Transcript Chapter 20 Electric Potential Energy and Potential

Physics 314
CHAPTER 20
ELECTRIC POTENTIAL
ENERGY AND POTENTIAL
Agenda 2/9/2015

Review Electrostatics Test
◦ Commonly missed questions
◦ Retake policy

Potential Energy Demonstration

Notes on Electric Potential Energy
◦ Practice problems for EPE
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.
Energy

Energy is the ability to do work.

Measured in joules
◦ 1J = 1 N*m

Forms of Energy:
1. Kinetic Energy: Energy of Motion
2. Potential Energy: Energy of Position
3. Nuclear Energy: Energy of the Atom
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
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.
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.
A Brief Comparison
GPE: Ug=mgh

Directly related to height
EPE: Ue = kq1q2/d

◦ 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 2/10/2015

Review Potential Energy
◦ EPE and GPE
◦ Practice Problems

Introduce Potential
◦ Gravitational Potential
◦ Electric Potential (Voltage)

HW: EPE Problem Set
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

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

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)
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
Electric Potential vs. UE

Potential same but
UE different
(dependent upon
test charge) at
same spot in space
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
Example Problem

How much work is needed to change the
potential of an electron exactly 5.00V?
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?
Agenda 2/11/2015

Review 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

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 Elevation
Think of a
topographical map.
 Each line
represents the
same elevation.
 Masses will roll
downhill, toward
smaller numbers

Equipotential

Places of
equal
potential in a
diagram with
electric
charges
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
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
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?
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?
Electric potential difference
Critical Thinking:
•In order for work to be done
on a charge, what must
happen?
e-
•How would an electron
move between these two
plates?
•What is it called when
electrons flow/move?
Agenda 2/13/2015

Parallel 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: 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?
4. P to T 3cm apart?
E = 5.0 V/m
_- - - - - - - - - - - - - - - - - - - - -
Q
Q
T
T
P
P
R
R
S
S
++++++++++++++++++++++
Agenda 2/17/2015

Potential Energy Warm-Up

What’s the difference?
◦ Potential Energy
◦ Electric Potential
◦ E-Fields

Begin Unit Review
◦ TEST ON WEDNESDAY

HW: Complete Unit Review
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
++++++++++++++++++++++
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

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^-8V
 B) 4.8 x 10^-8V
 C) 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 V
 3C = 3.4 x 10^11 V
 Total = 1.42 x 10^12 V
