Welcome back to PHY101: Major Concepts in Physics I. Photo: J. M. Schwarz

Transcription

1 Welcome back to PHY101: Major Concepts in Physics I Photo: J. M. Schwarz

2 Announcements Course Website: HW 7 on Chapters 9 and 16 is due on Friday at 5PM in your TA s mailbox. No lab this week, but there is lab next week. After Chapter 16, we head to Chapter 17. Again, the Physics Clinic in PB112 is open from 9-9 Monday-Thursday and 9-5 Friday. It is staffed by at least one physics graduate student to answer any of your PHY101 questions if you cannot come to my office hours or your TA's office hours.

3 Seventh set of Three Big Questions What is electric charge? How do electric charges interact? What is an electric field? 3

4 16.4 THE ELECTRIC FIELD If a point charge q is in the vicinity of other charges, it experiences an electric force. The electric field (symbol ) at any point is defined to be the electric force per unit charge at that point. See falstad.com for examples of electric fields in 3D Slide 4

5 16.5 MOTION OF A POINT CHARGE IN A UNIFORM ELECTRIC FIELD The simplest example of how a charged object responds to an electric field is when the electric field (due to other charges) is uniform that is, has the same magnitude and direction at every point. Slide 5

6 16.9 Slide 6

7 It s Demo Time!

8 Physics Problem Solving 1 DAP Draw a picture 2 KNU Knowns and unknowns 3 EQN Equation(s) 4SSF Solve symbolically first 5 CYA Check your answer 6PIK Plug in knowns

9 16.9 A cathode ray tube (CRT) is used to accelerate electrons in some televisions, computer monitors, oscilloscopes, and x-ray tubes. Electrons from a heated filament pass through a hole in the cathode; they are then accelerated by an electric field between the cathode and the anode (next slide). Suppose an electron passes through the hole in the cathode at a velocity of m/s toward the anode. The electric field is uniform between the anode and cathode and has a magnitude of N/C. (a) What is the acceleration of the electron? (b) If the anode and cathode are separated by 2.0 cm, what is the final velocity of the electron? Slide 9

10 Strategy 16.9 Slide 10

11 Solution (a) 16.9 Slide 11

12 Solution (b) 16.9 Slide 12

13 Physics Problem Solving 1 DAP Draw a picture 2 KNU Knowns and unknowns 3 EQN Equation(s) 4SSF Solve symbolically first 5 CYA Check your answer 6PIK Plug in knowns

14 16.6 Two point charges are located on the x -axis. Charge q 1 = μc is located at x = 0; charge q 2 = 0.50 μc is located at x = 0.40 m. Point P is located at x = 1.20 m. What is the magnitude and direction of the electric field at point P due to the two charges? Slide 14

15 16.6 Strategy We can determine the field at P due to q 1 and the field at P due to q 2 separately using Coulomb s law and the definition of the electric field. In each case, the electric field points in the direction of the electric force on a positive test charge at point P. The sum of these two fields is the electric field at P. Slide 15

16 Solution 16.6 Slide 16

17 Solution 16.6 Slide 17

18 Solution 16.6 Slide 18

19 An electric field due to a positive charge is represented by the diagram. Between which of the following two points does the electric field do zero work on a moving charge? A. A and B B. B and C C. C and D D. D and E

20 An electric field due to a positive charge is represented by the diagram. Between which of the following two points does the electric field do zero work on a moving charge? A. A and B B. B and C C. C and D D. D and E

21 It s Demo Time! (including a clicker question)

22 We have talked about potential energy before..

23 We have talked about potential energy before.. Gravitational potential energy

24 We have talked about potential energy before.. Gravitational potential energy Elastic potential energy

25 We have talked about potential energy before.. Gravitational potential energy Elastic potential energy These energies are related to forces, so what about electric potential energy related to electric forces?

26 Eighth set of Two Big Questions What is electric potential energy and electric potential? How does one store/harness the electric potential energy? 26

27 Electric potential energy is the energy stored in an electric field. Slide 27

28 For both gravitational and electric potential energy, the change in potential energy when objects move around is equal in magnitude but opposite in sign to the work done by the field: Slide 28

29 CONNECTION The electric force and the electric potential energy for a pair of point particles are proportional to the product of the charges of the particles: Slide 29

30 CONNECTION The electric force and the electric potential energy for a pair of point particles are proportional to the product of the charges of the particles: vs. Slide 30

31 CONNECTION Some of the many similarities between gravitational and electric potential energy include: In both cases, the potential energy depends on only the positions of various objects, not on the path they took to get to those positions. Only changes in potential energy are physically significant, so we are free to assign the potential energy to be zero at any one convenient point. For two point particles, we usually choose U = 0 when the particles are infinitely far apart. Slide 31

32 CONNECTION Both the gravitational and electrical forces exerted by one point particle on another are inversely proportional to the square of the distance between them (F 1/r 2 ). As a result, the gravitational and electric potential energies have the same distance dependence (U 1/r, with U = 0 at r = ). The gravitational force and the gravitational potential energy for a pair of point particles are proportional to the product of the masses of the particles: Slide 32

33 Slide 33

34 Electric Potential Energy due to Several Point Charges To find the potential energy due to more than two point charges, we add the potential energies of each pair of charges. For three point charges, there are three pairs, so the potential energy is The potential energy is the negative of the work done by the electric field as the three charges are put into their positions, starting from infinite separation. Slide 34

35 ELECTRIC POTENTIAL Just as the electric field is defined as the electric force per unit charge, the electric potential V is defined as the electric potential energy per unit charge. Electric potential is often shortened to potential. It is also informally called voltage. Slide 35

36 Potentials do not have direction in space; they are added just as any other number (scalar). Potentials can be either positive or negative and so must be added with their algebraic signs. If the potential at a point due to a collection of fixed charges is V, then when a charge q is placed at that point, the electric potential energy is Slide 36

37 Potential Difference When a point charge q moves from point A to point B, it moves through a potential difference The potential difference is the change in electric potential energy per unit charge: Slide 37

38 THE RELATIONSHIP BETWEEN ELECTRIC FIELD AND POTENTIAL Slide 38

39 The SI unit for electric potential energy is equivalent to A. N m. B. V C. C. N/m. D. C m 2.

40 The SI unit for electric potential energy is equivalent to A. N m. B. V C. C. N/m. D. C m 2.

41 To prep for Lab 6..

42 Definition of electric current: The SI unit of current, equal to one coulomb per second, is the ampere (A). Slide 42

43 Conventional Current According to convention, the direction of an electric current is defined as the direction in which positive charge is transported or would be transported to produce an equivalent movement of net charge. Slide 43

44 EMF AND CIRCUITS Electromotive force (battery) in an Electric Circuit Slide 44

45 Circuit Symbols for a Battery Of the two vertical lines, the long line represents the terminal at higher potential and the short line represents the terminal at lower potential. Slide 45

46 Lab 6: How does a zinc-copperacetic acid battery work? Here is an image depicting the electrochemistry of a zinccopper-sulfric acid battery.