Q.A molecule of a substance has a permanent electric dipole moment of magnitude $10^{-29}\ \text{C m}$. A mole of this substance is polarised (at low temperature) by applying a strong electrostatic field of magnitude $10^{6}\ \text{V m}^{-1}$. The direction of the field is suddenly changed by an angle of $60^\circ$. Estimate the heat released by the substance in aligning its dipoles along the new direction of the field. For simplicity, assume 100% polarisation of the sample.

Dipole Alignment Energy — From Intuition to Formula

Imagine you have a tiny bar magnet — a compass needle. You know it always turns to point north. But what if you try to hold it pointing east? You feel a torque, a twisting force that wants to rotate it back. If you let go, it snaps to align with the field.

That "snap" releases energy. The energy that was stored in the misaligned configuration is called dipole alignment energy (or potential energy of a dipole in an external field).

The Core Intuition

A dipole (like a compass needle or a polar molecule) has two opposite "poles" — a north and a south, or a positive and a negative charge. When placed in an external field:

• Aligned (parallel to the field): the dipole is in its lowest energy state — like a ball at the bottom of a valley.
• Anti-aligned (opposite to the field): the dipole is in its highest energy state — like a ball balanced at the top of a hill.
• Perpendicular: the energy is somewhere in between.

The energy depends on how much the dipole is twisted away from the field direction. The more you force it to point against the field, the more energy you store — like winding a spring.

The Precise Statement

For an electric dipole with dipole moment $\vec{p}$ placed in a uniform external electric field $\vec{E}$, the potential energy of alignment is:

where $\theta$ is the angle between $\vec{p}$ and $\vec{E}$.

For a magnetic dipole (like a current loop or a compass needle) with magnetic moment $\vec{\mu}$ in a magnetic field $\vec{B}$:

Why the Negative Sign?

This is the part that confuses most students. Let's break it down.

When $\theta = 0^\circ$ (aligned), $\cos\theta = 1$, so $U = -pE$. This is the minimum energy — the most stable configuration.

When $\theta = 180^\circ$ (anti-aligned), $\cos\theta = -1$, so $U = +pE$. This is the maximum energy — the least stable.

The negative sign is a convention that makes the aligned state the lowest energy. Think of it this way: the field does positive work to rotate the dipole from anti-aligned to aligned, so the dipole loses potential energy. The formula captures that loss as a negative value relative to the zero-energy reference (which is usually taken at $\theta = 90^\circ$, where $U = 0$).

What It Physically Means

The alignment energy tells you:

1. How much work an external agent must do to rotate the dipole from aligned to some angle $\theta$.
2. How stable the dipole is in a given orientation — the deeper the energy well (larger $p$ or $E$), the harder to knock it out of alignment.
3. The torque on the dipole: $\tau = -\frac{dU}{d\theta} = -pE\sin\theta$, which matches the familiar $\vec{\tau} = \vec{p} \times \vec{E}$.

A Quick Example

A water molecule has a permanent electric dipole moment $p = 6.2 \times 10^{-30} \text{ C·m}$. In an electric field of $10^6 \text{ N/C}$ (a strong laboratory field): …