Lecture 4.1 : Electric Potential

Transcription

1 Lecture 4.1 : Electric Potential Lecture Outline: Electric Potential Energy Potential Energy of Point Charges Electric Potential Textbook Reading: Ch Feb. 4,

2 Announcements Exam #1 in class Thu. (Feb. 6) covers Ch material. You are responsible for bringing your own calculator and writing utensils. 1 sheet of notes (front and back) allowed. Seating assignment will be posted by entrance doors to Stolkin. You must sit in your assigned seat. Optional review session TONIGHT from 7-8pm in Stolkin. If you attend, please bring your clickers! (Note: I know a few of you have an overlapping PHY222 lab. My apologies...i will try to schedule around this in the future. You can join us late. I actually have the room booked until 9pm and can stay late to cover anything you miss before you join.) HW4 due next Tue. (2/11) at 9am on Mastering Physics. During recitation on Friday you will go over exam solutions. 2

3 From HW3... 3

4 Last Lecture... Electric field must be zero at all points within a conductor in electrostatic equilibrium. Any excess charge has to reside on the outside surface. 4

5 From HW3... 5

6 Last Lecture... 6

7 From Recitation 3.2 Infinite Slab of Charge (27.48) 7

8 Electric Potential Energy So far in this course we have described electric charge in terms of fields and forces. We now need to consider Energy. Reminders from Mechanics: E mech = K + U =0 Conservation of Energy U = U f U i = W Work/Potential Energy Relation W = i j F ds Work done by a Force. 8

9 Electric Potential Energy Potential Energy in a Uniform Gravitational Field. U grav = mgy f mgy i U grav = U 0 + mgy = mgy 9

10 Clicker Question #1 Two rocks have equal mass. Which has more gravitational potential energy? A. Rock A. B. Rock B. C. They have the same potential energy. D. Both have zero potential energy. 10

11 Electric Potential Energy Consider a positive charge inside a parallel-plate capacitor: W elec = F elec rcos 0 = qe s f s i = qes i qes f U elec = U f U i = W elec = qes f qes i Define s=0 at negative plate, and U 0 is potential at s=0. 11

12 Electric Potential Energy Signs can be confusing here, so remember these pictures: 12

13 Clicker Question #2 Two positive charges are equal. Which has more electric potential energy? A. Charge A. B. Charge B. C. They have the same potential energy. D. Both have zero potential energy. 13

14 Electric Potential Energy Example problem: Proton/Electron in a 2.0 cm x 2.0 cm parallel plate capacitor 14

15 Clicker Question #3 A positive charge moves as shown. Its kinetic energy A. Increases. B. Remains constant. C. Decreases. 15

16 Potential Energy of Point Charges 16

17 Potential Energy of Point Charges Two like-sign point charges launched at each other: 17

18 Potential Energy of Point Charges Two opposite-sign point charges launched away from each other: E mech <0 implies a bound system. Can you think of any other bound systems in nature? 18

19 Clicker Question #4 A positive and a negative charge are released from rest in vacuum. They move toward each other. As they do: A. A positive potential energy becomes more positive. B. A positive potential energy becomes less positive. C. A negative potential energy becomes more negative. D. A negative potential energy becomes less negative. E. A positive potential energy becomes a negative potential energy. 19

20 Potential Energy of a Dipole 20

21 Electric Potential We introducted Electric Field to indicate an electric charge s alteration of space. Now we need a concept of potential energy at all points in space due to a source charge. Electric Potential: V U q+sources q 1 volt = 1 V 1 J/C Alessandro Volta 1.5 V Battery 21

22 Electric Potential V = Potential Difference, or Voltage. U = q V 22

23 Electric Potential What is the speed of a proton that has been accelerated from rest through a potential difference of -1000V? 23

24 Reminders Exam #1 on Thursday! Bring note sheet and calculator! Review session tonight from 7-8pm. 24