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Ch 17: Electric Potential and Electric Potential Energy

Ch 17: Electric Potential and Electric Potential Energy. Electric Potential Energy: Parallel Plates. Gravitational PE is similar to Electric PE Height*Gravity is similar to Electric Potential. Electric Potential: Parallel Plates.

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Ch 17: Electric Potential and Electric Potential Energy

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  1. Ch 17: Electric Potential and Electric Potential Energy

  2. Electric Potential Energy: Parallel Plates • Gravitational PE is similar to Electric PE • Height*Gravity is similar to Electric Potential

  3. Electric Potential: Parallel Plates • The electric field between two charged parallel plates is uniform. • Work has to be done on a charge to move it.

  4. Electric Potential: Parallel Plates

  5. Electric Potential • Electric potential, like electric field, is a property of the source charges. • Electric potential is the ability of charges to have an interaction if other charges show up.

  6. Electric Potential Difference • The potential energy of a particle is determined by knowing the charge…and the electrical potential. • We only care about potential difference. • Charged particles change energy (speed up or slow down) when they move through a potential difference. • Unit is Volts.

  7. Sample Problem • Two parallel plates, connected to a 200V power supply, are separated by an air gap. How small can the gap be if the air is not to become conducting by exceeding its breakdown value of E = 3 x 106 V/m?

  8. Conservation of Energy • Electric potential links electricity to conservation of energy. • A (+) potential difference will slow down a positive charge. • Will do the opposite for a (–) charge. • A (–) potential difference will speed up a positive charge. • Will do the opposite for a (–) charge. • WE ALREADY KNOW HOW TO DO THIS.

  9. Sample Problem • A proton with a speed of 2.0 x 105 m/s enters a region of space in which there is an electric potential. What is the proton’s speed after it moves through a potential difference of +100V?

  10. Sample Problem • An electron with a speed of 2.0 x 105 m/s enters a region of space in which there is an electric potential. What is the electron’s speed after it moves through a potential difference of +100V?

  11. Electron Volt • If an electron "falls" through a potential difference of it gains a kinetic energy equal to electron volt). • Because elementary particles such as electrons and protons are pretty small, the SI unit of the Joule is generally too large to easily deal with these particles. • The electron volt is a UNIT OF ENERGY.

  12. Conceptests

  13. Equipotential Surfaces • Places where electric potential is equal.

  14. Equipotential Surfaces • These contour lines are analogous to equipotential lines. • If you move between contour lines, you will be changing your gravitational potential. • The closer together the lines are, the steeper the slope you are climbing up.

  15. Electric Potential: Point Charges • The electric field is not uniform near a point charge.

  16. Electric Potential: Point Charges • Electric potential is a SCALAR! • That means in order to find the total potential at a given point….add ‘em up! • Forces and Field = vectors • Potential and PE = scalars

  17. Conceptests

  18. Sample Problem • Three point charges are arranged in the following manner. • What is the potential at the fourth corner taking V=0 at a great distance? • How much work must be done to move an electron from infinity to point A?

  19. Things Due • Electric Forces (Friday @ start of class) • Ch 16: 1, 6, 7, 12, 23, 27, 28 • *Think everyone is almost done with this.* • Electric Fields (Friday @ start of class) • Ch 16: 13, 14, 17, 34, 37, 67 • Electric Potential and (Friday @ start of class) • Ch 17: 11, 15, 16, 21, 24 • AP: 1993 B2, 1996 B6, 1999 B2 • Tuesday:Quiz (Chapter 16 and 17) • Equations and problems • I am free every day this week 7:40-3:15 • I am afterschool Friday and Monday

  20. ConcepTests 14-27

  21. Capacitors • A capacitor is a device which can store electric charge via two conducting objects placed very near each other, but not touching. • Examples of capacitors: • Camera flash • Key on a keyboard

  22. Charging a Capacitor • While going through a battery, work is done on charges to increase their potential. • The plates are initially neutral and of area E. A battery removes charge Q; from one plate and place it on the other. One plate has a positive charge, the other has a negative charge. • Once our capacitor is fully charged each terminal of our battery is at the same electrical potential (voltage) as our respectively charged plates on our capacitor.

  23. Simulations • Camera flash and keyboard key

  24. Capacitors • Capacitance: how “good” something is at storing charge. • Measured in Farads (usually picofarad-millifarad) • The capacitance of a capacitor is only based on the geometry. • Capacitors store energy in the separation of charge.

  25. Sample Problem • Two circular plates of radius rare separated by an air gap of width d. What is the magnitude of the charge on each plate when connected to a battery of magnitude V?

  26. Sample Problem • Calculate the capacitance of a parallel-plate capacitor whose plates are 20 cm x 3.0 cm and are separated by a 1.0 mm air gap. • What is the charge on each plate if a 12V battery is connected across the two plates. • What is the electric field between the plates? • Estimate the area of the plates needed to achieve a capacitance of 1F, given the same air gap.

  27. Dielectrics • A dielectric is made of an insulating material. • When placed in a capacitor….the capacitance goes up because of the rearranging of charge in the insulator.

  28. Capacitors • Capacitors are useful because they store energy by the separation of + and - charge.

  29. Sample Problem • A camera flash unit stores energy in a 150 microfarad capacitor at 200 V. How much electrical energy can be stored?

  30. Why do we Physics? • To save lives. • Defibrillators work using the charging and discharging of capacitors.

  31. Why do we Physics? • To watch TV (when we were younger) • Cathode Ray Tubes (CRT). The electrons are "aimed" by placing a certain potential difference on each of the deflection plates.

  32. Why do we Physics? • A standard issue TV works by making the electron beam sweep across 525 lines filling the entire screen in 1/30 sec • In an HDTV, the electrons move rapidly through a greater number of lines to create a better looking picture.

  33. Why do we Physics? • Again…to save lives. • Electrocardiogram (EKG) records potential changes of a person’s heart.

  34. Concept Recap • Electric potential is the ability for there to be potential energy. • Potential and Potential Energy are scalars (add or subtract them) • Capacitors store energy by separating charge.

  35. Practice • Electric Potential and Capacitors (Tuesday @ start of class) • Ch 17: 11, 15, 16, 21, 24, 37, 39 {can skip one} • I’d suggest about 2 hours • Graded • AP: 1993 B2, 1996 B6, 1999 B2, 2010 B3 (Tuesday @ start of class) • I’d suggest about 1 hour • To study for quiz • Read Chapter 16 + 17 to see if there’s anything you missed. • I’d suggest about 1 hour

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