Lecture Notes Ch27 Blackboard

8/2/2019 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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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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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


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

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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

2/24/2012 Warwick Bowen PHYS1002 Lectures, CH. 27 8 ...


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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


_

+ +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


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


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


A. AB. B

C. C

D. D

E. More than one of the above is O.K.

A. B. C. D. E.

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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


(a) (b)

(c)A. B. C. D. E.

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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


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)

2/24/2012 18Warwick Bowen PHYS1002 Lectures, CH. 29 ... ...


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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


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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Demonstration: Electric field

shapes



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4 Sample exam question



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z

x

y

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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


A. B. C. D. E.

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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


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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5 Demonstration:

Electroplating



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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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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


A. B. C. D. E.

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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:


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


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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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

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