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square. 45 ELECTRIC POTENTIAL FROM MULTIPLE POINT CHARGES – 1302Lab2Prob3 3. Determ ine the total electric potential at the point of interest by ad d ing the electric
potentials from each point charge. Rem em ber that the charges are of d ifferent sign. PRED ICTION
Restate the problem . What are you trying to calculate in your pred iction? H ow d o you
calculate a total electric potential from a collection of point charges? EXPLORATION
In the fold er P hysLab o n the d esktop, open EM
Field a nd click anyw here in the w ind ow for the
instructions. From the S ources p ull-d ow n m enu,
select 3D point charges.
Drag any positive
charge to the center of the w ind ow of EM Field .
From the Field and Potential p ull-d ow n m enu
(show n to the right), select Potential. To find the potential at precise points on the screen / you can use the show grid a nd
constrain to grid features of the program (from the display p ull-d ow n m enu ). It m ight
also be helpful to expand the d isplay w ind ow to fill the entire com puter screen. You
can d o this by clicking on the sm all box in the upper right -hand corner of the d isplay
Another useful w ay to view electric potential involves equipotential surfaces. Select
Equipotentials with number from the Field and Potential p ull-d ow n m enu. Move the cursor
w here you w ould like to d eterm ine the electric potential and click the m ouse. H ow is
this d ifferent from the Potential s etting?
Try d ifferent m agnitud es of charge. What range of charge values allow s you to
accurately m easure the electric potential at a large num ber of locations on the screen?
Try using negative charges. H ow d oes this change the electric potential? Look at the
cases of (a) equal and opposite charges and (b) tw o id entical charges. Does the potential
behave as you pred ict in each case? Does it go to zero w here you pred ict it?
To further check w hether or not you get the correct behavior of the electric field from a
point charge d o the follow ing:
1. Pick a useful charge value and d eterm ine several locations at d ifferent d istances r
from the center of the single point charge. (H int: Choose your locations at regular 46 ELECTRIC POTENTIAL FROM MULTIPLE POINT CHARGES – 1302Lab2Prob3
intervals.) At each location , m easure r a nd the value of the electric potential. In your
no tebook, record the data and sketch a plot of the vector length as a function of r.
2. N ow , calculate pred icted potential d ue to th e point charge as a function of r and
sketch the values for the field strength vs. d istance (r) on t he same graph.
3. Com pare the shape of the graph to that based on the Coulom bs’ law and record your
You have to calibrate t he com puter sim ulation program in ord er to translate potential
values on the screen to potential values that you calculated using your pred iction
equation, w hich are in SI units (assum e Q = 1C). You can use the d ata that you just
collected to d o this. On the com puter, plot the electric potential in SI units vs. the
electric potential values you obtained from the screen. Fit a line to the d ata. If the fit is
good , then you can use the fit equation to convert from the potential values on the
screen to potential valu es in SI units. If it is not good , then you need to go back and
repeat steps 1-3 above.
N ow , explore the d istribution of three charges. Drag tw o equal negative charges and
one positive charge of 1/ 3 the value of one negative charge onto the screen in the
configu ration specified in the problem statem ent above. H int: m ake sure the charges
are accurately placed using the ‘constrain to grid’ feature. Note the value of the electric
potential in the fourth corner of the square. What param eter can you easily vary to
change the value of the potential in that corner w hile preserving the other cond itions of
the problem ? In your notebook, note w hether or not such m anipulations change the
d irection of the electric field at that corner, and record the d irection. Determ ine a
m easurem ent plan. M EASUREMEN T
M easure the electric potential at the point of interest for several d ifferent values of the
varying param eter accord ing to your m easurem ent plan . Record the d ata in your
no tebook. A N ALYSIS
U se the d ata for the follow ing analysis (perform on Excel):
1. Using your calibration equation, convert the potential values prod uced by the
com puter sim ulation into potential values in SI units.
2. Using your pred iction equation , calculate the expected potential values in SI for your
chosen values of the variable param eter. 47 ELECTRIC POTENTIAL FROM MULTIPLE POINT CHARGES – 1302Lab2Prob3
3. Com pare the calculated potential to that from the com puter sim ulation on a plot.
Includ e uncertainties. Without them , the results are nearly m eaningless. CON CLUSION
H ow d id your expected result com pare to your m easured result? Explain any
d ifferences. From your results, w hich general properties of the electric potential does
the sim ulat...
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This document was uploaded on 02/23/2014 for the course MANAGMENT 2201 at University of Michigan.
- Spring '14