# Fermilab

## Inquiring Minds

 Questions About Physics

Electric Fields

You asked:
I have a question about electric fields. I have seen in many texbooks that show that for a negative charge, the electric field goes into itself or inwards, and it shows arrows that go into the core of the negative charge. And for the positive charge, the field goes outwards and has arrows that go outwards. But this doesn't explain how the two charges attract.

Who is doing the attracting. You could say, that the positive charge is moving towards the negative charge because its like the negative charge has a hoover vacuum effect, because the negative charge is sucking or taking any field which it finds and draws into itself. That could make a lot of sense. But it doesn't explain why the negative charge moves if you do electrostatic experiments.

If you charge 2 pith balls, one negative and one positive, they both move towards each other. But if you follow the implication of the electric fields movement, only the negative charge is making anything move. Why does the positive charge move, since it is only extending outwards. Also, how does the electric field move anything, because all the negative charge is doing, is grabbing on to the positive charges of arrows of energy going outwards, but then its only grabbing on to the arrows and bringing them into itself. How does this move the actual positive charge. It seems that only the arrows of one charge is moving the arrows of the opposite charge.

Also, if the proton is stuck because of its weight, why then does it have the same charge as an electron, since the electron is always closer to any object since it is spinning and it can leave its orbit easily with metals. And how can a negative charge, which means it has an excess of electrons, make anything positive move.

And, if you bring a glass rod that is positively charged close to a pith ball, then it is attracted to it. But if the proton is so heavy and doesnt move ever, then how can the positive glass rod push away the protons that are there. Or is it that the electrons go to the surface of the object. Is the answer have to do with the fact that we are dealing with a 3 d effect on the two charges. Like if the proton or positive charge has a line of directional field that goes outwards, it finds the negative charge, and it pushes the negative charge because it goes into it. But that doesn't explain why it pushes it. I mean the positive energy can just go into the negative and disappear. Why is it pushing or moving it. And why does the proton move

Dear Daniel,

I'll try to answer your questions regarding the attraction of electric charges and the meaning of field lines.

As you already now, opposite charges attract each other. Whether the object with a negative charge moves towards the object with positive charge or visa versa depends on the circumstances. The arrows on the field lines DO NOT indicate which object is moving towards the other one!!! The arrows on the field lines have ALWAYS the convention that they point from the positive charge to the negative charge. That's it, nothing more to it. So if you get a drawing with dots, which represent charged objects, and field lines with arrows, you can figure out which dots represent a positive charge and which ones indicate a negative charge.

The forces created by electric charges always work in both directions. (That is one of Newton's laws: action equals reaction.) It's like two people pulling on a rope. The positive charge pulls on the negative one, the negative one pulls on the positive on. Which charge is eventually going to move to the other one is a question that depends on several other factors. Again, think of two people pulling on a rope. Who will win? It depends on how heavy each person is, who has the better traction, etc, etc. Similarly, for charges. If one charged object is really heavy (has a large mass) and the other one is light, then the lighter one will move easier towards the heavy one than vice versa. And if one charged object is attached to something bigger, it is even less likely that it will move.

Another example you may want to think of are the forces acting between magnets. Magnets have a magnetic north pole and a magnetic south pole. If you bring two identical poles together they repel, if you bring two opposite poles together they attract each other. Charged objects do the same thing. Now to the meaning of the electric field lines. The field lines surrounding charged objects indicate the path which a tiny object with a tiny charge would take if it would be inserted into the field. If the tiny object has a small positive charge, it would move in the direction of the arrow on the line on which it was put. If the tiny object has a small negative charge, it would follow the same line, but in the opposite direction, against the direction of the arrows. I hope this answer clarified some of the questions you raised. You also might want to check the question and answer section on Fermilab's webpages.

In particular, I found some correspondence that is also related to charges: www.fnal.gov/pub/inquiring/questions/bob.html

Good luck,

Kurt Riesselmann
Fermilab Public Affairs

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 last modified 5/7/2000   physicsquestions@fnal.gov