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Today 3/12

Today 3/12. Plates if charge E-Field Potential HW: “Plate Potential” Due Friday, 3/14 . (-). E net = 0. E net = 0. How big is E?. (+). Charged conducting plate. A = Area of one side.  0 = Q/A. What’s wrong with this picture?. Charged conducting plate.  L =  0 /2.

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Today 3/12

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  1. Today 3/12 • Plates if charge • E-Field • Potential • HW: “Plate Potential” Due Friday, 3/14

  2. (-) Enet = 0 Enet = 0 How big is E? (+)

  3. Charged conducting plate A = Area of one side 0 = Q/A What’s wrong with this picture?

  4. Charged conducting plate L = 0/2 R = 0/2 Free charge always goes to surface of conductor.

  5. Charged conducting plate L = 0/2 R = 0/2 EL = L/2e0 = 0/4e0 What is the electric field inside the conductor?

  6. Charged conducting plate ER = R/2e0 = 0/4e0 L = 0/2 R = 0/2 What is the electric field inside the conductor?

  7. Charged conducting plate L = 0/2 R = 0/2 The electric field is zero everywhere inside the conductor.Always, any conductor, no exceptions. What is the electric field inside the conductor?

  8. What happens if I let it go? q=+1C

  9. Assume the particle gains 100 joules of kinetic energy as it moves from A to B. KE = 0 KE = 100 J q=+1C B A

  10. Now I stop it at B. How much work must I do to move it back to A? q=+1C B A +100 J How does the potential energy change in moving from B to A? DPEBA = +100 J +100 J DPEAB = -100 J

  11. What if q=+2C? How much work must I do to move it back to A? q=+2C B A +200 J How does the potential energy change in moving from B to A? DPEBA = +200 J +200 J DPEAB = -200 J

  12. DPEBA = DVBA q DVBA tells us how much PE changes when +1C is moved from B to A. Now we are back to our original definition. q=+1C B A DVBA = +100 J/C DPEBA = (+100 J/C)x(q)

  13. What if q = -1C? First I must turn my hand around. B A q= -1C

  14. DVBA tells us how much PE changes when +1C is moved from B to A. What if q= -1C? How much work must I do to move it back to A? B A q= -1C -100 J How does the potential energy change in moving from B to A? DPEBA = DVBA q DPEBA = (+100 J/C)x(q) -100 J DPEBA = (+100 J/C)x(-1)

  15. DVBAdoes not depend on the sign of the point charge but DPEBAdoes!!!!! What if q= -1C? How much work must I do to move it back to A? B A q= -1C -100 J How does the potential energy change in moving from B to A? DPEBA = DVBA q DPEBA = (+100 J/C)x(q) -100 J DPEBA = (+100 J/C)x(-1)

  16. A proton is released from rest at A. What is its speed when it reaches B? m= 1.7 x 10-27kg q= 1.6 x 10-19 C DVAB = -100 J/C DVAB = -100 volts   B A DPEAB = q DVAB DPEAB = q (-100 J/C) DPEAB = -1.6 x 10-17 J What happens to the kinetic energy?

  17. A proton is released from rest at A. What is its speed when it reaches B? m= 1.7 x 10-27kg q= 1.6 x 10-19 C DKEAB = +1.6 x 10-17 J   B A 1/2 mv2 = +1.6 x 10-17 J v = 1.4 x 105 m/s What happens to the kinetic energy? DPEAB = -1.6 x 10-17 J

  18. F E What direction is the force on an electron?   B B A A

  19. An electron is released from rest at B. What is its speed when it reaches A? m= 9.1 x 10-31kg q= -1.6 x 10-19 C DVAB = -100 J/C DVBA = +100 J/C   B A DPEBA = q DVBA DPEBA = q (+100 J/C) DPEBA = -1.6 x 10-17 J What happens to the kinetic energy?

  20. An electron is released from rest at B. What is its speed when it reaches A? m= 9.1 x 10-31kg q= -1.6 x 10-19 C DKEBA = +1.6 x 10-17 J   B A 1/2 mv2 = +1.6 x 10-17 J v = 5.9 x 106 m/s What happens to the kinetic energy? DPEBA = -1.6 x 10-17 J

  21. DVAB = ?   A B

  22. How does doubling the E-field affect DVAB ? DVAB = ?   DVAB doubles A B

  23. How does moving point B affect DVAB ? D DVAB = ? DVAB is halved    Bold A Bnew Anything else? How does DVAB depend on E and D?

  24. KEA=1/2 mv02 constantspeed v0 v0 How much work must I do to move the charge from A to B? KEB=1/2 mv02 DKEAB = 0 D A B

  25. FHq FE = qE How much work must I do to move the charge from A to B? A B WAB = FE x D WAB = qED WAB = FHq x D

  26. WAB = qED FHq FE = qE WAB = qED What is the change in potential energy in going from A to B? A B Only applies when the field is uniform over the distance. DVAB‘s sign depends on the direction of E. In this case it’s positive. PEAB = qED PEAB = qDVAB DVAB = EDAB

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