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CHAPTER 20CHAPTER 20ELECTRIC POTENTIAL ELECTRIC POTENTIAL ENERGY AND ENERGY AND POTENTIALPOTENTIAL
Physics 314
Agenda 2/9/2015Agenda 2/9/2015Review Electrostatics Test
◦Commonly missed questions◦Retake policy
Potential Energy Demonstration
Notes on Electric Potential Energy◦Practice problems for EPE
Retake PolicyRetake PolicyMust 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.
EnergyEnergyEnergy is the ability to do work.
Measured in joules◦1J = 1 N*m
Forms of Energy:1.Kinetic Energy: Energy of Motion2.Potential Energy: Energy of Position3.Nuclear Energy: Energy of the Atom
WorkWork
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
Gravitational Potential Gravitational Potential EnergyEnergy
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.
Electric Potential EnergyElectric 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.
A Brief ComparisonA Brief Comparison
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
Agenda 2/10/2015Agenda 2/10/2015Review Potential Energy
◦EPE and GPE◦Practice Problems
Introduce Potential◦Gravitational Potential◦Electric Potential (Voltage)
HW: EPE Problem Set
EnergyEnergyAbility to accomplish workMeasured in joules (J)Variable shown as U in textForms: kinetic-energy of motion
(KE, UK)
potential-energy of position (PE, UG, UE)
nuclear-energy of the atom
WorkWorkEqual to the change in energy of
a systemW = 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
Increasing Electric Potential Increasing Electric Potential EnergyEnergy
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)
Electric Potential (Voltage)Electric Potential (Voltage)Ratio of potential energy to charge
V = Electric potential energy test charge’s chargeV = 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
Electric Potential vs. UElectric Potential vs. UEE
Potential same but UE different (dependent upon test charge) at same spot in space
Electric potential Electric potential differencedifferenceSame as voltage in circuitsIf potential difference exists, work will
be done.◦Charges will be moved (current)
ΔV = Vb – Va = +W/qo = +ΔPE/qo
W = +q ΔVIf no potential difference exists between
points no work is done…no current◦Equipotential
Example ProblemExample ProblemHow much work is needed to
change the potential of an electron exactly 5.00V?
Example ProblemExample ProblemWhat 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?
Agenda 2/11/2015Agenda 2/11/2015Review EPE Homework
◦Exit Slip
Review Electric Potential◦Equipotential◦Gravitational Potential Analogy PhET
Sheet Demo◦Where is work done??
HW: Electric Potential Problem Set
Electric Potential 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?
Potential is like ElevationPotential is like Elevation
Think of a topographical map.
Each line represents the same elevation.
Masses will roll downhill, toward smaller numbers
Equipotential Equipotential
Places of equal potential in a diagram with electric charges
Equipotentials vs. Electric Equipotentials vs. Electric Field LinesField LinesNo arrows on equipotentialsList Potential somewhere on equipotential
Have equal increments between shown equipotentials
All equipotentials strike electric field lines at right angles
Agenda 2/12/2015Agenda 2/12/2015Potential Difference Warm Up
Review Electric Potential PS
Can you feel the Potential Difference?◦Equipotential Between plates◦EP Lines between plates
HW: Potential Difference Practice
Potential Difference Potential Difference ProblemProblemWhat 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?
Electric Potential Difference in Electric Potential Difference in a Constant Electric fielda Constant Electric field
Between two Between two plates the E-Field plates the E-Field lines run from + lines run from + to – plates.to – plates.
How should the How should the EP lines be EP lines be drawn?drawn?
Electric potential Electric potential differencedifference 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-
Agenda 2/13/2015Agenda 2/13/2015Parallel Plate Warm-Up
◦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
Example: Example: ΔΔV vs. V vs. ΔΔEPEEPE1. Find the change inpotential (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?4. P to T 3cm apart?
E = 5.0 V/m
P
S
T
Q
RP
S
T
Q
R
_- - - - - - - - - - - - - - - - - - - - -
++++++++++++++++++++++
Agenda 2/17/2015Agenda 2/17/2015Potential Energy Warm-Up
What’s the difference?◦Potential Energy◦Electric Potential◦E-Fields
Begin Unit Review◦TEST ON WEDNESDAY
HW: Complete Unit Review
Example: Example: ΔΔV vs. V vs. ΔΔEPE EPE 1. Find the change inpotential 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
P
S
T
Q
R
_- - - - - - - - - - - - - - - - - - - - -
++++++++++++++++++++++
Agenda 2/18/2015Agenda 2/18/2015Test Moved to ThursdayReview Questions from the Study Guide
◦Specific Topics
Review Equation Sheets◦Specific Variables and Units
New Review Sheet
HW: Complete New Unit Review
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?
How much work energy is required to raise the electric potential of an electron exactly 5.00 V?
8 x 10-19J
What is the electric potential exactly 2.00 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^-8VB) 4.8 x 10^-8VC) 2.4 x 10 ^-8 V
What is the total electric potential upon a point Z located 5.00 cm from a charge of 6.00 C if another charge with magnitude 3.00 C is located on a line 8.00 cm beyond the first one?
6C= 1.08 x 10^12 V3C = 3.4 x 10^11 VTotal = 1.42 x 10^12 V