By: Engr. Hinesh Kumar Lecturer IBT , LUMHS Coulomb’s Law By: Engr. Hinesh Kumar Lecturer IBT , LUMHS

History First experiment to investigate quantitative law of forces between localized charges was out by Charles Augustin Coulomb (French scientist). The results of experiment can be stated the form of a law called Coulomb’s Law.

Coulomb’s Law The interaction force between two charges is: directly proportional to the size of each charge (q1 and q2)

Coulomb’s Law q2 q1 r F The interaction force between two charges is: inversely proportional to the square of their Separation distance (d) F q2 q1 r

Coulomb’s Law k = Coulomb’s Constant = 9.0x109 Nm2/C2 q1 = charge on mass 1 q2 = charge on mass 2 r = the distance between the two charges e0 = permitivity of free space

Coulomb’s Law in Vector q1 q2 F12 r12 r12 Where is the force F12 exerted by q1 on q2 and r12 is the unit vector along the line joining the two charges from q1 to q2. The magnitude as well as the direction of force can be represented by the vector equation

Coulomb's Law The force between charges is directly proportional to the magnitude, or amount, of each charge. Doubling one charge doubles the force. Doubling both charges quadruples the force.

Coulomb's Law The force between charges is inversely proportional to the square of the distance between them. Doubling the distance reduces the force by a factor of 22 = (4), decreasing the force to one-fourth its original value (1/4). This relationship is called an inverse square law because force and distance follow an inverse square relationship.

Inverse Square Law It is state that a physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity. In equation form:

Coulomb’s Vs Newton Look carefully at the Coulomb’s Law equation This equation looks very much like Newton’s Law of Universal Gravitation! Gravitational force concerns MASS Coulomb force is about CHARGE Both have an INVERSE SQUARE relationship between FORCE and DISTANCE

A negative answer shows that the Example # 1 What is the electrostatic force between these two objects? r = 2.0 m q1 = -2C q2 = +2C A negative answer shows that the force is ATTRACTIVE

A positive answer shows that the Example # 2 What is the electrostatic force between these two objects? r = 5 nm q1 = -5μC q2 = -2μC A positive answer shows that the force is REPULSIVE

Problem # 1 Two 40 gram masses each with a charge of 3μC are placed 50cm apart. Compare the gravitational force between the two masses to the electric force between the two masses. 3μC 40g 50cm

Problem # 2 Three charged objects are placed as shown. Find the net force on the object with the charge of -4μC. -5μC 20cm 45º F1 5μC - 4μC F2

Applications Coulomb’s law is employed in meteorology to test models of thunderstorm charge distributions. Atmospheric physics, planetary physics, astrophysics, and plasma physics are vast domains of application of coulomb’s law. Another application of Coulomb's law is in the study of crystal structure. Crystals are made of charged particles called ions.

Applications Ions arrange themselves in any particular crystal (such as a crystal of sodium chloride, or table salt) so that electrical forces are balanced. By studying these forces, mineralogists can better understand the nature of specific crystal structures.

Questions..!!!

Facts are many but Truth is One… (Rabirnath Tagore)