1. Chapter 29

2. ΔK + ΔU
mgh
Fd= FdcosΘ
1/2mv2

3. Conservative force is one for which the work done as a particle moves form its initial position to its final position independent of the path that is followed.
Electric force (electricity)is a conservative force!!!

4. The uniform electric field between places of a parallel-plate capacitor look like the uniform gravitational field near the earth’s surface. The major difference is that gravity always points down whereas the electric field inside of a capacitor can point in any direction.

5. A proton in a capacitor speeds up and gains kinetic energy as it moves toward the negative plate of the capacitor.
The electric field exerts a constant force on the charged particle in the direction of motion…thus doing work.
F=qE and W=Fd or qEs where s is the distance the particle is traveling.

6. The work done by the electric field causes the particle to experience a change in potential.

8. The potential energy of a point charge is dependant on distance that separates it from another point charge.
Uelec = (kq1q2)/ r
*** Notice that this is different that force between 2 particles…distance here is not squared!!

9. Homework page 905 # 1,3,5,7

10. Uelec = (kq1q2)/ r
Electric potential for two like charges is positive and for two opposite charges the potential is negative.
The formula for electric potential is valid for two charged spheres in which case “r” would represent the distance from their centers.

12. Electric force is considered to be a conservative force because work is independent of the path traveled.
In the previous slide we saw that two particles interact on a “cycle” base. As two like particeles shot apart from each other they slow down as their distance increases, losing KE, until they reach maximum separation and then reverse motion until they get close enough to interact again. And then the cycle happens all over again.

13. This continuous interaction (cycle) is known as a bound system.
The energy of the system is continuously shifting between potential and kinetic.
Energy in the system is staying negative, or at a level at which these particles will stay close enough to interact.
This cycle will continue unless one particle reaches its escape velocity!!!

14. Escape velocity….
Two oppositely charged particles with total positive energy can escape the “pull of the other charge.
Two like charged particles repel, slow down and coast past the point (threshold) of potential and reach a state of infinite KE.
The initial velocity that allows a particles to reach KInfinity with vf=0 is called escape velocity.

15. Rank in order from largest to smallest the potential energies of the following pairs…

16. B=D>A=C

17. Dipole Moment….
The force or torque needed to turn a dipole into the electric field is shown by
W=T=-pEsinΘ = -qsEsinΘ
p = qs
E=electric field
This is negative because the dipole is “trying” to turn clockwise

18. Udipole = -pEcosΘ
Dipole potential is the derivative of work!!!

19. Homework page 905 #8

20. Electric Potential or Voltage
The electric potential energy (voltage=V) is due to the interactions of each charge with all of the surrounding charges.
1V= 1 volt = 1J/C

21. Electric Potential or Voltage
2 key ideas…
1. potential only depends on source and geometry. Potential is the ability to have an affect regardless if the test particle is there or not.
2. If we know the electric potential in an area, we immediately know the potential energy of any particle entering that area via U=qV

22. Potential energy is an interaction between source charges and the probe in units of Joules.
Electric potential in the change in electric field as you move from the positive plate (high potential) to the negative plate (low potential)

23. A particle moves through a potential difference between an initial potential and a final potential. The potential difference between two points is called voltage.

25. Pause and summarize!!!

27. What good is electric potential??
If given a source which has high electric potential (batteries of different voltage) you can get electrons to flow (transforming potential into kinetic) and when electrons flow…you get “power”
Ki + Ui = Kf + Uf
Ki + qV = Kf + qV

28. Example…
A proton with a speed of 200,000m/s enters an area with an electric potential of 100V. What is the protons speed after it moves through this battery? What is its acceleration rate? What would happen if the proton was replaced by an electron?

29. Homework page 905 #11,12,15