How to find force on charge +q by uniformly charged rod(or ring) of charge +Q. I know how to find electric field but I am confused whether we can use the formula:- $$F=qE$$ Because the charged object will get polarized.
Note:- I haven't yet done electrostatics chapter completely but I tried to find the answer(which I didn't find) so please try to explain answer like you are explaining it to someone new to this chapter.
The charged rod or ring has a bunch of + point charges in it, each at a different position. Each point charge creates an E field around it.
The test charge is at some position. To get the E field at the test charge position, you figure out the E from each charge at that position. You add up all these E fields using vector addition. Once you have the total E, you can use $F=qE$ to find the force on the test charge.
There are some ways to simplify this. First electron-size charges are a pretty good approximation of an infinitesimal charge, and a charged rod or ring has a pretty good approximation of an infinite number of them. Integration is how you add an infinite number of infinitesimal numbers. You will get to how to handle charge distributions like this.
Second, some distributions work out to create fairly simple electric fields. An infinite wire or plane with a uniform charge are typical examples. No real charge distribution extends to infinity, of course. But it makes for useful exercises for learning how to handle charge distributions. And some real applications are good approximations to this. For example, up close a capacitor looks a lot like two infinite charged planes.
An object like a rod or ring is usually neutral - it has equal numbers of electrons and protons. It is possible to add more electrons or remove electrons, making the object charged.
Protons are in the nucleus of each atom. In a solid the atoms are fixed in place. Protons don't move.
Electrons may or may not be able to move. The cannot move in insulators such a glass. They can in conductors such as copper.
In a charged conductor, electrons repel each other and spread out. As they do, they leave behind positive charges. This attracts electrons. Electrons spread out until the repulsive and attractive forces on each electron balance. The resulting distribution isn't always intuitive, but it can be calculated.
If you put a + charge near a conductive object, it will attract electrons and alter the distribution. Again, electrons will spread out until attractive and repulsive forces balance. There will be more electrons near the + charge and fewer far away. The part of the object near the + charge will be negatively charged, and the part far away positively charged. The conductive object will be polarized.
You can add or remove electrons from an insulator. When you do, they stay where they are. Depending on how you added them, the insulating object may or may not be polarized.
If you put a + charge near the object, electrons don't move. The polarization of the object doesn't change.
That is a little simplistic. Some molecules are neutral, but one end is positive and the other negative. If such molecules can rotate in place, a solid made of them can be polarized. You might read about that in a chapter or two. Most insulators don't do this.
