Force is a word that is used in everyday language to mean many different things, but in physics, it has a very specific meaning. In physics, a force is an interaction between two objects that has the ability to change the motion of one or both of the objects. One of the fundamental forces of the universe is the electric force. This is the force that exists between all charged particles. The electric force is responsible for such diverse phenomena as making your hair stand up on a cold, dry day, creating chemical bonds, and allowing you to see when you turn on a lamp on a dark night.
There are two types of charge, positive and negative, and they interact with each other in predictable ways. Unlike charges exert attractive forces on each other, while like charges exert repelling forces on each other. This means that if two objects that are both positively charged come close to each other, they will repel, or push each other away. If a positively charged object comes close to a negatively charged object, the two objects will attract each other and try to come together.
Inside an atom there are very small particles called protons and electrons. Each proton has a positive charge, and each electron has a negative charge. Protons and electrons are the smallest charged particles that exist. All other objects (which are, in turn, made up of atoms) become charged because of an imbalance in the number of protons and electrons inside those atoms.
Two objects with the same type of charge (either both positive or both negative) will repel each other, while two objects with different charges will attract each other
Protons are very tightly held in the nucleus of each atom, so they are not able to move around at all. In contrast, electrons are away from the nucleus and are free to move around within the atom. It is also relatively easy for electrons to move from one atom to another, creating an imbalance in the number of positively charged protons and negatively charged electrons and causing the atom to become charged. When this happens to many atoms in an object, the entire object becomes charged.
The normal force is one of several forces which act on the object. In the simple situations so far considered, the most important other forces acting on it are friction and the force of gravity.
This imbalance of protons and electrons is what causes your hair to stand up after you brush it on a cold, dry day. As you brush your hair, electrons from your hair are transferred to the brush or comb. This means that each strand of hair is now positively charged. Because two objects (hair strands) with the same charge repel each other, each strand of hair pushes away from all the others, causing your hair to stand up. This phenomenon is more likely to happen when the air is very cold and dry. It can happen even on a hot, humid day, but when there is a lot of water in the air, your hair picks up charges from the air more easily and loses its charge quickly.
The strength of the electric force between any two charged objects depends on the amount of charge that each object contains and on the distance between the two charges. As the amount of charge gets bigger, the force gets bigger, and as the distance between the two charges gets larger, the force gets smaller. This is known as Coulomb's Law and can be written mathematically as shown below.
In this equation, the symbol 'q1' represents the amount of charge on object 1, 'q2' represents the amount of charge on object 2, r represents the distance between the two objects, and k is a constant known as the electrostatic constant. 'F sub 12' simply means, 'the force of object 1 on object 2', and 'F sub 21' means 'the force of object 2 on object 1.' The objects exert equal forces on each other (F sub 12 = F sub 21).
