1. Chapter 17 Electric Potential Energy and the Electric Potential

2. Calculate the change of PE g for: a)A 10.0 kg object falling 10.0 m b)A 5.0 kg object falling 10.0 m Calculate the change of PE g per mass for each object. Make a conclusion: what does this ratio depend on? What does it characterize? Which force is doing work on the ball? Is the work positive or negative? Does it lead to increase or decrease of potential energy?

3. Analogy Which force is doing work? Which energy is changing? Is the energy increasing or decreasing?

4. Analogy Gravitational field’s strength Electric field’s strength What characterizes the object What characterizes the motion

5. Analogy Formula for

6. White Board Practice 1.Draw a uniform electric field. E = ___________. 2.Make up two positive charges and place them in the field. 3.Let them travel __________ cm. 4.Calculate the change in EPE for each charge. 5.Calculate the ratio

7. DEFINITION OF ELECTRIC POTENTIAL DIFFERENCE Potential difference – ratio of the change of EPE per charge SI Unit of Electric Potential: joule/coulomb = volt (V) DEFINITION OF ELECTRIC POTENTIAL Potential difference – ratio of EPE per charge

8. BACK TO THE BOARD 1.Return the two charges to their original position in the field. 2.Mark that point with a V 1 to indicate potential at that point 3.Mark a place 10.0 cm down the direction of the field. Mark it with V 2 4.For each charge, calculate a)Change of EPE b)Potential difference between V 1 and V 2 c)Compare the magnitudes of V 1 and V 2 (which one of them is greater) What is the natural direction of a positive charge moving in an electric field: - in the direction of the field / - opposite the direction of the field - towards a lower potential / towards a higher potential

9. White Board Practice 1.In the center of the field, place a positive and a negative charge (make up their magnitudes). 2.Show the direction of the electric force on each charge. 3.Allow the force move the charge 5.0 cm. 4.Calculate the change of EPE for each charge. 5.Calculate potential difference that each charge naturally went through.

10. Conclusion: 1.EPE characterizes _________________________ (charge or field) 2.Potential characterizes ______________________ (charge or field) 3.______________________ force works on charges as they move in EF 4.As charges move in an electric field naturally, their EPE _______________ 5.If EPE increases, __________________ force has worked on the charge 6.Positive charges naturally move towards ___________________ potential 7.Negative charges move towards _____________________ potential.

11. Check your understanding + + + + A +5.0 nC charge moves 5.0 cm at a 60-degree angle to the direction of the field. Calculate the change in its potential energy.

12. Review Charges have _________________________ associated with their position in an ____________________________ Charges that are moving naturally in an ____________________, lose / gain _______________________________ _____________________ characterizes the charge Ratio of __________________ per charge is called _____________________ Ratio of ___________________ per charge is called __________________________________________________ Positive charge naturally moves through a _____________________________ Negative charge naturally moves through a ____________________________

13. Unit Play

14. Potential in EF of a point charge White board practice: + 5.0 C 1.0 m 2.0 m 1.Find the magnitude of the field at point A and at point B. 2.Potential energy of a +2.0nC charge at each point (refer to the analogy with PE g ) 3.Find the change of EPE as the charge moves AB

15. Potential in EF of a point charge White board practice: + 5.0 C 1.0 m 2.0 m 1.Determine potential difference between the two points. 2.Make a conclusion about the potential (magnitude) as the distance to the charge increases / decreases. 3.As you get closer to the positive charge, V is ______________________ 4.As you move away from the positive charge, V is ___________________ AB

16. Potential in EF of a point charge White board practice: - 5.0 C 1.0 m 2.0 m 1.Find the magnitude of the field at point A and at point B. 2.Find the change of EPE for a +2nC charge as it moves: AB

17. Potential in EF of a point charge White board practice: - 5.0 C 1.0 m 2.0 m 1.Determine potential difference between the two points. 2.Make a conclusion about the potential (magnitude) as the distance to the charge increases / decreases. 3.As you get closer to the positive charge, V is ______________________ 4.As you move away from the positive charge, V is ___________________ AB

18. Deriving the formula for potential near a point charge Potential around a positive charge is _________________ Potential around a negative charge is ________________

19. Potential is a scalar! 2.0 m Find the potential midway between the two charges: a)+2.0 nC and – 4.0 nC b)-2.0 nC and +4.0 nC c)-6.0 nC and +6.0nC

20. Midpoint problems revisited 2.0 m Is there a net force on the point between the two charges? Is there a net field between the two charges? Is there a net potential between the two charges?

21. Zero V vs. ES equilibrium

22. Midpoint problems revisited 2.0 m If you know the net field between the charges, you can predict _________________________________________________________________ If you know the net potential between the charges, you can predict _________________________________________________________________

23. EPE of a system of Charges Does a singular electric charge have EPE if there is no other charge or EF in the proximity? Explain

24. Consider two charges: +5nC +2nC A +5.0 nC charge is fixed in pace. What would be the EPE of the system of these two charges? How much work would need to be done to bring another +2.0 nC form an infinitely far-away point to 1.0 m proximity of the charge?

25. Whose EPE is this? +5nC +2nC Think gravity Potential energy – energy of the system of two objects (charges)

26. p. #26(textbook) Four identical charges (+2.0 μC each) are brought from infinity and fixed to a straight line. The charges are located 0.40 m apart. Determine the electric potential energy of this group.

27. p. 27 (textbook) Let’s start with +8.00 μC - 15.0 μC moves in. Predict: did PE of the system rise or drop? +20.0 μC moves in. Predict: did PE of the system rise or drop? Determine the electric potential energy for the array of three charges shown in the drawing, relative to its value when the charges are infinitely far away.

28. An equipotential surface is a surface on which the electric potential is the same everywhere. The net electric force does no work on a charge as it moves on an equipotential surface.

30. The plates of the capacitor are separated by a distance of 0.032 m, and the potential difference between them is V B -V A =-64V. Between the two equipotential surfaces shown in color, there is a potential difference of -3.0V. Find the spacing between the two colored surfaces.

31. A charged sphere A charged sphere is handled like an equipotential sphere around a point charge, providing V ∞ =0 Consider two spheres: both are missing 5.0x10 13 e R 1 = 15 cm, R 2 = 30 cm Predict the ratio of V 1 /V 2 Find it mathematically, using numbers.

32. A charged sphere What would happen IF these two spheres were connected with a wire? Find the final charge on of each sphere