JEE Main 2022 · 26 June · morning · Single correct · +4 / −1
Given below two statements : One is labelled as Assertion (A) and other is labelled as Reason (R).
Assertion (A) : Non-polar materials do not have any permanent dipole moment.
Reason (R) : When a non-polar material is placed in an electric field, the centre of the positive charge distribution of it's individual atom or molecule coincides with the centre of the negative charge distribution.
In the light of above statements, choose the most appropriate answer from the options given below.
- (a) Both (A) and (R) are correct and (R) is the correct explanation of (A).
- (b) Both (A) and (R) are correct and (R) is not the correct explanation of (A).
- (c) (A) is correct but (R) is not correct.
- (d) (A) is not correct but (R) is correct.
Answer and solution
Answer (c) (A) is correct but (R) is not correct.
Solution
Physics sir · 4 steps
Watch when the centres coincide: only without a field. Switch the field on and they pull apart.
- Check the Assertion. In a non-polar molecule such as O₂ or CO₂, the centre of positive charge sits exactly on the centre of negative charge when no field acts. So there is no permanent dipole moment, and (A) is correct.
- Check the Reason. Once the material is placed in an electric field E⃗, the field pushes the positive charge one way and the electron cloud the other way. The two centres separate, so they no longer coincide.
p⃗_(induced) = α E⃗
- Induced dipole. This separation creates an induced dipole moment along the field, with α the polarisability. That is how a non-polar dielectric gets polarised. (R) says the centres coincide inside the field, which is wrong.
- So. (A) is correct but (R) is not correct, which is option (c).
The idea to keep. Non-polar means zero dipole moment with no field present. Put it in a field and the charge centres shift, giving an induced dipole. That is why non-polar dielectrics still polarise.
Answer from the NTA final answer key. Solution by PrepWiser.