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Electric Potential

Electric Potential. A. GPE = (mg)Δh. GPE = mgh A – mgh B. F = mg. GPE = Work (W) required to raise or lower the book. - Where W = (F gravity )( Δh). B. h A. h B. Gravitational Potential Energy. + + + + + + + + +. d A. A. B. +. +. d B.

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Electric Potential

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  1. Electric Potential

  2. A GPE = (mg)Δh GPE = mghA – mghB F = mg GPE = Work (W) required to raise or lower the book. -Where W = (Fgravity)(Δh) B hA hB Gravitational Potential Energy

  3. + + + + + + + + + dA A B + + dB Fe = qoE Fe = qoE - - - - - - - - - - Electric Potential Energy ΔPEE = (qoE)Δd = Fd = W ΔPEE = qoE(dA – dB ) W = DET = ΔPEE • Does a proton at rest at point A have more or less potential energy than it would at point B? More

  4. F F +q -qo F F +q +qo Electric Potential Energy of Point Charges and Work • Much like the book is attracted to the earth due to gravity, two unlike charges are attracted to one another. • Conversely, like charges repel. • It takes positive work to move unlike charges away from one another and like charges closer together.

  5. -qo +q Electric Potential Energy • What would happen if the charged particle q was fixed in place and then particle qo was suddenly released from rest? • It would accelerate away from q. • It would accelerate towards q. • It would stay where it is. • How would the potential energy of this system change? • It would increase. • It would decrease. • It would remain the same.

  6. Electric Potential We know that W = Fd = qEd. We can define the amount of work per unit of charge as This is also called V (voltage) or potential difference. (You could conceive of an analogue as work / unit mass, although I know of no use for it: )

  7. Electric Potential SI Units: joule/coulomb = 1 volt (V) • The Electric Potential is the energy per unit of charge (J/C). • We may write it as to emphasize the fact that it is a potential difference and that the “zero” is arbitrary (like gravity)

  8. Example 1: Electric Potential • An object with 2.5C of charge requires 1.00x10-3 Joules of energy to move it through an electric field. What is the potential difference through which the charge is moved?

  9. + + + + + + + + + + + ++++ - - - - - - - - - - - ---- qo B Uniform Electric Field Two equal and oppositely charged plates qo C qo A Characteristics of a Capacitor E • Since the electric field is constant, the force acting on a charged particle will be the same everywhere between the plates. • Fe = qoE FA = FB = FC

  10. + + + + + + + + + + + ++++ - - - - - - - - - - - ---- B F = qoE dB dA qo Electric Potential and Work in a Capacitor D WAB = F·dB - F·dA A qo WAB = qoEd F = qoE WAB qo V = qo C If WAB = qoEd, then what is WCD? WCD = 0 Joules because the force acts perpendicular to the direction of motion. • Do you remember that W = F·d·cos?

  11. + + + + + + + + + + + ++++ - - - - - - - - - - - ---- d Electric Potential of a Capacitor – An alternative • From mechanics, W = Fd. • From the previous slide, W = qoEd • From the reference table, V = W/qo Two equal and oppositely charged plates A B qo F = qoE Uniform Electric Field V = WAB/qo = Fd/qo = qoEd/qo= Ed

  12. d Example 2:Parallel Plates A spark plug in an automobile engine consists of two metal conductors that are separated by a distance of 0.50 mm. When an electric spark jumps between them, the magnitude of the electric field is 4.8 x 107 V/m. What is the magnitude of the potential difference V between the conductors? V = Ed V = (4.8 x 107 V/m)(5.0 x 10-4m) V = 24,000V

  13. Example 3: Parallel Plates A proton and an electron are released from rest from a similarly charged plate of a capacitor. The electric potential is 100,000 V and the distance between the two plates is 0.10 mm. • Which charge will have greater kinetic energy at the moment it reaches the opposite plate? • Determine the amount of work done on each particle. • Determine the speed of each particle at the moment it reaches the opposite plate. • Determine the magnitude of the force acting on each particle. • Determine the magnitude of the acceleration of each particle.

  14. + + + + + + + + + + + ++++ - - - - - - - - - - - ---- d Example 3: Parallel Plates(cont.) • Begin by drawing a picture and listing what is known: • V = 100,000V • d = 0.10 mm = 1.0 x 10-4m • qe = qp = 1.6 x 10-19C (ignore the sign. We are only interested in magnitude.) p+ e-

  15. Example 3: Parallel Plates(#1 & #2) • For #1, you could answer #2 first to verify. • The answer is that the kinetic energy of both particles will be the same • Why? • because of the formula needed in question #2 applies to both charges, and work = energy. • Hence: Wproton = Welectron qprotonV = qelectronV Wproton = Welectron = (1.6x10-19C)(100,000V) Wproton = Welectron = 1.6x10-14 J

  16. Example 3: Parallel Plates(#3) • Apply the work-energy theorem to determine the final speed of the electron and proton. W = KE • Since the initial kinetic energy is equal to 0J: W = KEf W = ½ mvf2 • Proton: • Electron:

  17. Example 3: Parallel Plates(#4) • Since F = qE, it will be the same for both particles because their charges are the same and the electric field is uniform between two parallel plates. • We also know that W = Fd. Since we know the distance between the plates and the work done to move either charge from one plate to another, we can determine the force as follows:

  18. Example 3: Parallel Plates(#5) • Since we have the force acting on each particle, we can now calculate the acceleration of each particle using Newton’s 2nd Law.

  19. Equipotential Lines • Equipotential lines denote where the electric potential is the same in an electric field. • The potential is the same anywhere on an equipotential surface a distance r from a point charge, or d from a plate. • No work is done to move a charge along an equipotential surface. Hence VB = VA (The electric potential difference does not depend on the path taken from A to B). • Electric field lines and equipotential lines cross at right angles and point in the direction of decreasing potential.

  20. + + + + + + + + + + + ++++ - - - - - - - - - - - ---- Lines of Equipotential Note: Electric field lines and lines of equipotential intersect at right angles. Equipotential Lines • Parallel Plate Capacitor Electric Field Lines Decreasing Electric Potential / Voltage

  21. Note: Electric field lines and lines of equipotential intersect at right angles. Lines of Equipotential + Equipotential Lines • Point Charge Electric Field Lines Note: A charged surface is also an equipotential surface! Decreasing Electric Potential / Voltage

  22. Equipotential Lines (Examples) • http://www.cco.caltech.edu/~phys1/java/phys1/EField/EField.html

  23. Key Ideas • Electric potential energy is the work required to bring a positive unit charge from infinity to a point in an electric field. • Electric potential (V) is the change in energy per unit charge as the charge is brought from one point to another. • The electric field between two charged plates is constant meaning that the force is constant between them as well. • The electric potential between two points is not dependent on the path taken to get there. (Similar to gravity and gravitational PE.) • Electric field lines and lines of equipotential intersect at right angles.

  24. Electric Potential We know that W = Fd = qEd. We can define the amount of work SI Units: joule/coulomb = 1 volt (V) • The Electric Potential Difference is equal to the Work required to move a test charge from infinity to a point in an electric field divided by the magnitude of the test charge. • The Electric Potential is the energy per unit of charge (J/C).

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