Case: Read the passage and answer the question.
Conductors contain mobile charge carriers. In metallic conductors, these charge carriers are electrons. In a metal, the outer (valence) electrons part away from their atoms and are free to move. These electrons are free within the metal but not free to leave the metal. The free electrons form a kind of ‘gas’; they collide with each other and with the ions and move randomly in different directions. In an external electric field, they drift against the direction of the field. The positive ions made up of the nuclei and the bound electrons remain held in their fixed positions. A dielectric (an insulator) behaves differently. Its charges are held to their atoms or molecules and cannot move freely through it. An external electric field can only shift these charges a little or turn the molecules. The dielectric is then said to be polarised.
Consider the following statements about dielectrics:
(A) In a polar dielectric, thermal energy tends to disturb the line-up of the permanent dipoles with the field.
(B) A water molecule is an example of a non-polar molecule.
(C) A dielectric develops a net dipole moment in an external field, whether its molecules are polar or non-polar.
(D) For polar molecules, the induced dipole effect is generally more important than the line-up of permanent dipoles.
Which of the statements given above are incorrect?
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B) and (D) only
A dielectric has no free charges. In an external field its molecules are stretched or turned, and the dielectric becomes polarised.
(A) In a polar dielectric, thermal energy tends to disturb the line-up of the permanent dipoles with the field. — True. The field tries to line up the dipoles, while thermal energy tends to break this line-up. How much the dielectric is polarised depends on which of the two is stronger.
(B) A water molecule is an example of a non-polar molecule. — False. In a water molecule the centres of positive and negative charge are separated. So water is a polar molecule, like HCl.
(C) A dielectric develops a net dipole moment in an external field, whether its molecules are polar or non-polar. — True. Non-polar molecules get induced dipoles, and polar molecules line up their permanent dipoles. Either way, the dielectric gets a net dipole moment along the field.
(D) For polar molecules, the induced dipole effect is generally more important than the line-up of permanent dipoles. — False. A polar molecule may also get an induced dipole. But the line-up of its permanent dipole is generally the bigger effect.
NCERT (2026-27):
Dielectrics and Polarisation – “… The extent of polarisation depends on the relative strength of two mutually opposite factors: the dipole potential energy in the external field tending to align the dipoles with the field and thermal energy tending to disrupt the alignment.”
Dielectrics and Polarisation – “An ionic molecule such as HCl or a molecule of water (H2O) are examples of polar molecules.”
Dielectrics and Polarisation – “Thus in either case, whether polar or non-polar, a dielectric develops a net dipole moment in the presence of an external field.”
Dielectrics and Polarisation – “There may be, in addition, the ‘induced dipole moment’ effect as for non-polar molecules, but generally the alignment effect is more important for polar molecules.”