Download Surfing An Electric Field Lab

Surfing An Electric Field
AP Physics
Name:
Period:
In this activity, we will map the voltage around two different
charge distributions: a pair of oppositely charged parallel plates
and a positive and negative charge. The voltage level at different
points in space tells us a lot about how any charge at that point will
behave. We will make graphs of our data and figure out what
those graphs tell us about the situation.
Materials: Parallel Plates
Power Supplies
Positive and Negative Charge
Voltage Probe
Procedure:
Parallel Plates:
1. Fill the container with about 0.5 cm of water and then attach the positive electrode to one plate and
the negative electrode to the other. Turn on the power supply to about 10 V.
2. Turn on the voltmeter and touch the negative probe to the negative plate – this is the level we will
call 0 V of electric potential.
3. Starting at the point (1,1) on the grid under the container, touch the positive probe of the voltmeter to
each point and record the electric potential reading at that point
Point Charges:
1. Fill the container with about 0.5 cm of water and then attach the positive electrode to one bolt and the
negative electrode to the other. Turn on the power supply to about 5 V.
2. Turn on the voltmeter and touch the negative probe to the negative bolt – this is the level we will call
0 V of electric potential.
3. Starting at the point (1,1) on the grid under the container, touch the positive probe of the voltmeter to
each point and record the electric potential reading at that point.
Analysis:
You will plug both of your sets of data into a spreadsheet file and use Excel to make several different
graphs of your data. For each set of data follow these steps:
1. From your teacher’s web page, download the Excel file called “Surfing An Electric Field.” Open the
file and save it under a different name – ideally, one that identifies which set of data you will be
analyzing.
2. Type in all of your data and print out the data form.
3. Turn to the tab titled “Multiple Views” and look at the plots of voltage shown there. A sense of the
shape of the graphs in three dimensions will help when you analyze your two dimensional graphs in the
next step.
4. You will now create one more graph of your data – a contour plot showing lines of equipotential.
a) Return to the “Data” tab and select your data values – you should have all of the white cells
highlighted, but none of the blue cells.
b) Under the “Insert” menu, choose the “Chart…” option.
c) A dialog box will pop up with a list of different Chart types on the left hand side.
•Choose the “Surface” chart type.
•You will then have a choice of four chart sub-types on the right hand side of the dialog box. If you
click on one of the sub-types of graph, the name for that sub-type will show up in the text box below
the sub-types. Choose the sub-type called “Contour.” Be sure you do not choose the “Wireframe
Countour”
•Then hit the “Next” button.
d) Hit the next button two more times until you get to the final dialog box – Chart Location. Choose the
button that opts to place the chart “As a new sheet” and then hit the finish button.
e) To increase the number of lines on your graph, right click on the legend and select the “Format
Legend…” option. Select the “Scale” tab in the dialog box that opens. On the “Scale” tab, find the
“Major Unit” box and enter 1 in that space. Hit return and close the dialog box
f) Examine the contour graph and print out a copy of it.
5. On the contour graph, use the equipotential lines to sketch in electric field lines for the situation
shown. Remember how electric fields behave around conductors for situations that have conductors.
6. On the back of the graph, write a brief description of your graph, noting the important features of the
graph – what do the equipotential lines look like in terms of shape and spacing – and what does that tell
you about the voltage and electric field around the charges. Why does the arrangement makes sense in
terms of the objects (charges, plates or conductors) in the experiment?
7. Calculate the approximate strength of the electric field at three locations – select different locations
around the graph so that you can see how the strength of the electric field changes with the appearance
of the equipotential lines.
Find two points on your field line where you can find the voltage – either from the data or from your
graph – and the distance between them.
Remember that the corners on your grid (your data points) are exactly 1 cm apart – if you measure
distances on your graph, you will need to scale your measurements to get the right data. Show your
work and the results for each location on your graph. Clearly mark each location on your graph where
you calculate the field strength.
8. Download the data for the other groups from your teacher’s website and perform the same analysis
on each set of data.