lecture02.ppt Wkwkk just got my firstly e

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Course Learning Assistance

Exam 1 is on Tuesday, February 14, from 5:00-6:00 PM.
According to the Student Academic Regulations “The period from 5:00 – 6:00 PM
daily [is] to be designated for common exams. If a class or other required
academic activity is scheduled during common exam time, the instructor of the
class that conflicts with the common exam will provide accommodations for the
students taking the common exam.”
• other time conflicts are the student’s responsibility
unless the student participates in a major university or
intercollegiate event
Exam 1

The spring career fair is on Tuesday, February 21, 2017. There
is no conflict between the exam and career fair.
If you have an interview or must attend a career fair activity
during your recitation time on February 21, contact your
recitation instructor.
We understand the importance of the career fair and
will work with you to make sure you can participate!
Career Fair

http://campus.mst.edu/physics/courses/2135lab/
“Odd” labs (3L01, 3L03, etc.) start the week of January 23
“Even” labs (3L02, 3L04, etc.) start the week of January 30
Purchase a lab manual in the Physics office. Students not
purchasing a lab manual will receive a lab grade of zero.
Lab manuals are not available at the bookstore.
Labs
Labs begin week of Jan 23!

Coulomb's Law:
+ -
Review of Lecture 1, Part I
q1 q2
r12
1 2
2
0 12
q q
1
F = ,
12 4πε r
Coulomb’s Law quantifies the force between charged particles.

Review of Lecture 1, Part II
Electric field:
• charges create electric fields
• electric field of a point charge:
magnitude:
direction: away from , towards
• force felt by charge in electric field:
positive charge feels force in direction of
negative charge feels force in direction opposite to
2
q
E=k
r
+ -
F= qE
E
E

Today’s agenda:
Electric field due to a charge distribution.
You must be able to calculate electric field of a continuous distribution of charge.
+
+
+
-
-
-

Electric Field
Due to a Continuous Charge Distribution
Problem:
• our equations for the Coulomb force and the electric
field hold for point charges only
Solution:
• decompose extended object into charge elements
• calculate electric field for each element
• sum up (integrate) contributions of all elements to
obtain total electric field

Electric Field
Due to a Continuous Charge Distribution
(worked examples)
finite line of charge
general derivation: http://www.youtube.com/watch?v=WmZ3G2DWHlg
ring of charge
disc of charge
infinite sheet of charge
infinite line of charge
semicircle of charge

Instead of talking about electric fields of charge distributions,
let’s work some examples. We’ll start with a “line” of charge.
Example: A rod* of length L has a uniformly distributed total
positive charge Q. Calculate the electric field at a point P
located a distance d below the rod, along an axis through the
left end of the rod and perpendicular to the rod.
Example: A rod* of length L has a uniformly distributed total
negative charge -Q. Calculate the electric field at a point P
center of and perpendicular to the rod.
I will work one of the above examples at the board in lecture.
You should try the other for yourself.
*Assume the rod has negligible thickness.

Example: A rod of length L has a uniformly distributed total
negative charge -Q. Calculate the electric field at a point P
center of and perpendicular to the rod.
d
L
-Q
P
Starting equation: 2
q
E = k
r
“Legal” version of starting
equation:
2
dq
dE = k
r
This is “better” because it tells you how
to work the problem! It also helps you
avoid common vector mistakes.

You should begin electric field of charge distribution problems
with this
2
dq
dE = k
r
because the equation “tells” you how to work the problem.
The equation says:
(1) pick a dq of charge somewhere in the distribution
(2) draw in your diagram the dE due to that dq
(3) draw the components of dE
(4) for each component, check for simplifications due to
symmetry, then integrate over the charge distribution.
This is a “legal” version of a
starting equation, so it is “official.”

Calculate the electric field at a point P.
d
L
-Q
P
Starting equation: 2
dq
dE = k
r
Pick a dq (best to not put
it at either end or in the
middle).
Draw the dE due to the dq.
Before I draw the components, I need to define axes!
x
y
dq dE
Now draw the components.
Do you see why symmetry tells me that Ex = 0?

d
L
-Q
P
Ex = 0, so calculate Ey
First, label an angle  in
the vector diagram.
To find sin , we need the x-coordinate of dq. If dq is at an
arbitrary position along the x-axis, what is a good name for its
coordinate? That’s right, we’ll call it x.
x
y
dq dE
The diagram is getting rather “busy,” but we are almost done
with it.

dEy = +dE sin 
x
yes, the + sign conveys
important information

d
L
-Q
P
To find sin , look at the green triangle. The sides have length
x and d, and hypotenuse r, where
x
y
dq dE
From the green triangle, we see that sin  = d / r.

x
2 2
r = x d



d
L
-Q
P
Now we start to put things together:
x
y
dq dE
To find Ey we simply integrate from one end of the rod to the
other (from –L/2 to L/2).

x
 
y 3/2
2 2 3 2 2
dq dq d dq dq
d
dE dEsin k sin k k kd
r r r r x d
           



d
L
-Q
P
x
y
dq dE
But wait! We are integrating over the rod, which lies along the
x-axis. Doesn’t there need to be a dx somewhere?

x
 
L 2
y 3/2
L 2 2 2
d dq
E k
x d






-Q
x
y
dq
dq is a tiny bit of charge on the uniformly charged rod.
x
I removed un-needed “stuff” from the figure.
If the charge is uniformly distributed, then the amount of
charge per length of rod is
(charge) Q
(linear charge density) = or
(length) L
 

-Q
x
y
dq
x
We use the symbol  for linear charge density. You
probably thought (based on Physics 1135) that  is
the symbol for wavelength. It is. But not today!
Q
L
 
What would be a good name for an infinitesimal length of rod
that lies along the x-axis? How about dx?
 
 
 
 
charge
charge on segment of rod = × length of segment of rod
length

-Q
x
y
dq
x
Thus, dq =  dx and
dx
     
L 2 L 2 L 2
y 3/2 3/2 3/2
L 2 L 2 L 2
2 2 2 2 2 2
dq dx dx
E kd kd kd
x d x d x d
  

   
  
  
 
L 2
y 3/2
L 2 2 2
Q dx
E kd
L x d



 The physics of the problem is all
done. The rest is “just” math.
We can take  outside the integral
because the charge is uniformly
distributed, so  must be constant.

-Q
x
y
dq
x
dx
 
L 2
y 3/2
L 2 2 2
Q dx
E kd
L x d




A note on the “just” math part. We expect you to remember derivatives
and integrals of simple power and trig functions, as well as exponentials.
The rest you can look up; on exams we will provide tables of integrals. We
would provide you with the above integral. It is not one that I could do in
5 minutes, so I would not expect you to do it.

Example: A rod of length L has a uniform charge per unit length
 and a total positive charge Q. Calculate the electric field at a
point P along the axis of the rod a distance d from one end.*
P
d L
To be worked at the board in lecture…
*Assume the rod has negligible thickness.

point P along the axis of the rod a distance d from one end.
P x
y
d L
It’s a good bet we will need x- and y-axes, so let’s just put them
in right now. Let’s put the origin at P.
After the previous example, we realize we will need to calculate
the linear charge density on the rod.
Q
= and Q = L
L
 
Note 1: both  and Q are given, so we can express our answer in terms of our choice of either one.
Note 2: we are told Q is positive, so we don’t need the absolute value signs as in the previous example.

point P along the axis of the rod a distance d from one end.
P x
y
d L
Because dq is positive, its contribution to the electric field
points away from the rod.
We start with our usual equation: 2
dq
dE = k
r
The equation says pick a dq of charge, so do it!
dE
Remember, this is the
best way to start a
problem like this one.
dq

P x
y
d L
The infinitesimal charge dq is a distance x away from the origin.
dE dq
The electric field at point P has no y-component (why?).
Therefore, Ey=0.
I could work the problem “all at once” using unit vector
notation, but for now I think it’s safer to work out each
component separately.
x 2 2 2
dq dx dx
dE = -k -k -k
x x x
 
 
Therefore
x
If I don’t know the sign of dq, I keep the absolute value signs but don’t know the direction of dEx. If dq is negative, then the
safest thing to do is change the – signs in the equation above to +’s, and keep the absolute value signs around dq.
The – sign is
important!
The rod has
no thickness.

P x
y
d L
dE dq
Now simply integrate over the rod.
x
d L d L
x x 2 2
rod d d
dx dx
E = dE = -k = -k
x x
 


  
 
 
d L
x
d
d d L
1 1 1
E = -k = k = k
x d L d d d L

 
 
   
     
     
 
     
 
x
k L
E = -
d d L



P x
y
d L
dE dq
The problem asks for the electric field at point P. Let’s make it
easy for a potential grader by writing down our complete
answer with a box around it.
x
 
y x
k L
E =0 E = -
d d L


Any of the boxed answers below is correct.
 
k L ˆ
E = - i
d d L


 
k L
E = , in the -x direction
d d L


On an exam, put a box around each part of an answer
when you finish it, so the grader can clearly see it. You can
copy parts of an answer and rewrite them together in one
place so you can put a box around the whole answer at
once (like I did here), but don’t make a mistake copying,
because you will lose points. Also, just box one answer. Do
not box different versions of the same answer.
If a problem says “express your answer in
unit vector notation,” you need to do that!

Example: calculate the electric field due to an infinite line of
positive charge.
There are two approaches to the mathematics of this problem.
One approach is that of example 21.10. See notes here. An
alternative mathematical approach is posted here. The result is
If a homework problem has an infinite line of charge, you would
need to repeat the derivation, unless I give you permission to
use it.
0
2k
E
2 r r
 
 

This is not an “official”
starting equation!
The above equation is not on your OSE sheet. In general, you
may not use it as a starting equation!

Example: A ring of radius a has a uniform charge per unit
length and a total positive charge Q. Calculate the electric field
at a point P along the axis of the ring at a distance x0 from its
center.
P
x0
x
Homework hint: you must provide this derivation in your solution to any
problems about rings of charge (e.g. 21.53 or 21.55, if assigned).
Visualization here (requires Shockwave, which downloads automatically):
http://web.mit.edu/viz/EM/visualizations/electrostatics/calculatingElectricFields/RingIntegration/RingIntegration.htm
To be worked at the
blackboard in lecture.

center.
An edge-on view of the ring would look like this:
x
y
The z-axis would be coming out of the screen at you.
I will use the perspective view of the ring in my solution.

center.
P
a
dq
r
x0
dE
x


y
Let’s add a y-axis to the
figure.
Starting equation:
2
dq
dE=k
r
Pick a dq of charge. Let’s put it on the y-axis for now.
Show the dE due to that positive dq.
We’ll need r and  later.

Show the x- and y-
components of dE. There
may also be a z-component,
which we’ll leave out
because it is difficult to draw
and visualize.
The y-components cancel pairwise! Same for the
z-components (not shown). Ey = Ez = 0.
r
P
a
dq
r
x0
dE
x


y
Consider the dq’ on the ring where it is intersected by the
negative y-axis.
dq’ gives rise to dE’ at P. Show the components of dq’.
dq′
dE′
All points on the ring are the same distance r from point P.
Also, x0 and  are the same for all points on the ring.

P
a
dq
r
x0
dE
x


y
From the diagram:
Back to our OSE…
2
dq
dE=k
r
2 2
0
r = x a

x
dE = +dE cos
0
x
cos
r
 
 
0
0 0 0
x x 3/2
2 3 3 2 2
ring ring ring 0
dq kx Q
x x x
E dE k k d q k Q
r r r r x a
 
    
 

 
  
For a given x0, r is a constant
for all points on the ring.
Only works because all Ex are in same direction.

 
0
0 0 0
x x 3/2
2 3 3 2 2
ring ring ring 0
dq kx Q
x x x
E dE k k d q k Q
r r r r x a
 
    
 

 
  
Some of you are wondering why all the absolute value signs.
You don’t really need them in this example, because Q is
positive.
When I draw the dEx and dEy in the diagram, the sign of Q
determines the directions of the components.
Because I used the sign of Q to determine the directions of the
components in my diagram, I don’t want to accidentally use the
sign again later and get the wrong direction in my final answer;
hence the absolute value signs, for safety.
If x0 is negative, then Ex points along the –x direction, as it should, so I don’t want
to put absolute value signs around the x0 in the answer.

P
a
x0 E
x
y
Back to our example…
 
0
x 3/2
2 2
0
kx Q
E
x a


y z
E E 0
 
Also “legal” answers:
 
0
3/2
2 2
0
kx Q ˆ
E i
x a

  
0
3/2
2 2
0
kx Q
E , away from the center
x a


Awesome Youtube Derivation: http://www.youtube.com/watch?v=80mM3kSTZcE
(he leaves out a factor of a in several steps, but finds it in the end).
These equations are only valid for P along the x-axis!
What would be different if
Q were negative? If P were
on the negative x-axis?

Example: A disc of radius R has a uniform charge per unit area
. Calculate the electric field at a point P along the central axis
of the disc at a distance x0 from its center.
P x
x0
R

P
r
x
x0
R
The disc is made of
concentric rings.
Caution! In the previous example, the radius of the ring was R. Here the radius of the disc is
R, and the rings it is made of have (variable) radius r.
The length is 2r, the
thickness is dr, so the
area of a ring at a radius
r is 2rdr.
Imagine taking a ring
and cutting it so you can
lay it out along a line.
dr
2r
The ring has infinitesimal thickness, so you can imagine it as
a rectangular strip.

P
r
dq
x
x0
R
The charge on each
ring is dq = (2rdr).
Let’s assume  is positive
so dq is positive.
We previously derived an equation for
the electric field of this ring. We’ll call
it dEring here, because the ring is an
infinitesimal part of the entire disc.
 
0 ring
ring 3/2
2 2
0
kx dq
dE
x r


dEring
     
charge on ring = charge per area × area
Skip to slide 47 for result.

P
r
dq
x
x0
R
so dq is positive.
 
0 ring
ring 3/2
2 2
0
kx dq
dE
x r


 
0
3/2
2 2
0
2 rd
k r
x ( )
x r


 
   
R
0
disc ring 0
3/2 3/2
0
2 2 2 2
disc disc 0 0
kx 2 rdr 2r dr
E dE kx
x r x r
 
   
 
  
dEring

 
 
R
1/2
2 2
0 0 0
disc 0 1/2
2 2
0 0
0
x r x x
E kx 2k
1/ 2 x x R

   

 
 
    
 
 
 
   
 
R
disc 0 3/2
0 2 2
0
2r dr
E kx
x r
 


You know how to integrate
this. The integrand is just
(stuff)-3/2 d(stuff)
P
r
dq
x
x0
R
so dq is positive.
dEring
Kind of nasty looking, isn’t it.

 
0 0
x y z
1/2
2 2
0 0
x x
E 2k E E 0
x x R
 
 
    
 

 
P

x
x0
R Edisc
As usual, there are
several ways to write
the answer.
Or you could give the magnitude and direction.
 
0 0
1/2
2 2
0 0
x x ˆ
E 2k i
x x R
 
 
  
 

 

Example: Calculate the electric field at a distance x0 from an
infinite plane sheet with a uniform charge density .
An infinite sheet is “the same as” disc of infinite radius.
Take the limit and use to get
0
1
k
4


sheet
0
E .
2



 
0 0
sheet 1/2
R 2 2
0 0
x x
E lim 2k
x x R

 
 
 
 
  
 
 

 
 
This is the magnitude of E. The direction is
away from a positively-charged sheet, or
towards a negatively-charged sheet.

Example: Calculate the electric field at a distance x0 from an
infinite plane sheet with a uniform charge density .
sheet
0
E .
2



Interesting...does not depend on distance from the sheet. Does
that make sense?
I’ve been Really Nice and put this on your starting equation sheet. You don’t have to
derive it for your homework!
This is your fourth Official Starting Equation, and the only one
from all of today’s lecture!

Example: calculate the electric field at “center” of semicircular
line of uniformly-distributed positive charge, oriented as shown.
R

d
ds
dE
y
x
To be worked at the
blackboard in lecture.
R
You don’t have to follow the steps in the exact
order I present here. Just let the problem tell you
what to. You may do things in a different order;
that’s probably OK.
+Q

y
x
Start with our usual OSE.
2
dq
dE = k
r
R
Pick an infinitesimal dq of charge.
dq subtends an arc length ds, and an angle d.
d
dq
ds
What is the charge dq?
   
dq = charge per length of arc length of the arc

dq = ds

+Q

y
x
Draw the dE due to the dq, and
show its components.
R
+Q
d
dq
ds
dE Do you see any helpful symmetry?
Pick a dq′ horizontally across the arc from dq. The x-
components of dq and dq′ will cancel. Because of this
symmetry, Ex = 0
Each dEy points downward so Ey will be negative.
dq′
dE′

y
x
Recall that dq and ds are
infinitesimal. dq is located at an
angle  along the semicircle from
the negative y-axis.
R

dq
ds
dE
dq
+Q
 is also one of the angles in the
vector triangle.

y
dE = -dE sin 

y
x
R

dq
ds
dE
dq
+Q

 
   
charge on arc Q Q
=
1
length of arc R
2 R
2
  


An arc of a circle has a length
equal to the circle radius times
the angle subtended (in radians):
ds = R d
Also,

y
x
R

dq
ds
dE
dq
+Q
 Q
=
R


 
2 2 2
Q
R d
ds R Q d
dE = k = k = k
R R R
 

 
  
 

Let’s summarize what we have
done so far.
ds = R d
2
dq
dE = k
r
dq = ds

Every dq is a distance R away from the arc center: r = R
y
dE = -dE sin 

y
x
R

dq
ds
dE
dq
+Q

y 2
kQ d
dE = - sin
R



y y 2
arc arc
kQ d
E = dE = - sin
R

 

 

 
 
y 2 2 2
0
0
arc
kQ kQ kQ
E = - sin d = - sin d = + cos
R R R


    
  
 
     
y 2 2 2
kQ kQ 2kQ
E = + cos cos 0 = + 1 1 = -
R R R
   
 
 
  
Awesome Youtube derivation: http://www.youtube.com/watch?v=L1n2EUvayfw

Help!
We covered a lot of material in a brief time.
If you want to explore a slightly different presentation of this at
your leisure, try the MIT Open Courseware site:
http://ocw.mit.edu/courses/physics/.

Homework Hints (may not apply every semester)
Your starting equations so far are:
(plus Physics 2135 starting equations).
1 2
2
12
q q
F k
12 r
 0
0
F
E =
q 2
q
E=k
r
2
dq
dE=k .
r
This is a “legal variation” (use it for charge distributions):
You can remove the absolute value signs if you know that dq is
always positive.
sheet
0
E .
2




Suppose you have to evaluate this integral in your homework…
 
a
2
0
dx
a r x
 

Let u=(a+r-x)2. Then du=-dx and u=a+r when x=0, u=r when
x=a.
Or look it up if you have tables that contain it.

 
3
2 2 2
dx
x a


The integrals below are in appendix B of your text.
 
3
2 2 2
x dx
x a


Your recitation instructor will supply you with needed integrals.
The above integrals may or may not be needed this semester.

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