Define the term potential energy of charge q at a distance r in an external electric field.

Explanation

**Step-by-Step Solution:**

1. **Definition of Potential Energy**:
The potential energy (U) of a charge \( q \) in an external electric field is defined as the work done in bringing the charge from a point at infinity to a specific point at a distance \( r \) from the source of the electric field.

2. **Understanding Work Done**:
The work done (W) in moving the charge \( q \) from infinity to a distance \( r \) in the electric field can be expressed mathematically. The work done against the electric field is given by:
\[
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