1. Chapter 20 Electric Potential Energy and Potential
Physics 314
Chapter 20 Electric Potential Energy and Potential

2. Agenda 2/9/2015 Review Electrostatics Test
Commonly missed questions
Retake policy
Potential Energy Demonstration
Notes on Electric Potential Energy
Practice problems for EPE

3. Retake Policy Must complete any missing work from the unit first!!
Must complete assigned problem set and review answers with Mr. Gustin.
Meetings must take place before the next test date.

4. Energy Energy is the ability to do work. Measured in joules
1J = 1 N*m
Forms of Energy:
Kinetic Energy: Energy of Motion
Potential Energy: Energy of Position
Nuclear Energy: Energy of the Atom

5. Work This is a measure of the change in energy in a system.
Represented by variable W.
Work = Force * distance
Both quantities must be present for work to be done
Demo: Physical Science Textbook

6. Gravitational Potential Energy
The energy of position is called potential energy
The higher you raise the book off the ground the greater the GPE of the object.
The more WORK the object CAN do.

7. Electric Potential Energy
A charged object can have a potential based on its position in an electric field or its position based on another charged particle.
Think about holding two positive charges very close together.

8. A Brief Comparison Directly related to height
GPE: Ug=mgh
EPE: Ue = kq1q2/d
Directly related to height
Larger height => Larger Energy
Based on gravitational pull
Greater G => Greater Energy
Inversely related to distance
Smaller dist. => Larger Energy
Based on strength of E-Field
Greater E => Greater Energy

9. Agenda 2/10/2015 Review Potential Energy Introduce Potential
EPE and GPE
Practice Problems
Introduce Potential
Gravitational Potential
Electric Potential (Voltage)
HW: EPE Problem Set

10. Energy Ability to accomplish work Measured in joules (J)
Variable shown as U in text
Forms: kinetic-energy of motion (KE, UK)
potential-energy of position (PE, UG, UE)
nuclear-energy of the atom

11. Work Equal to the change in energy of a system W = ΔU
Work has to be done to a charge to increase its potential energy
Work is done by a charge to decrease its potential energy

12. Increasing Electric Potential Energy
Just as an object raised to a higher height has more of an ability to exert a greater force onto the ground at landing, a negative charge moved in the direction of a field has more UE (EPE)

13. Electric Potential (Voltage)
Ratio of potential energy to charge
V = Electric potential energy
test charge’s charge
V = UE/qo
For potential created by a point charge…
V = kq/r
**like with E fields, potential exists and is the same at a given location no matter which charge is there and even if NO charge is there to feel it
SCALAR QUANTITY

14. Electric Potential vs. UE
Potential same but UE different (dependent upon test charge) at same spot in space

15. Electric potential difference
Same as voltage in circuits
If potential difference exists, work will be done.
Charges will be moved (current)
ΔV = Vb – Va = +W/qo = +ΔPE/qo
W = +q ΔV
If no potential difference exists between points no work is done…no current
Equipotential

16. Example Problem
How much work is needed to change the potential of an electron exactly 5.00V?

17. Example Problem
What is the electric potential difference for a positive charge of value 2.0 x 10-6 C if it is moved closer to another positive charge, requiring 4.0 x 10-4 J of work?

18. Agenda 2/11/2015 Review EPE Homework Review Electric Potential
Exit Slip
Review Electric Potential
Equipotential
Gravitational Potential Analogy PhET
Sheet Demo
Where is work done??
HW: Electric Potential Problem Set

19. Electric Potential
What is the potential at a point that is located 3.3mm left of a 20C source?
What about to the right?
Above?
Below?

20. Potential is like Elevation
Think of a topographical map.
Each line represents the same elevation.
Masses will roll downhill, toward smaller numbers

21. Equipotential
Places of equal potential in a diagram with electric charges

22. Equipotentials vs. Electric Field Lines
No arrows on equipotentials
List Potential somewhere on equipotential
Have equal increments between shown equipotentials
All equipotentials strike electric field lines at right angles

23. Agenda 2/12/2015 Potential Difference Warm Up
Review Electric Potential PS
Can you feel the Potential Difference?
Equipotential Between plates
EP Lines between plates
HW: Potential Difference Practice

24. Potential Difference Problem
What is the electric potential difference for a positive charge of value 2.0 x 10-6 C if it is moved closer to another positive charge, requiring 4.0 x 10-4 J of work for this to happen?

25. Electric Potential Difference in a Constant Electric field
Between two plates the E-Field lines run from + to – plates.
How should the EP lines be drawn?

26. Electric potential difference
Critical Thinking:
In order for work to be done on a charge, what must happen?
How would an electron move between these two plates?
What is it called when electrons flow/move? e-

27. Agenda 2/13/2015 Parallel Plate Warm-Up Potential Difference Demo
Potential Difference and Potential Energy
Potential Difference Demo
w/ Van DeGraff Generator
Potential Pennies Lab
What are the differences between V and J
Practice Work and Potential Energy
HW: TBD

28. Example: ΔV vs. ΔEPE 1. Find the change in potential (ΔV) of a 2.0 C
Charge moved from point P
to Point Q they are 3cm apart.
2. What is ΔV for this charge
going from P to R 3cm apart?
3. P to S 3cm apart?
P to T 3cm apart?
E = 5.0 V/m _ Q Q T P R T P R S S

29. Agenda 2/17/2015 Potential Energy Warm-Up What’s the difference?
Electric Potential
E-Fields
Begin Unit Review
TEST ON WEDNESDAY
HW: Complete Unit Review

30. Example: ΔV vs. ΔEPE
1. Find the change in potential energy (ΔEPE) of a 2.0 C charge moved from point P (0,0) to Point Q (0,3). 2. What is ΔEPE for this charge going from P to R (3,0)? 3. P to S (0,-3)? 4. P to T (-3,0)? E = 5.0 V/m _ Q T P R S

31. Agenda 2/18/2015 Test Moved to Thursday
Review Questions from the Study Guide
Specific Topics
Review Equation Sheets
Specific Variables and Units
New Review Sheet
HW: Complete New Unit Review

32. What is the magnitude of an electric field existing between two electrodes of a parallel-plate capacitor with a separation distance of 7.50 mm and potential difference of 300 V?

33. How much work energy is required to raise the electric potential of an electron exactly 5.00 V?
8 x 10-19J

34. What is the electric potential exactly cm away from a bare proton? What is the electric potential exactly 3.00 cm away from a bare proton? What is the potential difference between the two locations?
A) 7.2 x 10^-8V
B) 4.8 x 10^-8V
C) 2.4 x 10 ^-8 V

35. What is the total electric potential upon a point Z located 5
What is the total electric potential upon a point Z located 5.00 cm from a charge of C if another charge with magnitude C is located on a line 8.00 cm beyond the first one?
6C= 1.08 x 10^12 V
3C = 3.4 x 10^11 V
Total = 1.42 x 10^12 V