Question

(a) Who discovered the fact that amber rubbed with wool or silk attracts light objects ?
(b) Define current density.
(c) What should be the angle between the velocity vector of the charged particle field to experience a maximum force, when a charged particle is moving in a field ?

Answer 1

Hint: When amber is rubbed with silk or wool, amber gets positively charged and this can be explained with the help of the static electricity concept. Thales of Miletus discovered the fact that amber rubbed with wool or silk attracts light objects.

Complete answer:
The current density of a conductor is defined as the amount of current passing per unit area of conductor. It is a vector quantity and is expressed as $J = \dfrac{I}{A}$. The S.I. the unit of current density is $A{m^{ - 2}}$. The angle between the velocity vector of the charged particle field to experience a maximum force, when a charged particle is moving in a field is ${90^ \circ }$ because $F = Bqv\sin \theta $ and the value $\sin 90$ is the maximum value applicable in this situation.

Therefore,
(a) Thales of Miletus discovered the fact that amber rubbed with wool or silk attracts light objects.
(b) The current density of a conductor is defined as the amount of current passing per unit area of conductor.
(c) The angle between the velocity vector of the charged particle field to experience a maximum force is ${90^ \circ }$.

Note: Amber becomes positively charged when it is rubbed with silk or wool and this can be explained with the help of static electricity. With the help of electric charge, the effects of static electricity can be explained. There are two types of charges: they are positive charge and negative charge. Like charges repel each other whereas the unlike charges attract each other. The force between charges decreases as the distance between the charges increases.