# Concept question about electric force

If I comb my hair with a rubber comb, the comb can be used to pick up small bits of paper on a desk. This says that there is a force between the comb and the paper bits. If the paper bits are uncharged why are they attracted? You can pick up paper bits by pulling a piece of scotch tape frm a roll and using that also.

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CWatters
Homework Helper
Gold Member
The comb becomes charged. When you bring that near to an uncharged bit of paper the charges in the comb repel like charges in the paper to the far side of the paper and opposite charges are pulled to the near side.

This seems reasonable if the bits of paper were metallic. However, since the paper is an insulator how could the charges move??

Doc Al
Mentor
This seems reasonable if the bits of paper were metallic. However, since the paper is an insulator how could the charges move??
Think of the electrons surrounding the nuclei of the molecules within the insulator as shifting to the side a bit, creating enough of a polarization charge to produce attraction.

The reason I was wondering about this is that, as a tutor, I wanted to show how electric force could exceed gravitational force and posed the question of how many columns would be required to be generated in a comb, rubbed through the hair, and lift a bit of paper off the ground at a distance of say 2 mm.I calculated that a bit of paper 1/8 inch square would have a weight of about 7E-5 N and then I went from here. The (q1)(q2) product came to about 3.4 E-21 giving the comb and the bit of paper a charge of about 5.8E-11 c. Then I asked myself where did the charge on the paper come from. I can visualize rubbing the comb might rub off electrons but did not know about the paper.

Maybe this is not a good problem to pose.

Doc Al
Mentor
One way to pose the problem is: How many electrons would you have to move from object B to object A so that their electrostatic attraction is equal to the weight of object B. Then express that as a fraction of the total number of electrons.

I am still struggling with why a comb, or balloon would pick up uncharged paper bits because nothing has been done to the paper bits to add or subtract electrons. We could assume that the molecules and atoms could orientate themselves so as to be attracted but on the average, it seems to me this is hard to imagine. I can visualize the comb losing or gaining electrons due to the physical rubbing. I have a friend here that has a Van de Graph generator. He places a cup of Styrofoam bits on the upper terminal and when turned on, the bits all are repelled from the cup. This is a matter of repulsion not attraction but here again, the Styrofoam is an insulator and should not allow electrons to move. I'll keep thinking on the subject but yourt suggestion is a good one.

Look up "dielectric polarization".
For example here
http://en.wikipedia.org/wiki/Dielectric

The attraction is due to interaction of the induced dipole of the paper and the electric field of the charged rod. The expression of the force is not so simple as Coulomb's law.

Doc Al
Mentor
I am still struggling with why a comb, or balloon would pick up uncharged paper bits because nothing has been done to the paper bits to add or subtract electrons. We could assume that the molecules and atoms could orientate themselves so as to be attracted but on the average, it seems to me this is hard to imagine. I can visualize the comb losing or gaining electrons due to the physical rubbing. I have a friend here that has a Van de Graph generator. He places a cup of Styrofoam bits on the upper terminal and when turned on, the bits all are repelled from the cup. This is a matter of repulsion not attraction but here again, the Styrofoam is an insulator and should not allow electrons to move. I'll keep thinking on the subject but yourt suggestion is a good one.
Do not think of the paper bits as gaining or losing electrons. They do not. Just think of the electrons of the paper shifting away a bit due to the negative charge of the comb. Once they have shifted, so that the paper becomes polarized, then the positive charges of the paper are slightly closer to the comb than the electrons are. Thus the attractive force on the positive charges is stronger than the repulsive force on the electrons. The net effect is an attractive force acting on the paper bits. As nasu says, to quantify the force is not so simple as the force between two charges.