Name Page 1 of 5 Period Electrostatic Fields and Forces Lab Go to http://tinyurl.com/337842 or http://linus.highpoint.edu/~atitus/physlets/em_field.html You should see the Electric Field Physlet. Your computer must support Java. If it doesn t, please find one that does! A. Click boxes at bottom to add a + charge (Red) and a test charge (Black). 1. Notice the arrow on the test charge. This is the NET force vector. 2. Drag the test charge around the field. What happens to the force vector as you approach the + charge (size and direction)? 3. Now click Show Fieldlines. a. As you move the test charge around, how does the direction of the field lines compare to the direction of the force vector? b. How does the space between the field lines relate to the strength of the force vector? (What happens to the spacing as you move the test charge away from the + charge?) c. Where do the field lines end? If they come in and out of the box away from the charge, we say they come from/go to infinity. Where do these lines go? 4. Now add a - charge (Blue). Drag it well away from the + change. a. Drag the test charge around the - charge. Ignore the + charge. What happens to the force vector (size and direction)? b. Now consider both the + and charges. As you move the test charge around, how does the direction of the field lines compare to the direction of the force vector? c. How does the space between the field lines relate to the strength of the force vector? (What happens to the spacing as you move the test charge away from both charges?)
Page 2 of 5 d. Would you agree that the field lines make a map of the force on the test charge at different locations? e. Would you agree that the test charge can be used to map out the field lines, and that one predicts the other? 5. Is the test charge + or -? How do you know? 6. Use the Refresh Button at the screen top to reset the physlet and erase the charges. Add a + charge and move it to the center. Add another + charge and move it about 4cm from the first, as perfectly horizontal as you can. Add a test charge. a. Drag the test charge around and observe what happens to the force vector. Can you find a place directly between the + charges where the force vector goes to (almost completely) zero? Sketch the position of the two + s and test charge here: b. Turn on the Fieldlines. i. Based on the direction of the field lines, what could tell you that the place you found should be zero force? ii. As you move the test charge around, how does the direction of the field lines compare to the direction of the force vector? 7. Use the Refresh Button at the screen top to reset the physlet and erase the charges. Don t add a test charge. Place a + charge and a charge about 4 cm apart. Turn on the fieldlines. a. Based on the field lines, can you see a place near the two charges where the force vector would be zero? Now add a test charge and see if you were correct. What did you find? b. Move the test charge around the + and - charges. Does the vector follow the electric field lines?
Page 3 of 5 8. Since there is a force on the test charge, it should move in the electric field. a. Consider this field map. If you dropped the test charge at point A and it was free to move, mark on the map another A where it would end up. (It must stay within the box.) A B C b. Do the same thing for a test charges dropped at B and C. c. Which test charge will experience the greatest force at its original location? i. How do you know? d. Which test charge will experience the least force at its original location? i. How do you know? e. Refresh the page and set up the picture as shown above. Insert a test charge and move it to A, B & C. Which location has the largest force vector? i. Were your answers to 3 a and 4 a about the forces correct?
Page 4 of 5 9. Using just 8 field lines around each charge, draw the field lines that will surround each of these two negative charges. Include arrow heads to show direction. a. Now set up the applet with two charges. Check you drawing, and redraw with corrections below.
Page 5 of 5 Summary Questions. You should have learned the answers from this lab. 1. A test charge is always (positive/negative). 2. A test charge will be (repelled/attracted) by a positive charge. 3. The force on the test charge (represented by the vector) points in the (same/opposite) direction as the field lines. 4. The field lines (will/will not) show the path a freely-moving test charge will take. 5. With two different charges, field lines start on (positive/negative) and point toward (positive/negative) 6. With two same charges or just one charge, field lines start on (positive or infinity/negative) and point toward (positive or infinity/negative). 7. The spacing between the field lines is important. When lines are close together, the field is and the force vector will be. When lines are far apart, the field is. This lab is over, said Tom, forcefully!