a) What are polar and non-polar molecules? (2)
b) Derive the expression for the capacitance of a parallel plate capacitor. (3)

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> [!TLDR]
> Polar molecules have a permanent dipole moment (centres of positive and negative charge do not coincide); non-polar molecules have none. For a parallel plate capacitor, $C = \dfrac{\varepsilon_0 A}{d}$.

**a) Polar and non-polar molecules**

- **Non-polar molecule:** A molecule in which the centre of positive charge coincides with the centre of negative charge, so it has **no permanent dipole moment** (e.g. $H_2$, $O_2$, $CO_2$).
- **Polar molecule:** A molecule in which the centres of positive and negative charge do **not** coincide, so it possesses a **permanent dipole moment** even in the absence of an external field (e.g. $H_2O$, $HCl$).

**b) Capacitance of a parallel plate capacitor**

Consider two large parallel plates each of area $A$, separated by a distance $d$, carrying charges $+Q$ and $-Q$. Let $\sigma = \dfrac{Q}{A}$ be the surface charge density.

The uniform electric field between the plates is …

Q.a) What are polar and non-polar molecules?

Concept understanding — Capacitance of a Parallel Plate Capacitor

Q.a) What are polar and non-polar molecules?

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