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Chapter 17

Chapter 17. Electric Potential Energy and the Electric Potential. Calculate the change of PE g for: A 10.0 kg object falling 10.0 m A 5.0 kg object falling 10.0 m. Calculate the change of PE g per mass for each object.

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Chapter 17

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  1. Chapter 17 Electric Potential Energy and the Electric Potential

  2. Calculate the change of PEg for: A 10.0 kg object falling 10.0 m A 5.0 kg object falling 10.0 m Calculate the change of PEg 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 object What characterizes the motion What characterizes the motion

  5. Analogy Formula for Formula for

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

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

  8. BACK TO THE BOARD Return the two charges to their original position in the field. Mark that point with a V1 to indicate potential at that point Mark a place 10.0 cm down the direction of the field. Mark it with V2 For each charge, calculate Change of EPE Potential difference between V1 and V2 Compare the magnitudes of V1 and V2 (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 In the center of the field, place a positive and a negative charge (make up their magnitudes). Show the direction of the electric force on each charge. Allow the force move the charge 5.0 cm. Calculate the change of EPE for each charge. Calculate potential difference that each charge naturally went through.

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

  11. Checkyourunderstanding + + 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 2.0 m A B 1.0 m Find the magnitude of the field at point A and at point B. Potential energy of a +2.0nC charge at each point (refer to the analogy with PEg) Find the change of EPE as the charge moves

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

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

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

  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: +2.0 nC and – 4.0 nC -2.0 nC and +4.0 nC -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: A +5.0 nC charge is fixed in pace. What would be the EPE of the system of these two charges? +5nC 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? +2nC

  25. Whose EPE is this? +2nC +5nC 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) 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. 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?

  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.

  29. The plates of the capacitor are separated bya distance of 0.032 m, and the potential differencebetween them is VB-VA=-64V. Between thetwo equipotential surfaces shown in color, thereis a potential difference of -3.0V. Find the spacing between the two colored surfaces.

  30. 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 5.0x1013 e R1 = 15 cm, R2 = 30 cm Predict the ratio of V1/V2 Find it mathematically, using numbers.

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

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