Download Chapter 23 – Electric Fields

Chapter 23 – Electric Fields
& Chapter 24 – Electric Potential
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Reading Assignment
Only Sections 23.0, 23.1, 23.2, 23.3, & 23.4 from Chapter 23
And Sections 24.0, 24.1, 24.4, & 24.6 from Chapter 24
Section 23.0: Introduction
23.0.1
We know that two charges do NOT have to be in physical contact with each other to exert a
force on each other. Isn’t this strange? Can you imagine being able to push someone without
touching them? How do charges convey this force between each other?
Electrostatic force is a non-contact force much like the force between 2 magnets or the gravity
attraction between two objects. They convey their mutual force via the electric field, a
mathematical means of understanding how one charge projects its force field onto a test charge.
23.0.2
Play with “Interactive 1” does the field point toward or away from a positive charge? How about
from a negative charge? What is the field at the exact point of the charge (try placing your
mouse DIRECTLY over the charge)? Field points always in the way a positive test charge
would act. Hence, the field points away from a positive charge and towards a negative charge.
The field is not defined exactly at the charge since then the distance is zero and 1/distance is not
defined or infinity.
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Section 23.1: Electric Fields
23.1.1
What units is the Electric Field measured in?
Newtons per coulomb (N/C) since field is force per charge
23.1.2
How are the electric field and the electric force related (mathematically)? What does this mean
(as the Electric Field at a specific point get stronger what happens to the force)?
Force = charge * field ------------- they are directly related, stronger field means stronger force
Section 23.2: Electric Fields and Coulomb’s Law
23.2.1
Calculate the electric field in the following situations (specify magnitude and direction, like does
the field point toward or away from the charge).
E= Force/charge but force = K* q1 * q2/ d2 so ….E= K* q2/d2
A) 4.5m away from a 2C charge---near a large + charge--- expect high fields
q2 = 2, d = 4.5 ……so E= K q2/d2 = (9e9 * 2 )/4.52 = 8.9e 8 N/C
B) 2.3mm away from a -4nC charge- close to a small negative charge- expect negative field
q2 = -4e-9C, d = 2.3e-3m …………..so E= K q2/d2 = (9e9 * -4e-9 ) / (2.3e-3) 2 = -6.78e3 N/C
C) 2nm away from a 6mC charge- very close- explect large positive field
q2 = 6e-3, d = 2e-9 ……so E= K q2/d2 = (9e9 * 6e-3 ) / 2e-9 = 1.35e16 N/C
Section 23.3: Interactive Checkpoint: Electric Field
Read through this problem, it is a pretty difficult problem but it is doable. First examine the
problem and predict, will the electric field at point 2 be stronger or weaker than at point 1. Then
work through the problem and write down what it is (you can definitely use all the help that the
book gives to get through the problem.)
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Section 23.4: Electric Field Diagrams
23.4.1
Does a negative charge feel a force in the same direction as the electric field lines, or in the
opposite direction? Opposite- field lines are drawn for positive test charges
23.4.2
How does an electric field diagram represent the strength of the electric field?
Longer lines/arrows mean larger strength
Also, closer denser lines mean stronger field
Section 24.0: Introduction
24.0.1
Interactive 1 has two positive charges. Describe what happens to the potential energy as you
drag the one positive charge around the screen.
The potential energy decreases as you separate the positive charges.
Potential energy seems to depend inversely on energy: PE = constant * (1/r)
Potential energy is positive and measured in joules
24.0.2
Interactive 2 has a positive and a negative charge. Describe what happens to the potential energy
as you drag the positive charge around the screen.
The potential energy increases as you separate the positive test charge from the negative charge.
Potential energy seems to depend inversely on energy: PE = constant * (1/r)
Potential energy is negative and measured in joules
24.0.3
What is the major difference between the first scenario and the second scenario?
Separating like charges results in lower energy while separating unlike charges increase energy
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Section 24.1: Electric Potential Energy
24.1.1
Electric Potential Energy describes how easy or difficult moving two charges together or apart is.
If the electric potential energy is zero then that means there is no force acting on a charge at all.
In each of the following situations describe if the potential energy is increased or decreased by
the following action.
A) Dragging one positive charge away from another positive charge.
Decreases potential energy- like charges have lowest energy when far apart
B) Pushing one negative charge toward another negative charge.
Increases potential energy- like charges have highest energy when closest together
C) Pushing a positive charge toward a negative charge.
Decreases potential energy- unlike charges have lowest energy when closest together
24.1.2
What would potential energy of a negative charge be if it was located EXACTLY in the middle
of two positive charges? What if it was located EXACTLY in the middle of two negative
charges? Zero potential energy- no force- balanced between two positive charges
Section 24.4: Electric Potential
24.4.1
How are electric potential and electric potential energy related?
Electric potential = energy/charge
24.4.2
What units is electric potential measured in?
joules/ coulomb= volts
24.4.3
Where is electric potential positive, where is it negative?
Simple! It’s positive around a positive charge, negative around a negative charge
24.4.4
Does an electron naturally want to move to areas of high potential or low potential?
Electrons are negative. All charges want to move to low potential. For an electron that means
towards a positive charge (lowest energy- happiest state!) for a proton the opposite is true!
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Section 24.6: Electric Potential Difference
24.6.1
What is the potential of a ground defined to be?
Ground means the potential = 0 Volts
24.6.2
What does it really mean to say I have a nine volt battery?
The battery provides a difference of 9V. 9V on the top +, 0V on the bottom (-).
Positive charges leave the top of the battery at 9V (high energy) and return to the bottom(0V).
24.6.3
Use the idea of moving electrons from section 4 and the idea that a current is “kind of” moving
electrons, and explain how a battery generates a current that can power your electronics.
Electrons are in wires which are attached to the bottom of the battery (-) . these electrons have a
high potential of -9V. Electrons are forced by the electric field to move in the wire away from
the (-) high energy end of the battery .Eventually they return to the top of the battery (+ side )
and return to ground (0) energy and 0 volts.
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Chap. 23 SUMMARY
Definition:
Electric field (E) is the force/ per unit charge
Imagine….
A group of charges (call them q2) will create an electric field “E”
A test charge q1 will experience a force if placed in this electric field
Concepts:
* Electric field lines are lines which point the direction of force on a positive test charge
* Closer lines mean a stronger electric field line- much like topographical or weather lines
* electrons and negative charge move the opposite direction of the electric field lines
Math:
general:
F= q1*E
or
E= F/q1
For simple point charges:
Recall coulomb’s law:
so…
F= q1*K*q2/r2
E=
K*q2/r2
Chap. 24 SUMMARY
Definition:
Potential energy (PE) is the work used to position charges = Joules
Electric potential (V) is potential energy (PE)/ unit charge= Joules/Coulomb=volts
Concept:
charges move from high to low potential energy
high potential energy: opposite charges placed far apart or like charges close together
low potential energy- when charges are happy----- unlikes close, likes far apart
MATH:
in general:
potential energy:
Potential:
Point charges:
PE = F*d = (q1*E)*d for moving a charge q1 a distance d
V= PE/q1 = E*d
potential energy: PE= K*q1*q2/r
potential: V = K*q2/r
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