Self-Force, Tension, and Electrostatic Pressure
Calculate force on an element from the field produced by all other charge, excluding its singular self-field. For conductor surfaces, electrostatic pressure is sigma^2/(2 epsilon_0).
Why this shows up in the exam
Tension in a charged ring · Force holding hemispheres together · Stress on a charged conductor
Learn the idea
A charged extended body can be held together only by balancing the repulsion of its own charge distribution. Every small element is pushed by the rest of the charged body. A cut or virtual displacement turns that distributed repulsion into a tension or holding force.
🧠 Memory hook: Use the field of the rest, never an element's own field.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- dF = dq E_other — force on an element due to the rest of the distribution
- P_e = sigma²/(2 epsilon₀) — outward pressure on a conductor surface in vacuum
How to approach it
- 1Choose a symmetric element or cut
- 2Find E due to the remaining charge
- 3Integrate the required component or pressure
Common slip-ups that cost marks
- •Including the singular self-field
- •Using total field instead of the other-charge field
- •Confusing pressure with total force
🌟 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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