INTRODUCTION

In Class XI, we studied several everyday forces -- gravitational force, tension, friction, and the normal force -- treating each of them, as Newton did, as though they were independent, unrelated forces of nature. But what is the actual origin of these forces? It is now understood that every one of them, gravity alone excepted, is really electromagnetic in origin, arising from interactions between the atoms of the objects involved. When an object is pushed, the atoms of the hand interact electromagnetically with the atoms of the object being pushed. The normal force that lets us stand on the ground -- keeping us from sinking through the floor even while gravity pulls us down -- arises the same way, from electromagnetic repulsion between the electron clouds of the atoms in our feet and the atoms of the ground. Static and kinetic friction, which resist an object's motion across a surface, have the same electromagnetic origin: an interaction between the atoms of the moving object and the atoms of the surface it slides on. Because nearly every everyday force other than gravity reduces, at the atomic scale, to electromagnetism, understanding electric charge and how charges interact is essential to understanding matter -- and the universe -- at every scale, from atoms to everyday objects. This unit restricts attention to charges that are stationary (static), which is why it is called electrostatics; the study of charges in motion is taken up separately, later in the XII syllabus.