Names:
In-Class Activity for Physics 7: Electric Field Hockey
1. On your laptop, open a web browser and go to the PhET site (http://phet.colorado.edu/en/simulation/electric-hockey) to
find the Electric Field Hockey simulation. Click “Run now!” to start the game. The display should look like this:
Make sure that “Practice” mode is selected in the green bar at the bottom, and that “Puck is Positive” is checked in the yellow
bar–this makes the “hockey puck” (represented by a black circle) a positive charge. Leave “Trace”, “Field”, and “Antialias”
unchecked for now.
The object of the game is to score goals, by getting the black “puck” (also called a test charge) into the goal (purple bracket
at the right of the screen). You can do this by judicious placement of positive (red) and negative (blue) charges, which you can
get from the cache at the top right of the screen. (The program treats the red and blue charges as fixed: you can move them
with the mouse, but they will not move on their own.) To see how this works, we’ll start off with a simple example:
a) Use the mouse to drag a negative (blue) charge out of the cache of charges, and place it directly behind (to the right of)
the goal. Now press Start. What happens? Why did this happen? (Explain in terms of electric forces.)
Hit the Reset button (this puts the puck/test charge back where it started). Drag the blue negative charge back into the cache.
Now get out a single positive (red) charge, and place it so that it will drive the puck into the goal.
b) Where did you place the positive charge in order to score a goal? Explain why this worked.
Press Reset to bring the test charge back to its original position, leaving the red positive charge where it is. Now turn on Trace,
Field, and Antialias in the yellow bar, and press Start to watch what happens. Trace shows the path (or trajectory) of the test
charge as a red dotted line; Field shows the electric field created by the red positive charge, as vectors at different points in
space. (Antialias is just a programming feature that increases the resolution and makes the vectors easier to see.)
When the test charge stops moving, print the display with the trace and field (one copy for each group member).
2. Press Reset again, and turn off Trace, Field, and Antialias. Place a red positive charge directly to the left of the test
charge/puck, at the edge of the screen. A red arrow will appear, showing the force that the red charge exerts on the test charge.
Now place a blue negative charge directly below the test charge, about the same distance away as the positive charge. This
causes a blue force arrow to appear. Your screen should now look something like this:
a) Make this prediction before you press Start: In which direction will the test charge initially move? Why?
Now turn on Trace, Field, and Antialias, and press Start. Watch the test charge until it leaves the screen. (It might leave
the screen and come back multiple times; you need only watch until it has left the screen at least once, though you may keep
watching if you like.) Print your result (again, one copy per group member).
b) Was your prediction for the initial direction of motion correct? If not, which way did the charge initially move, and why?
Try pressing Reset, moving your red or blue charge to a slightly different position, and pressing Start again. Repeat this a few
times. Notice that the path may be very complicated, even though the test charge is just following a few relatively simple rules
(F~net = m~a, with the force from each charge given by Coulomb’s law). Also notice that making a small change to the setup
can have a huge effect on the outcome. This situation–two fixed sources, with a free particle moving under their influence–is
called a restricted three-body problem. Problems of this type can almost never be solved using formulas (which is one reason
computer simulations can be very useful in physics). These problems often display extreme sensitivity to small changes in the
initial conditions, which is known as chaos.
c) As the test charge moves, does it always move in the direction of the electric field at its current location? That is, do the
electric field vectors always point along the path taken by the charge? If so, explain why this happens. If not, explain why not.
3. Press Reset again, and select difficulty level 2 in the green bar. (This adds some barriers to make it harder to reach the goal.)
Place some source charges (red and/or blue) until you can get a path that leads to the goal–this will probably take several tries!
Once you achieve a goal, print your result (one copy per group member), with Trace, Field, and Antialias turned on.
a) Describe, in detail, how you found your solution. What did you try first, and how did you come up with that idea? If your
first arrangement didn’t work, how did you change it? Did you find it more effective to have the Field display on or off during
your search? Why?