Question

When a comb rubbed with dry hair attracts pieces of paper. This is because the

• A. comb polarizes the piece of paper
• B. comb induces a net dipole moment opposite to the direction of field
• C. electric field due to the comb is uniform
• D. comb induces a net dipole moment perpendicular to the direction of field
• E. paper acquires a net charge

Answer 1

The correct option is B comb induces a net dipole moment opposite to the direction of field

$\text{Step 1: Charge on comb after rubbing}$

On rubbing the comb with dry hair, the electrons from the hair are transferred to the comb.

Hence comb becomes negatively charged.

$\text{Step 2: Polarization of paper}$

The neutral paper constitutes of atomic dipoles arranged randomly, so that the net dipole moment of the paper is zero. $\text{[Refer Fig. 1]}$

When the negatively charged comb is brought near to the piece of neutral paper, it creates an Electric Field around the paper.

Then, as a property of dipoles(or by coulomb's force), the atomic dipoles tries to align themselves parallel to the electric field. $\text{[Refer Fig. 2]}$

Effectively, there is a slight shift in positive charges towards the comb, and negative charges away from the comb. $\text{[Refer Fig. 3]}$

$\text{Step 3: Force on the paper}$

Now since, the positive charge of paper is nearer to the comb as compared to the negative charge.

Hence the Coulombic force of attraction between comb and paper is greater than the force of repulsion.

Hence the paper is attracted towards the comb.