Dielectrics & Polarisation ⚡

1. What are dielectrics?

Dielectrics are non-conducting materials with practically no free charge carriers. When an external electric field is applied, they cannot neutralise the field the way conductors do. Instead, the field stretches or re-orients their molecules so that surface charges are induced, creating an opposing field that reduces (but does not cancel) the original field.:contentReference[oaicite:0]{index=0}

2. Molecular picture 🔬

  • Non-polar molecules: Centres of positive and negative charge coincide, so there is no permanent dipole moment. Examples: O2, H2.:contentReference[oaicite:1]{index=1}
  • Polar molecules: Centres of positive and negative charge are separated even with no field, giving a permanent dipole moment. Examples: HCl, H2O.:contentReference[oaicite:2]{index=2}

3. How an external field acts 🌐

• Non-polar molecules: The field displaces charges in opposite directions until internal restoring forces balance the push, producing an induced dipole moment.:contentReference[oaicite:3]{index=3}
• Polar molecules: Randomly oriented dipoles tend to align with the field; thermal agitation tries to randomise them, so the final alignment depends on the tug-of-war between field energy and thermal energy.:contentReference[oaicite:4]{index=4}

4. Polarisation vector \( \mathbf{P} \) 📏

The polarisation of a dielectric is the dipole moment per unit volume. For a linear isotropic dielectric (common in exams!), it obeys

\( \mathbf{P} \;=\; \varepsilon_0 \, \chi_e \, \mathbf{E} \)   …(2.37)

Here \( \varepsilon_0 \) is the permittivity of free space, \( \chi_e \) is the electric susceptibility (a material constant), and \( \mathbf{E} \) is the applied field.:contentReference[oaicite:5]{index=5}

5. Induced surface charge & field reduction 🧲

Uniform polarisation makes the dielectric behave like two oppositely charged surfaces with surface charge densities \( +\sigma_p \) and \( -\sigma_p \). These bound charges set up a field opposing the applied field, so the net field inside the dielectric is smaller than it would be without the dielectric.

High-Yield NEET Pointers 🎯

  • Know the distinction between polar and non-polar molecules and how each gets polarised.
  • Remember the polarisation formula \( \mathbf{P} = \varepsilon_0 \chi_e \mathbf{E} \) for a linear isotropic dielectric.
  • The idea of induced surface charge density \( \sigma_p \) explains why a dielectric weakens (but does not nullify) an external field.
  • Bound vs free charges: in dielectrics, induced charges are bound, not free to move like in conductors.