# What is a charge?

The first law of electrostatics says: like charges are repelled, and unlike charges attract. Recall that the charge formed on the glass, worn about silk is called positive, and the charge that appeared on the sealing wax, rubbed against the flannel, is negative. But, we are used to the fact that friction is electrified only by dielectrics. It is believed that conductors can not be electrified at all. It turned out that if the conductor is well insulated, then it can be charged with electricity.

But, is it necessary to rub the bodies together? And most importantly, what is the cause of electrification? To answer these questions, you can do with simple experience.

Take two brass discs, which are attached to the handles of insulating material. To one of them glue a circle of flannel or silk, equal to the diameter of the disk, to the other - the same circle of plexiglass. Unloaded discs are stacked on top of each other and rub them against each other, turning in opposite directions. Bring the first disc a distance of 1-2 cm to the rod of the electroscope and note how deviated the petals. Approaching the second disk to the electroscope, the angle of divergence of the petals will be the same as in the previous case.

Then, bring the discs, not in contact, close to each other and bring them closer to the rod of the electroscope. If the leaves of the electroscope do not deviate, then the charges of the disks are opposite and their actions are compensated.

This fact allowed us to formulate the following statement: if two dissimilar bodies first touched, and then were removed from each other, then each of them acquires a charge equal in magnitude but opposite in sign.

Thus, the electrization by friction is explained by the appearance of charges upon contact of dissimilar bodies. In this case, friction provides a close contact between the contacting bodies.

The first quantitative law of electrostatics was discovered in 1785 by Coulomb. He found that the force of interaction between point charges is inversely proportional to the square of the distance between them.

To confirm this law, one can use the idea of Cavendish: the action of all charges located on the sphere on any charge inside the sphere will be mutually compensated only if the interaction force is inversely proportional to the square of the distance between interacting charges.

Based on the idea of Cavendish, you can conduct the following experiment. The charged metal ball on the insulating handle is inserted into the uncharged metal sphere and touches its inner surface. Charges from the ball go to the sphere, and the ball will be uncharged. Clearly, if the law of the inverse square does not work, then on the removed ball there will be some charge. It can be detected with an electroscope.

Maxwell did the experiment with great accuracy and received that the deuce in Coulomb's law has an error of no more than ±1/2160.

The first practical application of knowledge of electrostatics was, perhaps, a lightning rod. It is known that a spark jumps between the point and the charged body - this was used to protect houses from lightning strikes.