The electromagnetic forces are the forces between charged particles. When the charges are at rest, the forces are called electrostatic forces. The forces between unlike charges are attractive and the forces between like charges are repulsive. These forces are governed by Coulomb’s Law.
The charges in motion produce magnetic effects and a magnetic field gives rise to a force on a moving charge. That is why electric and magnetic forces, in general are inseparable. Hence the name electromagnetic forces.
The magnitude off electrostatic force F between two point charges q1 and q2 separated by
a distance r in air/vacuum is given by Coulomb’s law:
where - = 9 x 109 N m2/C2 is a constant
and E0 = 8-85 X 1CT12 C2 N-1 m-2, is absolute electrical permittivity of free space.
The smallest magnitude of charge found in nature is the charge of an electron or a proton represented by
e = 1-6 X 10_19C
The proton has charge + e and electron has charge — e. All charges are found to have magnitude where n is an integer. Electric charge is, therefore, said to be quantized.
As is known, matter consists of elementary charged constituents electron and protons. As electromagnetic force is so much stronger than the gravitational force, it dominates all phenomena at atomic and molecular scales. The structure of atoms and molecules, the dynamics of chemical reactions etc. are governed by electromagnetic forces. Some of the macroscopic forces like the forces the forces between two surfaces in contact, force of friction, tension in a spring/rope, forces due to spring etc. are also explained in terms of electromagnetic forces.
(1) Forces Between Two Surfaces in Contact
When we put two bodies in contact, the atoms of the two surfaces come close to each other. The charged constituents of the atoms of two bodies exert strong forces between them.
Generally, the forces between the two bodies in contact are along the common normal i.e. perpendicular to the surface of contact. Also, these forces are in the form of a push or repulsion. For example, when a book is placed on a table, the book pushes the table downwards (away from it) and the table pushes the book upwards (again away from it).
(02) Force of Friction
The two bodies in contact may have a component of force parallel to the surface of contact. This component is called friction. Obviously, the surfaces in contact will be frictionless, when they exert forces only along the direction perpendicular to them. Bodies with smooth surfaces in contact can exert only small amount of forces parallel to the surface of contact.
For example, we can cling on to the trunk of a tree, which is quite rough. The trunk exerts an upward frictional force parallel to the surface of the tree, on us to hold us there. This is the reaction of an equal frictional downward force, we exert on the tree due to our weight. On the contrary, it is difficult to stay on a smooth metallic lamp post, as it cannot exert enough force parallel to the surface of the lamp post.
(3) Tension in a String/Rope.
When a heavy block hangs from a ceiling by a string, the string is in a state of tension. The electrons and protons of the string near the lower end exert forces on the electrons and protons of the block. The resultant of these forces is the force exerted by the string on the block, that supports the block.
Infact, a string or rope under “tension exerts electromagnetic forces on the bodies attached at the two ends to pull them.
(4) Forces due to a Spring.
When extention or compression is not too large, the force exerted by the spring (F) is proportional to change in its length (/) i.e. F directly proportional to l
F = kl, where k is spring constant.
If the spring is extended, the force will be directed towards its centre, and if compressed, the force will
be directed away from the centre. This force comes into picture due to the electromagnetic
electromagnetic forces between
the atoms of the material of the spring.
Note that two bodies—not in contact with each other do not exert appreciable electromagnetic forces though both contain a very large number of electrons and protons. This is because the charged particles of one body and those of other body have both attractive and repulsive nature. Therefore, these forces largely cancel out.
Some of the salient features of electromagnetic forces are:
1. These forces may be attractive or repulsion like charges repel each other and unlike charges attract each other.
2. These forces are governed by Coulomb’s laws which are similar toNewton’s law of gravitation.
3. They obey inverse square law.
4. Electrostatic forces (between two protons) are 1036 times stronger than gravitational forces between them, for any fixed distance.
5. They operate over distance which are not very large.
6. They are central forces.
7. They are also conservative forces.
8. The field particle of electromagnetic forces is photon which carries no charge and has zero rest mass.