lecture notes ch27 blackboard
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UMass Amherst Biochemistry Teaching Initiative
Electricity and Magnetism
Lecture Notes
Chapter 27: The Electric Field
2/24/2012 1Warwick Bowen PHYS1002 Lectures, CH. 27
We gratefully acknowledge the University of Colorado forproviding many of the concept questions utilised in this
course
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UMass Amherst Biochemistry Teaching Initiative
Lecture 2
Sections 26.5 & 27.1-2: Basics of
electric fields, principle ofsuperposition
2/24/2012 Warwick Bowen PHYS1002 Lectures, CH. 26 2
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UMass Amherst Biochemistry Teaching Initiative
1 Demonstration: Electric Field
Van de Graaff Generator
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A.
B.
C. 0
D.
E. None of these
Charges -Q and +Q are located on the x-axis as shown.
What is the magnitudeof the electric field at point A?
2 Question 3
2/24/2012 Warwick Bowen PHYS1002 Lectures, CH. 27 7
kQ
R2
11
9
kQ
R2
11
4
kQ
R2
1
91
x
-Q +Q
R R R
A
A. B. C. D. E.
20% 20% 20%20%20%
A. A
B. B
C. C
D. DE. E
10
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Electric field of a dipoleWe can represent an electric dipole by two opposite charges q separated by
the small distance s.
The dipole moment determines the electric field strength and is defined as
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Electric field of a dipole
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A. A
B. B
C. C
D. D
E. E
Three charges (equal magnitude) are arranged as shown.
I) What is the direction of the E-field at point A?
2 Question 4A
2/24/2012 Warwick Bowen PHYS1002 Lectures, CH. 27 10
_
+ +A
A. B. C. D. E.
20% 20% 20%20%20%
Yellow
Purple: zero
Pink
Blue
Green
A.
D.
B.
C.
E. zero 10
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A. A
B. B
C. C
D. D
E. E
2 Question 4B
2/24/2012 Warwick Bowen PHYS1002 Lectures, CH. 27 11
Yellow
: some other direction
Bluen
A.
B.
D.C.
E. some other direction
A. B. C. D. E.
0% 0% 0%0%0%
Three charges (equal magnitude) are arranged as shown.
II) An electron is fired into the region of the three
charges from the lower right as shown. What is the
direction of the acceleration of the electron when
it is at point A?
_
+ +
A
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A. A
B. B
C. CD. D
E. E
A. B.
C. D.
E: none of these/dont know
Two IDENTICAL positive charges (protons) are fixed in place on the x-
axis. Which of the following sketches best represents the electric field
lines around these protons?
2 Question 5
2/24/2012 Warwick Bowen PHYS1002 Lectures, CH. 27 12
A. B. C. D. E.
0% 0% 0%0%0%
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There are no charges in the regions shown. Which of the following are
physically possible electrostatic field line configurations?
2 Question 6
2/24/2012 Warwick Bowen PHYS1002 Lectures, CH. 27 13
A. AB. B
C. C
D. D
E. More than one of the above is O.K.
A. B. C. D. E.
0% 0% 0%0%0%
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There are no charges in the regions shown.
Which of the following electrostatic field configurations are physically
possible?
2 Question 7
2/24/2012 Warwick Bowen PHYS1002 Lectures, CH. 27 14
(a) (b)
(c)A. B. C. D. E.
0% 0% 0%0%0%
A. None are possible
B. (a) only
C. (b) only
D. (c) only
E. More than one of the above is o.k. 10
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Pictorial representation of
electric field
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Field-vector diagram Field-line diagram
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Tactics: drawing and using
electric field lines
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Summary of E-field model
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18UMass Amherst Biochemistry Teaching Initiative
Vector Algebra
If youre not confident with vector
algebra, download the vectors worksheetfrom blackboard and go through it in your
PASS session this week.
Rm 7-209 (same room as prac labs)
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19UMass Amherst Biochemistry Teaching Initiative
Lecture 3
Sections 27.3-5: Electric fields of
continuous charge distributions andof basic objects
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20Warwick Bowen PHYS1002 Lectures, CH. 27
Aims
To understand and be able to calculate surface chargedensity.
To understand the method of determining the electric field
of continuous charge distributions.
To know and be able to calculate the electric fields of
basic objects (lines, rings, disks, planes, capacitors, and
spheres).
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3 Question 1
2/24/2012 Warwick Bowen PHYS1002 Lectures, CH. 27 21
A piece of plastic is uniformly charged with surface charge density 1.
The plastic is then broken into a large piece with surface charge
density 2 and a small piece with surface charge density 3.
Rank in order, from largest to smallest, the
surface charge densities 1 to 3.
A. B. C. D. E.
20% 20% 20%20%20%
A. 1
> 2
> 3
B. 1 > 2 = 3
C. 1 = 2 = 3
D. 2 = 3 > 1
E. 3 > 2 > 110
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Surface charge density
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24UMass Amherst Biochemistry Teaching Initiative
Demonstration: Electric field
shapes
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4 Sample exam question
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4 Sample exam question
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z
x
y
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4 Sample exam question
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4 Sample exam question
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4 Sample exam question
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4 Sample exam question
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Rank in order, from largest to smallest, the electric field strengths Ea to
Ee at these five points near an infinite plane of charge.
4 Question 2
2/24/2012 Warwick Bowen PHYS1002 Lectures, CH. 27 37A. B. C. D. E.
20% 20% 20%20%20%
10
A. E a
= Eb
= Ec
= Ed
= Ee
B. Ea > Ec > Eb > Ee > Ed
C. Eb = Ec = Ed = Ee > Ea
D. Ea > Eb = Ec > Ed = Ee
E. Ee > Ed > Ec > Eb > Ea
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Getting to an infinite plane
1) Sum field of point charges around a ring
2) Sum field of concentric rings on a plane
3) Let radius of plane go to infinity
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Electric field strengths
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The equation for the electric field outside an object with a net charge Q
can be expressed using E= kQ/r2
4 Question 3
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A. B. C. D. E.
20% 20% 20%20%20%
10
A. if an only if the object is a point charge
(like an electron or proton).
B. if the object is stationary.
C. if the object is a point charge or a
uniformly charged sphere.
D. if the object is a sphere, a disk, or a ring.
E. regardless of the shape of the object.
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The electric charge per unit area is +for plate 1 and for plate 2.
The magnitude of the electric field associated with plate 1 is
/2o, andthe electric field lines for this plate are as shown.
When the two are placed
parallel to one another,
the magnitude of theelectric field is:
5 Question 2
2/24/2012 Warwick Bowen PHYS1002 Lectures, CH. 27 43
A. B. C. D. E.
0% 0% 0%0%0%
A. /o between, 0 outside.
B. /o between, /2o outside.
C. zero both between and outside.
D. /2o both between and
outside.
E. none of the above. 10
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44UMass Amherst Biochemistry Teaching Initiative
5 Demonstration:
Electroplating
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45UMass Amherst Biochemistry Teaching Initiative
Vector Algebra
If youre not confident with vector
algebra, download the vectors worksheetfrom blackboard and go through it in your
PASS session this week.
Rm 7-209 (same room as prac labs)
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46UMass Amherst Biochemistry Teaching Initiative
Lecture 4
Sections 27.6-7, 28.1-3: Motion of
charged particles in electric fields,
electric dipoles, symmetry, electric
flux
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47Warwick Bowen PHYS1002 Lectures, CH. 28
Aims
To understand and be able to predict the motion of
charged objects and dipoles in the presence of electric
fields.
To understand the three fundamental symmetries
(translation, rotation, and reflection).
To understand that symmetries which apply to charged
objects also apply to the shape of their electric field.
To understand the concept of electric flux, and be able
to calculate the flux through a surface.
To know the definition of the direction of positive flux
through a closed surface.
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A. A
B. B
C. C
D. D
E. E
Which electric field is responsible for the trajectory of
the proton?
6 Question 2
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A. B. C. D. E.
0% 0% 0%0%0%
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Motion of charged particles
in an E-fieldThe electric field exerts a force:
on a charged particle. If this was the only force acting on q, it causes the
charged particle to accelerate with:
In a uniform field, the acceleration is constant:
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analogous to constantacceleration due togravity g near theEarths surface
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(1) (2)
(3) (4)
A dipole is placed in an external field as shown. In which situation(s)
is the net force on the dipole zero?
7 Question 1
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A. B. C. D. E.
20% 20% 20%20%20%
A. (1)
B. (3)
C. (1) and (2)
D. (3) and (4)
E. (2) and (4)
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Dipoles in an electric field
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Dipoles in an electric fieldTorque rotates dipole to align to electric field. The net force on
the dipole is then always in the direction of increasing field
magnitude.
Dipoles experience a net force towards any charged object.
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The dipole force allows trapping of neutral particles in regions of high
electric field strength, such as at the focus of an intense laser beam.
E.g. optical tweezers: important technique in biology for investigating the
properties of DNA and understanding the properties of cells.
Optical dipole traps
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Optical dipole traps have also become an important tool for research in the
areas of ultra-cold atoms and quantum computing.
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Optical dipole traps
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Optical dipole traps
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7 Sample exam question
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64UMass Amherst Biochemistry Teaching Initiative
Chapter 27. Summary Slides
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General Principles
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General Principles
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Applications
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Applications