What is the interaction between charged bodies? Let's do an interesting experiment first: cut half a dry newspaper into dozens of narrow strips, each of which is not cut to the end, so that their upper parts are still connected. Then spread the cut newspaper on the door panel, hold it with one hand, and brush it back and forth for a while with the other hand. Take off the newspaper, hold the upper part of the newspaper with your hands and form a circle. At this time, these notes are not hung vertically, but scattered around, like an open "skirt". If you put a negatively charged rubber rod into the skirt tube from below, the skirt tube will open wider; If the positively charged glass rod is inserted into the skirt tube from below, the paper strips will gather on the glass rod immediately before insertion, and the rubbed paper strips will be negatively charged, which is the result of negative charge interaction. Experiments show that objects with the same charge repel each other and objects with different charges attract each other. This unique interaction property of charged bodies is called like-minded repulsion and opposite-minded attraction for short.

The magnitude of the interaction between charged bodies was first measured by Coulomb, a French physicist, in 1785, and he summed up the law of the interaction between two charges. When the size of charged bodies is much smaller than the distance between charged bodies, these charged bodies can be regarded as point charges, that is, the charges seem to be concentrated at one point.

Coulomb pointed out that the interaction between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them, and the direction of the interaction is on the connecting line of the two point charges. The point charge is represented by q 1 and q2, and the distance is represented by r, and the formula is:

F=kq 1q2r2

This is Coulomb's law. Force f is called coulomb force, and k is a proportional constant related to unit selection. When the signs of charges are the same, the coulomb force is positive, indicating repulsion; When the charges are different, the coulomb force is negative, indicating gravity.

How does this interaction between charges occur? At first, it was thought that it was transmitted directly between two charged bodies at infinite speed, which was called "action at a distance". Later, Faraday believed that the interaction between charges was not directly transmitted, but was transmitted at a limited speed through an intermediate medium. This interaction is called "intermediary" and is the origin of the concept of electric field. Electric field is a special substance, and there is always an electric field around the charge; Force acts on other charges in the electric field through the electric field. The electric field formed around the electrostatic charge is called electrostatic field.

The acting force of electrostatic field on electrostatic charge in the field is called electrostatic force, that is, coulomb force.

The establishment of Coulomb's law makes the study of electricity enter the quantitative stage, which lays the foundation for electromagnetism as a precise science.

Newton's third law

Newton's third law means that the action and reaction between two objects are always on the same straight line at the same time, with equal magnitude and opposite directions. That is, f 1 =-F2(n = n'), which means: ① The action of forces is mutual, appearing and disappearing at the same time. ② The interaction force must be the same force. (3) The acting force and reaction force act on two objects, and the resulting effects cannot cancel each other out. ④ Action can also be called reaction, but the selected reference objects are different. ⑤ The acting force and the reaction force cannot be combined, because the acting point is not on the same object.