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Phys 133 – Chapter 301 Chapter 28 Electric Potential.

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Presentation on theme: "Phys 133 – Chapter 301 Chapter 28 Electric Potential."— Presentation transcript:

1 Phys 133 – Chapter 301 Chapter 28 Electric Potential

2 Phys 133 -- Chapter 292 Looking ahead New force (Chapter 25) New potential energy (Chapter 28)

3 Phys 133 -- Chapter 293 Electric Potential Energy (U elec ) Potential energy ~ Gravitational ~ Uniform electric field ~ Point charge

4 Phys 133 -- Chapter 294 Gravitational potential (energy) Defined Then defined For the gravitational force In general

5 Phys 133 -- Chapter 295 Uniform field: Electric potential energy where

6 Phys 133 -- Chapter 296 s Uniform field: Electric potential energy s Direction of lower potential energy depends on type of charge Lower potential energy Positive chargeNegative charge

7 Phys 133 -- Chapter 297 Do Workbook 29.6

8 Phys 133 -- Chapter 298 A proton is released from rest at the positive plate of a parallel plate capacitor. It crosses the capacitor and reaches the negative plate with a speed of 50,000 m/s. What will be the speed if the experiment is repeated with double the amount of charge on the capacitor plates? (Ans: 7.07x10 4 m/s) Problem 29.3

9 Phys 133 -- Chapter 299 Electrical Potential Energy of Two Point Charges where

10 Phys 133 -- Chapter 2910 Question b) Two point charges (+/- Q) have their separation increased. Does the electric potential energy of the system (A) increase, (B) decrease or (C) remain the same? a) Two point charges (both +Q) have their separation increased. Does the electric potential energy of the system (A) increase, (B) decrease or (C) remain the same?

11 Phys 133 -- Chapter 2911 Question ans Like charges Opposite charges

12 Phys 133 -- Chapter 2912 Do Workbook 29.3

13 Phys 133 -- Chapter 2913 An electron (q=-e) is sent from very far away towards a charged sphere (Q=-0.25 nC). It gets to a distance of 1.10 mm from the center of the sphere before turning around. What is the initial speed of the electron? (Ans: 2.68x10 7 m/s) What was the speed of the same electron with same initial conditions when it was 2.20 mm away? e=1.6x10 -19 C, m e =9.1x10 -31 kg, K=9x10 9 Nm 2 /C 2 Problem

14 Phys 133 -- Chapter 2914 Problem ans

15 Phys 133 -- Chapter 2915 Electric Potential (V elec ) Potential ~ Gravitational ~ Uniform electric field ~ Point charge

16 Phys 133 -- Chapter 2916 Electric Potential (V elec ) Potential only depends on source charges Charge creates potential define Look at Gravitational Uniform electric field Point charge

17 Phys 133 -- Chapter 2917 Gravitational Potential environment object Environment only

18 Phys 133 -- Chapter 2918 Electric potential capacitor s 0 (from before) environment object Environment only

19 Phys 133 -- Chapter 2919 Capacitor: potential representations

20 Phys 133 -- Chapter 2920 Draw V(x) for the two equipotential surfaces Draw U(x) for a proton in these potentials Draw U(x) for an electron in these potentials

21 Phys 133 -- Chapter 2921 Workbook 29.18b extension A proton is at x=0, moving to the right with 5x10 -18 J of kinetic energy. What is its kinetic energy at point a? What if it were an electron?

22 Phys 133 -- Chapter 2922 Workbook 29.18b extension ans proton electron Not enough energy to reach a

23 Phys 133 -- Chapter 2923 a)In the figure which capacitor plate, left or right, is the positive plate? b)What is the electric potential energy of a proton at the midpoint of the capacitor? c)What is the electric field strength inside the capacitor? Problem 29.21 Modified In general d)What is the electric field strength inside the capacitor in terms of  and  o ? e)What is the electric field strength inside the capacitor in terms of ∆V and d (the distance between the plates?

24 Phys 133 -- Chapter 2924 Only potential difference matters

25 Phys 133 -- Chapter 2925 Electric potential two point charges (from before)

26 Phys 133 -- Chapter 2926 Point particle: potential representations

27 Phys 133 -- Chapter 2927 Do Workbook 29.21

28 Phys 133 -- Chapter 2928 Electric Potential: multiple charges No vectors

29 Phys 133 -- Chapter 2929 Do Workbook 29.25

30 Phys 133 -- Chapter 2930 a) Find an expression for the electric potential V(x,y) for a dipole with point charges +/- Q at y= +/- a. Problem b) Use the result for the dipole to find the electric potential at x=2 m, y=7 m when a=2 m, and Q=2 nC. c) What is the potential energy of a proton placed there?

31 Phys 133 -- Chapter 2931 Problem ans r+r+ y-a xx y+a r-r-

32 Phys 133 -- Chapter 2932 Problem plot

33 Phys 133 -- Chapter 2933 Problem ans c) What is the potential energy of a proton placed there? b) Use the result for the dipole to find the electric potential at x=7 m, y=2 m when a=2 m, and Q=2 nC.

34 Phys 133 -- Chapter 2934 Potential of continuous charge distribution where Then let solve

35 Phys 133 -- Chapter 2935 The figure shows a thin rod of length L and charge Q. Find an expression for the electric potential a distance x away from the center of the rod on the axis of the rod. Problem 29.69

36 Phys 133 -- Chapter 2936 Problem 29.69 x y P (d,0) riri i th xixi d Find potential at P due to i th bit of charge Sum up contribution from each charge at P


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