Presentation: charges and materials

Static electricity is never your friend. Not at the gas station. Nope. (Video link: "Fuel Pump Fire.")

In this presentation we will discuss how charges move differently through certain materials, and the effects of this behavior in certain situations.

We will assume that you already familiar with certain basics of electrostatics, in that there are positive and negative charges...

...and that two charges with opposite signs (positive-negative, or negative-positive) will attract, while two charges with the same sign (positive-positive, or negative-negative) will repel.

First, the mobility of these charges in certain materials.

We will also assume that you are already familiar with a simple atomic model of solids, where the positive charged nuclei of atoms are held at fixed locations, while their negatively charged outmost electrons are relatively free to move. The degree to which these electrons can move depends on the type of material.

For an insulator, the outermost electrons are more or less fixed to their atomic locations, but at least are able to move somewhat around these locations. (As in the gas station spark movie shown above, electrons can be exchanged when two different types of insulators that have different affinities for electrons are rubbed against each other.)

In contrast, the outermost electrons in a conductor are much more free to move about the entire material.

For a polar molecule, while the molecule itself may be neutrally charged, the electrons are distributed such that certain atoms at each end of the molecule have a permanent small positive or small negative charge. In a liquid state, these molecules would be free to align in different orientations.

Second, the response, or polarization for these different materials to external charged objects held nearby.

When a positively charged object is brought near a neutral insulator, the outermost electrons at each atom will be drawn to the side facing the positively charged object. Note that for each atom, the outermost electrons are slightly closer to the positively charged object (and feel an attractive force), while the positively charged nuclei are slightly farther away from the positively charged object (and feel a repulsive force). But since the outermost electrons are slightly closer to the positively charged object, and the positively charged nuclei are slightly farther away from the positively charged object, then the attractive forces will be slightly greater than the repulsive forces, such that there is a net attraction between the neutral insulator and positively charged object.

When a negatively charged object is brought near a neutral insulator, the outermost electrons at each atom will move to the side facing away from the negatively charged object. Note that for each atom, the outermost electrons are slightly farther way from the negatively charged object (and feel a repulsive force), while the positively charged nuclei are slightly closer to the negatively charged object (and feel a attractive force). But since the outermost electrons are farther away from the negatively charged object, and the positively charged nuclei are slightly closer to the negatively charged object, then the repulsive forces will be slightly less than the attractive forces, such that there is a net attraction between the neutral insulator and negatively charged object.

An example of this is when an insulating object (here, a cat) acquires a charge, due to rubbing or sliding against a different type of insulator. It doesn't matter whether the cat lost electrons (and thus becomes positively charged) or gained electrons (and thus becomes negatively charged) from this rubbing, as in either case the object will still attract neutral insulators (such as these styrofoam packing peanuts).

Convince yourself that a similar effect occurs when a charged object (positive or negative) is brought near a neutral conductor. While outermost electrons are much more mobile than in an insulator, they will still move to the side facing towards a positively charged object, or away from a negatively charged object, such that net attraction will result in either case.

Again, let's start with an insulating object (here, a balloon) acquiring a charge, due to rubbing or sliding against a different type of insulator (a fabric curtain). It doesn't matter whether the balloon lost electrons (and thus becomes positively charged) or gained electrons (and thus becomes negatively charged) from this rubbing, as in either case the object will still attract neutral conductors (such as this aluminum can). (Video link: "Aluminium can static roll.")

Once again a similar effect occurs when a charged object (positive or negative) is brought near a a liquid containing polar molecules. However, in this case the molecule will reorient itself such that the negative end will move to the side facing towards a positively charged object, or away from a negatively charged object, such that net attraction will result in either case.

Once again, start with an insulating object (here, a comb) acquiring a charge, due to contact with a different type of insulator (hair). It doesn't matter whether the comb lost electrons (and thus becomes positively charged) or gained electrons (and thus becomes negatively charged) from this rubbing, as in either case the comb will still attract neutral polar molecules (such as in this water stream). (Video link: "Water Bending.")