Working with Electric Charges and Lines of Equipotential
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Introduction:
In this lab we will be examining electric fields and lines of equipotential. We won’t define that term at the beginning instead you will be discovering it yourself in this mostly qualitative, conceptually driven lab. In other words, not a lot of math calculations to do.
Instructions:
This lab is composed of several tasks which utilize a PhET (Physics Education technology) simulation.
Go to the web page https://phet.colorado.edu/en/simulation/charges-and-fields
Once you have the screen up, go to the selection area found in the upper right of the screen and check the boxes for “Electric Field” and “Grid”.
You should then move one positive charge and one negative charge
out onto the field as shown below. Your screen should look similar this one:
Next place your cursor over the blue equipotential box on the right. You can move the box around with your cursor. The “bullseye” will locate a point through which you can plot Lines of Equipotential. Plot at least 10 lines. Staring close to the negative charge, move up towards the positive charge.
Save your work by taking a screen shot. There are several ways to do this. One of the simplest methods is to just use the <shift> + <PrntScn> button on your keyboard, this copies the screen onto the clipboard, and you can just paste it into the space provide below. (Keep the screen open though, you will still need it)
Once you have pasted the screen shot into the box, answer the following questions. (If you need to, you may use the internet, your physics book, or other published sources. Please include a source reference.)
1) What is the definition of an E-field?
2) What interesting observations can you make about the direction of the E-field arrows on your screen?
3) What do the lines of equipotential represent?
4) What interesting observations can you make about the lines of equipotential, especially how they relate to the E-field arrows?
Now go back to the interactive screen. Press the “Clear All”
button.
Next, arrange 10 positive charges and 8 negative charges in the following manner:
Then use the Equipotential Box to plot a total of 15-20 equipotential lines, above, between, and below the charges.
Copy and paste this screen into the box below.
Answer the following questions:
5) What do you notice about the “density” or “closeness” of the equipotential lines at different locations on the screen? Be specific?
6) Explain in some detail how these equipotential lines are similar to the contour lines on a topographic map? What is the variable that remains constant along an equipotential line? What is the variable that remains constant along a contour line?
If you think of the negative charges as analogous to a lower elevation and the positive charges as analogous to a higher elevation you may be better able to visualize a comparison. If you are not completely familiar with topographic maps, take a moment to look at the following web page:
https://adventure.howstuffworks.com/outdoor-activities/hiking/how-to-read-a-topographic-map2.htm
Now go back to the interactive screen. Uncheck all the boxes in the selection window, except grid lines but DO NOT press the Clear All button
Move the equipotential box around until you get a reading of 0 V (getting an exact reading of zero may be a bit difficult but get as close to that value as you can. You may find it easier to move your equipotential box some distance away from the charges.) Now, plot an equipotential line at that point.
7) What does this line represent?
Clear the equipotential box but DO NOT press the Clear All button. Place three more positive charges on the screen so that you have this configuration:
Again, move the equipotential box around until you get a reading as close to 0V as you can. Plot an equipotential line at that point.
Now check the Voltage box in the upper right of
the screen.
You will now see blue and red areas “light up”. Though the analogy isn’t exact, you can think of the blue areas as being similar to the lower elevations on a contour map and the red areas as being similar to the higher elevations.
As you know, like charges repel and unlike charges attract. Now imagine that you are particle with a positive electric charge, walking across the field on your screen.
8) Where on the field would you have to walk to feel no net sensation of either attraction or repulsion?
9) If you were to walk toward the positive charges, into the red areas, what kind of force, push or pull, would you feel? Which direction would you be pushed or pulled? What kind work would you have to do moving that direction?
10) If you were to walk toward the negative charges, into the blue area, what kind of force, push or pull, would you feel? Which direction would you be pushed or pulled? What kind work would you have to do moving that direction?
Now, move the equipotential box around to different places on the screen and notice that as you move into the blue area, the voltage becomes increasingly negative and in the red area the voltage becomes increasingly positive.
Next, move the cursor so that you get a positive value of approximately +50 V and plot an equipotential line at that point. Next find a point in the blue area that is approximately -50 V and plot an equipotential line there.
11) What can you say about the forces that you would experience as a positive particle walking along either of these two lines? (consider direction, magnitude, and anything else you can think of)
Review and thinking it through:
12) How would you explain what an equipotential line is to someone who hasn’t heard the term before?
13) Explain in your own words why equipotential lines must be perpendicular to electric field lines.
14) Can different equipotential lines cross? Explain.
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