Now that we've seen how electric charges interact, we can discuss the concepts of electric force fields and lines of force.

I've heard those terms before, but I'm not sure I understand them. There seems to be a "force field" in every episode of Star Trek; it's like an invisible wall that nothing can penetrate. Is that what a force field really is?

Not exactly. In physics, a force field is a way to picture the effects that electric charges have on one another. Instead of talking about the force a positive (+) charge exerts on an electron, we can say the charge creates a force "field" in the empty space around it. An electron put down at any place in this force field is pulled towards the + charge; a positive charge set down at the same place is pushed away. Try putting down "test" electrons with the mouse to see which way the field points and how strong it is; the line points away from the direction in which the electron will move, and the length of the line tells you the strength of the force at its current location. You can drag the mouse, or you can press the "R" key to have your computer put down electrons.

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Click to place an electron. Click and drag to place lots of them. Press "Delete" to start over. Press 'R' to add a bunch of random electrons. Press 'L' to show the entire force field.

OK, I get what a force field is (although it's a stretch to think of a force field as a property of empty space). What is a "line of force"?

You can visualize "lines" of force by looking at the forces created by the field in many different places; imagine connecting the lines from all the electrons you have placed. Press "L," and the computer will show you the pattern formed by all these connected lines. The lines in this pattern are known as "lines of force." Force field lines "coming out of" the big + charge "go into" the big - charge, so those two charges are "connected" by field lines.