Show that the energy of an ideal dipole p in an external electric field E is U = -p E Start by calculating the energy of a physical dipole of charges ±q separated by a displacement a, with p = qa. Then take the limit a → 0, q + o with qa constant (equal to p).

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**Energy of an Ideal Dipole in an External Electric Field**

The energy \( U \) of an ideal dipole \( \mathbf{p} \) in an external electric field \( \mathbf{E} \) is given by:

\[ U = -\mathbf{p} \cdot \mathbf{E} \]

**Derivation:**

1. Begin by calculating the energy of a physical dipole consisting of charges \( \pm q \) that are separated by a displacement \( \mathbf{a} \). The dipole moment is defined as \( \mathbf{p} = q\mathbf{a} \).

2. Take the limit as \( |\mathbf{a}| \to 0 \), \( q \to \infty \), while maintaining the product \( q\mathbf{a} \) constant (equal to \( \mathbf{p} \)).

This derivation leads to the expression for the energy of an ideal dipole in an electric field.
Transcribed Image Text:**Energy of an Ideal Dipole in an External Electric Field** The energy \( U \) of an ideal dipole \( \mathbf{p} \) in an external electric field \( \mathbf{E} \) is given by: \[ U = -\mathbf{p} \cdot \mathbf{E} \] **Derivation:** 1. Begin by calculating the energy of a physical dipole consisting of charges \( \pm q \) that are separated by a displacement \( \mathbf{a} \). The dipole moment is defined as \( \mathbf{p} = q\mathbf{a} \). 2. Take the limit as \( |\mathbf{a}| \to 0 \), \( q \to \infty \), while maintaining the product \( q\mathbf{a} \) constant (equal to \( \mathbf{p} \)). This derivation leads to the expression for the energy of an ideal dipole in an electric field.
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