How do insulators become charged

However, the conductor now has a charge distribution ; the near end the portion of the conductor closest to the insulator now has more negative charge than positive charge, and the reverse is true of the end farthest from the insulator. The relocation of negative charges to the near side of the conductor results in an overall positive charge in the part of the conductor farthest from the insulator.

We have thus created an electric charge distribution where one did not exist before. This process is referred to as inducing polarization —in this case, polarizing the conductor. The resulting separation of positive and negative charge is called polarization , and a material, or even a molecule, that exhibits polarization is said to be polarized. A similar situation occurs with a negatively charged insulator, but the resulting polarization is in the opposite direction.

Neutral objects can be attracted to any charged object. The pieces of straw attracted to polished amber are neutral, for example. If you run a plastic comb through your hair, the charged comb can pick up neutral pieces of paper. When a charged rod is brought near a neutral substance, an insulator in this case, the distribution of charge in atoms and molecules is shifted slightly. Opposite charge is attracted nearer the external charged rod, while like charge is repelled.

Since the electrostatic force decreases with distance, the repulsion of like charges is weaker than the attraction of unlike charges, and so there is a net attraction. Thus, a positively charged glass rod attracts neutral pieces of paper, as will a negatively charged rubber rod.

Some molecules, like water, are polar molecules. Polar molecules have a natural or inherent separation of charge, although they are neutral overall. Polar molecules are particularly affected by other charged objects and show greater polarization effects than molecules with naturally uniform charge distributions.

The duster is rubbed back and forth along the rod. Electrons carrying a negative charge are transferred and now reside on the duster. When a polythene rod is rubbed with a duster, the friction causes electrons to gain energy. If the rod is swapped for a different material such as acetate , electrons are rubbed off the acetate and onto the duster. Both of the rods and the duster are made of insulating materials. Insulators prevent the electrons from moving and the charge remains static.

Conductors , on the other hand, cannot hold the charge, as the electrons can move through them. A charged object will experience a non-contact force from another charged object. Superconductors allow the movement of charge without any loss of energy. Salty water and other similar conducting materials contain free ions that can move through them. An ion is an atom or molecule having a positive or negative nonzero total charge.

In other words, the total number of electrons is not equal to the total number of protons. Other substances, such as glass, do not allow charges to move through them. These are called insulators. Electrons and ions in insulators are bound in the structure and cannot move easily—as much as 10 23 times more slowly than in conductors.

Pure water and dry table salt are insulators, for example, whereas molten salt and salty water are conductors. Figure 2 shows an electroscope being charged by touching it with a positively charged glass rod. Because the glass rod is an insulator, it must actually touch the electroscope to transfer charge to or from it.

Note that the extra positive charges reside on the surface of the glass rod as a result of rubbing it with silk before starting the experiment. Since only electrons move in metals, we see that they are attracted to the top of the electroscope. There, some are transferred to the positive rod by touch, leaving the electroscope with a net positive charge. Figure 2. An electroscope is a favorite instrument in physics demonstrations and student laboratories.

It is typically made with gold foil leaves hung from a conducting metal stem and is insulated from the room air in a glass-walled container. Like charges in the light flexible gold leaves repel, separating them.

Electrostatic repulsion in the leaves of the charged electroscope separates them. The electrostatic force has a horizontal component that results in the leaves moving apart as well as a vertical component that is balanced by the gravitational force.

Similarly, the electroscope can be negatively charged by contact with a negatively charged object. It is not necessary to transfer excess charge directly to an object in order to charge it. Figure 3 shows a method of induction wherein a charge is created in a nearby object, without direct contact.

Here we see two neutral metal spheres in contact with one another but insulated from the rest of the world. A positively charged rod is brought near one of them, attracting negative charge to that side, leaving the other sphere positively charged. This is an example of induced polarization of neutral objects. Polarization is the separation of charges in an object that remains neutral.

If the spheres are now separated before the rod is pulled away , each sphere will have a net charge. Note that the object closest to the charged rod receives an opposite charge when charged by induction. Note also that no charge is removed from the charged rod, so that this process can be repeated without depleting the supply of excess charge.

Figure 3. Charging by induction. Another method of charging by induction is shown in Figure 4.