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

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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

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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