Electrostatic Field Energy
In a linear isotropic medium, electrostatic energy density is one half E dot D; in vacuum D=epsilon_0 E. Integrate over the field region for total energy when appropriate.
Why this shows up in the exam
energy-density comparisons · field energy between plates · connecting particle and field descriptions
Learn the idea
Electrostatic energy is stored throughout the electric field. A charged system does not keep energy only on its conductors; the surrounding field occupies space and carries an energy density proportional to E squared.
🧠 Memory hook: Energy density follows E squared, so its sign is never negative.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- u = (1/2) E dot D — linear electrostatic medium
- u = (1/2) epsilon₀ E² — vacuum
- U = integral u dTau — field description over all relevant space
How to approach it
- 1Identify the medium
- 2Find E region by region
- 3Integrate u over the correct volume
Common slip-ups that cost marks
- •Using u=epsilon E squared without one half
- •Treating vector E squared as signed
- •Integrating only where charge is located
🌟 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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