electrostatics electric field electric field lines...
TRANSCRIPT
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Electrostatics
Electric Field
Electric Field Lines
Shielding and Charging by Induction
Electric flux and Gauss’s law
Electrical energy
potential difference and electric potential
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Assignments: For next class: Read Ch. 20 HW4 Set due next Wed, 9/18 Exam 1 (Wed 9/25): (Ch 19-20)
Question/Observation Mondays
Research Q/O with HW (Wednesday)
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Assignments: For next class: Read Ch. 20 HW4 Set due next Thurs 9/19 Exam 1 (Thurs 9/26): (Ch 19-20)
Question/Observation Thurs
Research Q/O with HW
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5
Problem solving steps
1. Visualize problem – labeling variables
2. Determine which basic physical principle(s) apply
3. Write down the appropriate equations using the variables
defined in step 1.
4. Check whether you have the correct amount of
information to solve the problem (same number of
knowns and unknowns).
5. Solve the equations.
6. Check whether your answer makes sense (units, order of
magnitude, etc.).
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From observations: one finds that whenever multiple charges are present, the net force on a given charge is the vector sum of all forces exerted by other charges.
Electric force obeys a superposition principle.
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How to work the problem?
Find the electrical forces between pairs of charges separately
Then add the vectors
Remember to add the forces as vectors
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8
F12 F13
Superposition Principle
Example: 3 charges in a
line
q1 = 6.00 uC q2 = 1.50 uC q3 = -2.00 uC d1 = 3.00 cm d2 = 2.00 cm K = 8.99 x 109 N.m2 /C2
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Consider three point charges at the corners of a triangle, as shown in the next slide. a) Find the resultant force on q3 b) Find the direction on f3 (this problem looks like the one presented in pg. 662-663 Walker)
q1 = 6.00 x 10-9 C
q2 = -2.00 x 10-9 C
q3 = 5.00 x 10-9 C
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The force exerted by q1 on q3 is
The force exerted by q2 on q3 is
The total force exerted on q3 is the vector sum of
and
13F
13F
23F
23F
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Consider three point charges at the corners of a triangle, as shown below. Find the resultant force on q3.
Solution:
The net force = 7.16 x 10-9 N Θ = 65.2˚
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Spherical Charge Distribution
Read and work on pg 663/664
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Field
Action/Force at a distance: Gravitational field
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Point charge
The lines radiate equally in all directions
For a positive source charge, the lines will radiate outward
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For a negative source charge, the lines will point inward
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An electric dipole consists of two equal and opposite charges
The high density of lines between the charges indicates the strong electric field in this region
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Two equal but like point charges
At a great distance from the charges, the field would be approximately that of a single charge of 2q
The bulging out of the field lines between the charges indicates the repulsion between the charges
The low field lines between the charges indicates a weak field in this region
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Unequal and unlike charges
Note that two lines leave the +2q charge for each line that terminates on -q
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The lines for a group of charges must begin on positive charges and end on negative charges In the case of an excess of charge, some lines will
begin or end infinitely far away
The number of lines drawn leaving a positive charge or ending on a negative charge is proportional to the magnitude of the charge
No two field lines can cross each other
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The electric field lines are not material objects
They are used only as a pictorial representation of the electric field at various locations
They generally do not represent the path of a charged particle released in the electric field
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Electric forces act through space even in the absence of physical contact.
Suggests the notion of electrical field (first introduced by Michael Faraday (1791-1867).
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Mathematically,
SI units are N / C
o
FE
q
Given: One finds:
2
o
e
q qF k
r
2e
qE k
r
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Apply Coulomb’s Law
For each charge, find the force on the charge of interest
Determine the direction of the force
Sum all the x- and y- components
This gives the x- and y-components of the resultant force
Find the resultant force by using the Pythagorean theorem and trig
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a) E will be constant (a charged plane)
b) E will decrease with distance as (a point charge)
c) E in a line charge, decrease with a distance 1/r
2
1
r
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An electron moving horizontally passes between two horizontal planes, the upper plane charged negatively, and the lower positively.
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- vo
- - - - - - - - - - - - - - - - - - - - - -
+ + + + + + + + + + + + + + + + + + + + + +
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A uniform, upward-directed electric field exists in this region. This field exerts a force on the electron. Describe the motion of the electron in this region.
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- vo
- - - - - - - - - - - - - - - - - - - - - -
+ + + + + + + + + + + + + + + + + + + + + +
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Observations:
Horizontally:
No electric field
No force
No acceleration
Constant horizontal velocity
0
0
0
x
x
x
x o
o
E
F
a
v v
x v t
28
- vo
- - - - - - - - - - - - - - - - - - - - - -
+ + + + + + + + + + + + + + + + + + + + + +
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29
- vo
- - - - - - - - - - - - - - - - - - - - - -
+ + + + + + + + + + + + + + + + + + + + + +
Observations:
Vertically:
Constant electric field
Constant force
Constant acceleration
Vertical velocity increase linearly with time.
2
/
/
1/
2
y o
y o o
y o o o
y o o o
o o o
E E
F q E
a q E m
v q E t m
y q E t m
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30
-
- - - - - - - - - - - - - - - - - - - - - -
+ + + + + + + + + + + + + + + + + + + + + +
Conclusions:
The charge will follow a parabolic path downward.
Motion similar to motion under gravitational field only except the downward acceleration is now larger.
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Question:
Charge q1=7.00 mC is at the origin, and charge q2=-10.00 mC is on the x axis, 0.300 m from the origin. Find the electric field at point P, which has coordinates (0,0.400) m.
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x
y
0.300 m q1 q2
0.4
00 m
P
E1
E2
E
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Observations:
First find the Electric field at point P due to charge q1 and q2.
Field E1 at P due to q1 is vertically upward.
Field E2 at due to q2 is directed towards q2.
The net field at point P is the vector sum of E1 and E2.
The magnitude is obtained with
2e
qE k
r
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Set up the problem:
q1=7.00 mC
q2=-10.00 mC
K = 8.99 x 109 N. m2 /C2
r1 = 0.400m
r2= ? What do we do?
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1) Calculate r2
2) Calculate the electric field at P
3) It looks like example in pg. 669
4) See solutions in the next pag.
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Solution:
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Question:
Is it safe to stay inside an automobile during a lightning storm? Why?
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Question:
Is it safe to stay inside an automobile during a lightning storm? Why?
Answer:
Yes. It is. The metal body of the car carries the excess charges on its external surface. Occupants touching the inner surface are in no danger.
SAFE
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When no net motion of charge occurs within a conductor, the conductor is said to be in electrostatic equilibrium
An isolated conductor has the following properties: The electric field is zero everywhere inside the
conducting material
Any excess charge on an isolated conductor resides entirely on its surface
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The electric field just outside a charged conductor is perpendicular to the conductor’s surface
On an irregularly shaped conductor, the charge accumulates at locations where the radius of curvature of the surface is smallest (that is, at sharp points)
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The electric field is zero everywhere inside the conducting material Consider if this were not true If there were an electric field inside the conductor, the
free charge there would move and there would be a flow of charge
If there were a movement of charge, the conductor would not be in equilibrium
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Any excess charge on an isolated conductor resides entirely on its surface
A direct result of the 1/r2 repulsion between like charges in Coulomb’s Law
If some excess of charge could be placed inside the conductor, the repulsive forces would push them as far apart as possible, causing them to migrate to the surface
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Field lines penetrating an area A perpendicular to the field
Compare the flux, Φ, to a water hose.
In general:
ΦE = E A cos θ
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ΦE = E A cos θ The perpendicular to the area A is at an angle θ to
the field
SI unit: N.m2 /C
If the surface is close, the sign is The flux is positive for field lines that leave the
enclosed volume of the surface
The flux is negative for field lines that enter the enclosed volume of the surface.
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Let’s calculate the total flux on the surface of a sphere (it has to be symmetrical).
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Gauss’ Law states that the electric flux through any closed surface is equal to the net charge Q inside the surface divided by εo
εo is the permittivity of free space and equals 8.85 x 10-
12 C2/Nm2
The area in Φ is an imaginary surface, a Gaussian surface
insideE
o
Q
SI unit;
N.m2 /C
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Consider a thin hollow shell of uniformly distributed charge Q. Find the electric field inside and outside the shell.
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The calculation of the field outside the shell is identical to that of a point charge
The electric field inside the shell is zero
2e
o
2 r
Qk
r4
QE
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Use a cylindrical Gaussian surface
On pg. 664, we learned that if a charge Q is distributed uniformly, then:
Q = σA
Where σ is the surface charge density
A
Q
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Note, the field is uniform
o2E
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The field must be perpendicular to the sheet
The field is directed either toward or away from the sheet
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The device consists of plates of positive and negative charge
The total electric field between the plates is given by
The field outside the plates is zero
o
E
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The Coulomb force is a conservative force
A potential energy function can be defined for any conservative force, including Coulomb force
The notions of potential and potential energy are important for practical problem solving
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The electrostatic force is conservative
As in mechanics, work is
Work done on the positive charge by moving it from A to B
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A B
E
d
cosW Fd
cosW Fd qEd
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The work done by a conservative force equals the negative of the change in potential energy, DPE
This equation is valid only for the case of a uniform electric field
allows to introduce the concept of electric potential
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PE W qEdD
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The potential difference between points A and B, VB-VA, is defined as the change in potential energy (final minus initial value) of a charge, q, moved from A to B, divided by the charge
Electric potential is a scalar quantity Electric potential difference is a measure of
electric energy per unit charge Potential is often referred to as “voltage”
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B A
PEV V V
q
DD
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Electric potential difference is the work done to move a charge from a point A to a point B divided by the magnitude of the charge. Thus the SI units of electric potential
In other words, 1 J of work is required to move a 1 C of charge between two points that are at potential difference of 1 V
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1 1V J C
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Units of electric field (N/C) can be expressed in terms of the units of potential (as volts per meter)
Because the positive tends to move in the direction of the electric field, work must be done on the charge to move it in the direction, opposite the field. Thus,
A positive charge gains electric potential energy when it is moved in a direction opposite the electric field
A negative charge looses electrical potential energy when it moves in the direction opposite the electric field
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1 1N C V m
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Electric Force (Coulomb’s force): Superposition of forces
l Electric Field (point charge):
Definition of electric Flux, :
Gass’s Law electric Flux (q is enclosed by a surface):
2
21
er
qqkF
2e
qE k
r
ΦE = E A cos θ
insideE
o
Q