A tiny scrap of paper is suspended motionless below a plastic pen that carries a net charge of Q. Focus on a single carbon atom in the scrap of paper. Suppose that the center of the outer electron cloud (with a charge of -4e) of this carbon atom shifts a distance s when the carbon atom is polarized by the pen. Required:
a. Use the momentum principle (Newton's 2nd Law) to derive a mathematical (algebraic) expression for s.
b. Using your result for s, derive an algebraic expression for the atom's polarizability, a.

B) AC current can be used in transformers while DC current cannot.

Explanation:

AC current is used for long distance transportation of electrical energy with amplifying the voltage and reducing the current in the conductors so that there is minimum loss of energy in the form of heat.

The relation between heat energy and electrical current is given by Joule's law as:

where:

The above mentioned variation  of current and voltage in the transmission lines of AC is achieved by a device called transformer. It consist of a rectangular shaped steel core common to two of the insulated wire winding of which one side acts as input (primary coil) and the other one acts as secondary (output).

AC i.e. alternating current can only be used in the transformers because they have the continuously varying amplitude which in turn provides the change in flux with respect to the time inducing an emf in the  nearby secondary coil of the transformer without any physically moving parts.