential energy (calar quantity) can be introduced. F grav mg grav mgy So the all can be alo decribed uing the conervation o mechanical energy W K i i = K K W - the work done by F g imilar G qe 0 qe Since F e ha the ame orm a F G which i conervative, then F e i alo conervative. And, a a reult, the electric potential energy can be introduced r k r Let derive thee electric potential energy expreion The cae o two point charge k
Potential energy i an energy o interaction, o there mut be at leat two interacting electric object.
Potential energy o q in a uniorm electric ield (in a capacitor)
Potential energy o q in a uniorm electric ield The work done on q i: F E d q 0 i Conider a poitive charge q inide a capacitor. It peed up a it all toward the negative plate. There i a contant orce F qe toward the negative plate W F d i 0 q E d q i E Recall rom Phyic I qe i Ed qe W i d K qe i i qe i i qe qe To get the mot general expreion, let introduce 0, which i a potential energy at the reerence point =0 i 0 qe 0 qe i Electric potential energy (the interaction energy) o charge q and a charged capacitor i
Potential energy o a poitive charge, +q i q 0 ( poitive charge), in which direction doe increae? 0 qe It i convenient to chooe the potential energy at the reerence point 0 0 near plate near plate plate qe plate lower PE higher PE I +q move in the direction o E, then I we ue the Conervation o energy We will get W K 0 K plate K 0 K plate 0 0 E K I q i releaed
i q near plate Potential energy o a poitive charge, -q 0 ( negative charge), in which direction doe increae? near plate plate plate q E higher PE (le negative) lower PE (more negative) I -q move in the direction oppoite o E, then I we ue the Conervation o energy We will get W K 0 K K plate K plate 0 0 E K So, intead o orce, PE can be ued to analyze motion. 0 I q i releaed
ConcepTet Potential energy 2 Two poitive charge are equal. Which ha more electric potential energy? A) Charge A B) Charge B C) They have the ame potential energy D) Both have zero potential energy qe
ConcepTet Potential energy 1 Two negative charge are equal. Which ha more electric potential energy? A) Charge A B) Charge B C) They have the ame potential energy D) Both have zero potential energy q E
ConcepTet Potential energy A poitive charge move a hown. It kinetic energy A) Increae. B) Remain contant. C) Decreae. qe increae 0 K 0 decreae K
Potential energy o two point charge
The potential energy o two point charge Thi i explicitly the energy o the ytem, not the energy o jut q or Q. Note that the potential energy o two charged particle approache zero a r.
ConcepTet Potential energy A poitive and a negative charge are releaed rom ret in vacuum. They move toward each other. A they do: A) A poitive potential energy become more poitive. B) A poitive potential energy become le poitive. C) A negative potential energy become more negative. D) A negative potential energy become le negative. E) A poitive potential energy become a negative potential energy.
Example 28.2
What you hould read Chapter 28 (Knight) Section 28.1 28.2
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