Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): Net dipole moment of a polar linear isotropic dielectric substance is not zero even in the absence of an external electric field. Reason (R): In absence of an external electric field, the different permanent dipoles of a polar dielectric substance are oriented in random directions. In the light of the above statements, choose the most appropriate answer from the options given below:

### Related Formula vecP_textnet = sum vecp_i$$\vec{P}_{\text{net}} = \sum \vec{p}_i$$ where: vecP_textnet$\vec{P}_{\text{net}}$ = net dipole moment of the dielectric vecp_i$\vec{p}_i$ = dipole moment of the individual i$i$-th molecule ### Core Logic No external electric field is present (E_textext = 0$E_{\text{ext}} = 0$). Due to thermal agitation, all molecular permanent dipoles are randomly oriented in space: vecP_textnet = 0 quad textwhen vecE_textext = 0$$\vec{P}_{\text{net}} = 0 \quad \text{when } \vec{E}_{\text{ext}} = 0$$ Thus: 1. Assertion (A) is false because it claims the net dipole moment is non-zero even without an external field. 2. Reason (R) is true because it correctly describes that different permanent dipoles are randomly oriented. ### Step 1: Final Conclusion Therefore, (A) is not correct but (R) is correct. ### Pattern Recognition Sees: "polar dielectric" + "no external field" → net bulk dipole moment is always zero. Trap: Confusing the molecular level with the macroscopic level. Each molecule in a polar dielectric has a permanent dipole moment, but the macro substance has zero net moment due to random thermal orientations. Shortcut: No external field means vectors cancel globally, which implies zero net moment. Thus (A) is false immediately. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 12 Physics: Electrostatics