UNIT I - Electric Field Exploration

brothersroocooElectronics - Devices

Oct 18, 2013 (3 years and 7 months ago)

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UNIT I
-

Electric Field Exploration


Launch the "EM Field" program. From the "Sources" menu, select [3D point charges]. This
will call up a screen that lets you select different quantities of negative and positive charges on
th
e screen. You simply "grab" the charge with your mouse and drag it to where you want it on
the screen.


If the screen begins to get too crowded with markings, simply go to the [Display] menu and
select [Clean up screen]
--

this removes
all

markings.


You
can improve your estimation of distances on the screen by adding a background of dots
spaced 1 cm apart. To do this, go to the [Display] menu and select [show grid]. (If the grid
becomes confusing, go the same menu and select [remove grid].)



I
-

Explor
ing field around a point charge


Place a +4 charge
centered on a grid dot

in the middle of your screen. From the [Field and
Potential] menu, select [Field Vectors].



1.

Holding the mouse button down,
drag

your pointer around the charge. What do you not
ice about:

a) the direction of the arrow at all times?


b) the magnitude of the vector (the length of the arrow) as you move farther away from the
charge?



What does this arrow represent?



2.

When you release the mouse button, the field vector will rema
in on the screen.



a)

Create three more vectors of identical length. What do the four points have in common?


b)

Place a vector 1 gridmark from the point charge. Now, place a second vector 2 gridmarks
from the charge. Compare the length of the two ve
ctors. What can you conclude about the
relationship between electric field strength and distance?




c)

If you place a vector 3 gridmarks from the point charge, how will it compare in size to the
first vector? Test your prediction.


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3.

Keep this pictur
e in mind; erase these markings from the screen. Go now to the [Field and
Potential] menu and select [Field lines]. Click on the screen
symmetrically

around the charge to
create field lines produced by the charge.

a)

What information is missing in the "
field line" diagram that was given by the
field

vector
diagram?



b)

Predict the relationship between the
field strength

and the
spacing of the field lines

at a given
point in this representation



4.

Erase the field lines and replace the positive charge

with a
-

4 charge. To do so, drag the +4
charge to the bottom of the screen and the reservoir of charges reappears. (Or go to the [Source]
menu and select [Add more charges].)


Repeat the investigations in steps 1


3. Record your observations.

Step
1



Step 2



Step 3



In what ways are the
field

vectors and field lines the same? How are they different?



5.

Erase the field lines. Place a +4 and a
-

4 charge on the screen about 8 gridmarks apart. Use
[field vectors] to probe the electric field ar
ound the two charges. After completing your
exploration, answer the following questions:

a)

In what place or places is the field the strongest?



b)

In what place or places is the field the weakest?



c)

Change the display to [field lines]. Click the cur
sor on at least 10 places on the screen. In the
space below, sketch a diagram showing the charges and the electric field lines.

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d)

Select [Field vectors] from [Fields and potential]. While holding the mouse button down,
trace along an electric field lin
e from + to
-

charge. Describe how the direction of the field
vector changes as you move from one charge to the other. Describe how the magnitude of
the field vector changes.





6.

Erase any field markings. Leaving the +4 charge in place, replace the
-

4 charge with a second
+4 charge. Use the "field vectors" to probe the electric field around the two charges. After
completing your exploration, answer the following questions:


a)

You will find the field is weak far from the positive charges. Where else
is the field very
weak (even zero) in strength?



b)

If you placed a small positive charge halfway between the two +4 charges, how would it
move?



c)

Describe as best you can the general direction of the field.



d)

The field lines around a single charge
are always straight. Why do the lines curve in this
case?



e)

In the space below, sketch a diagram showing the two charges and the shape of the electric
field.

©Modeling Workshop Project 200
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II
-

Exploring field around parallel plates


While the software does not create plates, we ca
n approximate a plate by lining up a number of
rods of charge. To save time this has been created for you.



1.

Under the [File] menu choose [Get charges or currents from file] and select [2
-
plates.emf]. This
opens a distribution of charged rods like the
one below (your diagram will not have the little
X
’s).

























2.

Select [Field Lines] from the [Field and Potential] menu and establish the field between the two
plates

in the are
a

where the x’s are located. Do your clicking at regular i
ntervals
.
Sk
etch your
results on
the diagram above
.

a) What is true about the density of the field lines in this area?



b) What does this indicate about the nature of the field in this area?




3. Now use the Field Lines to explore the region above and b
elow the
X
’s. Sketch those lines on the
diagram above.

a) What is true about the density of the field lines in these areas?



b) What does this indicate about the nature of the field in these areas?





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4
.

Switch to [Field vectors]; while holding the mou
se button down, explore the region midway
between the two plates (at each of the places marked by an
X
).

Leave a field vector with the tail
at each X.

a)

What can you tell about the strength (and direction) of the electric field?





5
.

Suppose you
were t
o place
a small positive charge close to the positive line of charge

a)

Describe the motion of the charge.






b)

Describe the energy changes of the charge as it moves.