So we understand how there’s a force generated between charges (Coulomb’s Law).
If we were to map the force vectors on a test charge near another charge, we would have a graphic representation of the strength of the electric force on the test charge at any point around the charges.
Since the Force at any point will be equal to KQ q /d^2, we can define a new physical quantity, Electric Field strength, E, as F/q or KQ/d^2
Here’s the Field strength applet.
Instructions:
Go to the applet and hit clear.
Click the + button to add a new positive charge. Make sure the contours, charge, labels are all checked.
Move the red charge into the center of the field by dragging it with the mouse.
1. What do you see? What do the blue arrows represent? What do the circles represent? If you aren’t sure uncheck the contours. What goes away?
2. Click the test button to put a test charge in the field. You should see a black dot with a vector line pointing away from the red positive charge. By convention the test charge is always a positive charge. What does the vector represent?
3. Clear the field and put click on the – button and move the negative charge to the middle of the field. Click the test button to put a test charge in the field. Which way is the vector pointed now? Why?
4. Click the – button again to get another negative charge. Move them so that they are along the horizontal centerline of the field, with each near a vertical side. Where do you have to put the test charge to minimize the force vector? What does that say about the field strength?
5. Clear all and place a positive and a negative charge in the field. How do the field lines change?
6. Play around with adding more charges and moving them around. Can you get an intuitive sense for how the field will change with each added charge?
7. Place a series of positive charges (say 5) in a line. How does the field differ from that of a monopole (single charge)?
8. Place a series of negative charges in a parallel line to the line of positives. How does the field change?
9. Now that you’ve played around, answer #1 again. How can you predict field strength from the field lines? What are the contour lines representing?
