Charge Sharing by Contact
Charge is conserved during contact. Identical spheres at large separation acquire q_f = (q_1 + q_2)/2; unequal conductors require equal-potential rather than equal-charge reasoning.
Why this shows up in the exam
Touch-and-separate sphere problems · Neutralization by contact · Force before and after contact
Learn the idea
Connected identical conductors share total charge equally after electrostatic equilibrium. When conductors touch, charge moves until their potentials match. Identical isolated spheres therefore finish with equal charges, not necessarily equal to either starting charge.
🧠 Memory hook: Add first, split second, then calculate the force.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- q_f = (q₁ + q₂)/2 — final charge on each identical sphere
- F = k q_f²/r² — later force after identical spheres separate
How to approach it
- 1Conserve signed total charge
- 2Impose equal potential
- 3Use the final charges in Coulomb's law
Common slip-ups that cost marks
- •Averaging charge magnitudes instead of signed charges
- •Assuming unequal conductors get equal charge
- •Using the original charges after contact
🌟 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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