Dipole Potential Energy
In a uniform field, U = -p dot E = -pE cos theta, up to the conventional zero at 90 degrees. Quasistatic external work equals Delta U.
Why this shows up in the exam
Stable-orientation questions · Work required to rotate a dipole · Energy-to-speed conversions
Learn the idea
A dipole has minimum energy parallel to the field and maximum energy antiparallel. The field favours alignment. Rotating away from alignment requires external work, while the field releases energy when the dipole turns toward alignment.
🧠 Memory hook: Parallel is the valley; antiparallel is the hill.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- U = -p E cos theta — dipole potential energy in a uniform field
- W_ext = Delta U — slow rotation with no kinetic-energy change
How to approach it
- 1Mark initial and final angles
- 2Compute U at both orientations
- 3Use the correct sign for field or external work
Common slip-ups that cost marks
- •Using +pE cos theta
- •Calling antiparallel stable
- •Equating field work with Delta U instead of -Delta U
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
Two point charges 1 microC and 2 microC are 1 m apart in vacuum. Take k = 9 x 10^9 SI. Find the force magnitude.
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