Experiment 1: Electrostatics
Part 1 - Electric Field Mapping
BACKGROUND: A DC power supply charges
two electrodes printed on a sheet which is coated
with a poor electrical conductor. The current
through this coating is large enough to give a
voltmeter something to measure, but small
enough that conditions are approximately static.
You will use the voltmeter to find several curves of constant potential. The direction of the field

vector, E , is perpendicular to these equipotentials. (With gravity, water runs in the direction that the

elevation changes fastest. Similarly, E points in the direction that the potential changes fastest). So,
you will sketch field lines by drawing them perpendicular to the equipotentials.
PROCEDURE:
1. Hook up two wires, one from each electrode on the sheet, to the DC terminals of a power supply.
(Red and black.) Hook up two more wires: One from minus on the digital voltmeter to minus on the
power supply, and one from plus on the meter to the probe. (The thing with the black handle.)
2. With the probe touching the sheet’s positive electrode, set the power supply’s knob so that the
meter reads 6.0 V.
3. Move the probe around the sheet to find points where the potential is 1.0 V. Plot enough on the
answer sheet to accurately sketch a curve of constant potential all the way across the picture.
(Maybe 2 or 3 cm apart; closer where the curve bends sharply.)
4 Repeat, obtaining equipotentials at 2, 3, 4 and 5 V.
5. Draw in some approximate lines of force by sketching smooth curves that cross
all equipotentials at right angles. The edges of the printed electrodes are your
equipotentials for 0 V and 6 V. Include arrowheads, and label which are the positive
and negative electrodes. Draw enough to show the field in all areas of the picture.
(Since all you did was to draw a picture, there is no conclusion to draw when you write your
discussion for this part.)
Part II - Coulomb's Law
Verify that the force between two charges varies as the inverse square of their separation.
APPARATUS: Two balls are charged by touching them to a Van de Graff electrostatic generator.
(The motion of a belt inside it charges the metal globe on top.) They are then placed in a box to
protect them from air currents. One ball is mounted on a movable base; the other hangs from a
string. You can observe the force by how far the string is
pushed to the side, x. This force is plotted against the
separation of the balls, r, on log-log graph paper. The slope of
this plot is equal to n in rn; that is, the slope is the power to
which r is being raised. This is compared to what appears in
Coulomb’s law.
PROCEDURE:
(Place the box right under an overhead for best illumination of the ruler. If it has gotten out of
square in storage, set weights at its corners to hold it in shape.)
1. Run the Van de Graff for a second or two by plugging it in. Charge the balls by touching them to
the globe, running the machine between balls. If they now touch you, the box, or anything else, the
charge will escape.
2. Put the hanging ball in the box. There is a mirror with a ruler just below it on
the wall behind the balls. Adjust the hanging ball’s height so you can see when
it is in front of its reflection and, at the same time, read its position on the ruler.
With the other ball outside, position the hanging ball so that its left side is at 15.0
cm on the ruler. The left side should line up with its reflection each time you
read either ball's position so that your eye is in the correct place. Cover the top of the box with old
transparencies. Be careful not to bump the box after this, or you might change the 15.0 cm.
3. Slide the movable ball into the box. If the balls are sufficiently charged, the hanging one should
be displaced over 2 cm when the other is close to it.
4. Record the positions of both balls, at 1 cm intervals as you slide the ball from 26 cm to 18 cm.
Before each reading, check that the balls are lined up parallel to the ruler.
Do not take data with the sliding ball's position, s, outside of the range 26> s > 18 cm. Further out,
stray charge on the floor and walls of the box is nearly as close as the sliding ball. Farther in, the
charge on the balls begins to polarize, superimposing an attractive force on top of the repulsion.
CALCULATIONS & ANALYSIS:
If the q1 and q2 remain constant, Coulomb’s law reduces to
. Your
n
objective is to measure n in F = ar to see if it comes out -2. Notice that if you plot log F as a
function of log r, F = arn is a straight line whose slope is n:
Take the log of both sides:
log F = log (arn )
The log of a product is the sum of the logs:
log F = log a + log rn
Since log rn = n log r ,
So, you will plot this graph and determine its slope. If Coulomb’s law is wrong, the graph will not
be a straight line, or its slope will not be -2.
1. Calculate x and r for each trial. Instead of measuring force in newtons or pounds, it's convenient
to make up your own force unit, defined such that one unit of force causes one centimeter of
displacement. This makes the force simply equal to x.
2. Graph the data on the log-log graph paper provided. Don’t punch up any logs; the graph paper
does the logs for you. Include error bars based on your own estimate; notice that the logarithmic
scale makes these look larger in the lower part of the graph. Draw what appears to be a best fit line
through your data.
3. Find its slope, measuring the rise and run with a ruler, not from your data or the scale of the
graph. Measuring the logarithmic graph with the linear ruler is what takes the logs of F and r.
Indicate which points on the line you measured between.
4. Round n to the nearest integer - it's only good to one significant figure. In your conclusion, state
whether your result agrees with Coulomb's.
5. After the instructor approves your results, put the hanging ball back in its container so it won't get
lost.
PHY 132
Part 1.
Report on Experiment 1
PART II:
H
(hanging ball)
S
(sliding ball)
r
Attach log F vs. log r graph.
Calculate n:
So, your result is that the force is proportional to r to the ________ power.
x = F