Name: ___________________________
Hr: __________
Questions to be investigated
What happens to the strength of a charge as the distance between charged objects or the
strength of the charge on the objects is changed.
Objectives
 To visualize forces between charges and the properties of electric fields.
 To experiment with simulations to make clear conclusions.
Procedure
Go to: http://physics.ius.edu/~kyle/physlets/electrostatics/electrostatics.html
Part A: Click on Simulation A at the left side of the window. Drag the charges around
with the mouse and note the magnitude and direction of the force at various locations.
1: Is there any way to tell from this simulation which charge is positive and which is negative?
Explain.
2: Is there any way to tell from this simulation which charge has the larger magnitude?
Explain.
3: Clearly state the direction of the force one charge exerts on the other.
Part B: Click on Simulation B at the left side of the window.
1: Which charges have like signs? How did you arrive at your answer?
Name: ___________________________
Hr: __________
2: Move charges a and b to the far left of the window and the other three charges to the far
right. What can you say about the force on charge a as compared to the force on charge b (is
Newton's third law obeyed)? Now move one of the other three charges closer to the pair on
the left. Why are the forces on a and b no longer equal and opposite?
3: Now move one of the other three charges closer to the pair on the left. Why are the
forces on a and b no longer equal and opposite?
Part C: Click on Simulation C at the left side of the window.
 Click on the Reset button.
 Click on the add button to add a positive charge.
 Click on the add button again to add a second positive charge.
1. With two positive charges describe the electric field around each charge. What happens if
you lay a positive charge exactly on top of a positive charge of the same magnitude?
• Reset and add equal and opposite charges
2. With a positive and a negative charge showing describe the electric field around each
charge. What happens if you lay a positive charge exactly on top of a negative charge of the
same magnitude? Both charges are still there so why don’t you see a field?
3. In this simulation there is a way to tell from the picture which charge is negative and
which is positive? Explain how.
Name: ___________________________
Hr: __________
Part D: Click on Simulation J at the left side of the window.
1: Start the charge at x=0, y=0. What happens? Describe why this happens.
2: Start the charge at x=0, y=2. How does the direction of the test charge’s motion compare
to the electric field vector’s direction when the charge first starts to move?
3: What is the relationship between the direction the test charge travels and the electric
field vectors after the charge has moved for a while?
4. How does the direction of the force vector on the test charge relate to the electric field
vectors?
5. Why does the charge not collide with the dipole in this case (why doesn’t the charge follow
the field vectors)?
Part E: Go to:
http://phet.colorado.edu/new/simulations/index.php?cat=Electricity_Magnets_and_Circuits
 Click on Charges & Fields and choose Run Now
 Place a positive 1 nano-coulomb charge at the center of the screen. Click on “Show
Numbers”.
 Bring out an electric field sensor.
1: What are the units of an electric field?
Name: ___________________________
Hr: __________
2: Is the electric field a scalar or a vector?
3: To do its job, the electric field sensor should have a charge. Why?
4. Does the field sensor have a positive or negative charge?
 Turn on the “Tape measure”.
 As you sample the electric field further and further from a charge, you find that the
field strength weakens. Do you think the E field vs. distance relation is an inverse
relation, an inverse square, or some other power relation? Use the Electric field sensor
and tape measure to collect data sets in order to answer this question.
 Use Graphical Analysis to find the equation that relates E field to separation distances.
 Staple your data collection table and correctly labeled graph to your answer sheet.
5. Clear the screen and place a -1 nano-coulomb charge at the center of the screen. How
does the field change?
6. Place a second -1 nano-coulomb charge on top of the first. How does the electric field
change?
7. What is the relationship between the amount of charge creating the field and the
strength of the field?
8. Click on Show E-Field. What field properties are shown by these arrows?
Name: ___________________________
Hr: __________
9. Clear the screen, place on + and one – charge on the screen about 3 cm apart. Use field
sensors to collect information about the field. Make a sketch that shows the location of your
field chares, and the magnitude and direction of the electric field around the charges. Show
the E-field. Explain why the field has this appearance.
SKETCH ON ANOTHER PAPER
Part F – Go to:
http://phet.colorado.edu/new/simulations/index.php?cat=Electricity_Magnets_and_Circuits
 Click on Electric Field Hockey and choose Run Now.
Electric Field Hockey is a computer-simulated game played with a charged puck (or ball) on a
frictionless horizontal surface. The object of the simulation, as it is traditional field hockey
game, is to score a goal by propelling the puck into a net. However, here the puck is positively
charged and moves only as a result of the influence of other charged particles which, once
you put them where you want them, are “glued” down on the playing surface. Hitting a blue
obstacle with end the simulation.
Take some practice shots by selecting Practice, adding a fixed charge, and pressing the start
button. If you miss, select the game again, add and reposition the fixed charge, and try
again.
1. What does the arrow that appears on the puck represent? Explain how you know.
2. When you have scored a goal and have a good feel for the playing field, move on to Game 1.
Score a goal in game one. Sketch the charge configuration that allows you to score a goal.
SKETCH ON ANOTHER PAPER
3. Turn on the Trace and Field. Does the + charge hockey puck’s path have any consistent
relationship to the electric field?
Name: ___________________________
Hr: __________
4. Change the mass of the puck. How does this affect its motion?
5. Change the sign of the puck to negative. How does this affect its motion?
6. Try to score with just one + and one - charge. If you are successful sketch your field
arrangement.
SKETCH ON ANOTHER PAPER
7. Move on to games 2. Score a goal in and sketch the charge configuration.
SKETCH ON ANOTHER PAPER
8. Suppose you changed the sign of the charges on the puck and on the fixed charges (turned
all positive charges into negative and vice versa) for a situation where you scored a goal.
What would be the effect on the trajectory of the puck? Explain.
9. Which direction do electric field lines point for positive charges?
12. Which direction do electric field lines point for negative charges?
13. What do the direction and strength of the field lines indicate for the (positively charged)
“puck?”
14. Did the (positively charged) puck always move in the same direction as the field lines it
was passing over?
15. How did the distance between the puck and the particles affect the motion of the puck?