When two conductors touch, charge flows between them until both reach the same electric potential (V). For two identical spheres, this means the total charge splits equally, so each ends with the average: q_new = (q1 + q2)/2. Memory hook: "Same touch, same V" — contact does not equalise charge, it equalises potential; charge equalises only when the conductors are identical.
Two identical spheres, one with +q and one neutral, are touched. Charge flows from high to low potential until both reach the same V; because the spheres are identical, the total charge splits equally, leaving +q/2 on each.
Your doubts, answered
When two conductors touch, does the charge become equal or the potential become equal?
The potential becomes equal. Charge keeps flowing across the contact as long as one side is at a higher potential; it stops the instant both are at the same V. Charge only ends up equal as a special case — when the two conductors are identical in size and shape. For a small sphere touching a big sphere, the big one takes more charge even though both reach the same potential.
A sphere with charge q touches an identical uncharged sphere. What charge does each have after?
Each ends with q/2. The total charge q is conserved (it cannot vanish), and because the spheres are identical they must reach the same potential with the same charge. So q splits into q/2 + q/2. This is the single most tested rule in NEET charge-sharing questions.
Why does an uncharged conductor NOT stay uncharged after touching a charged one?
Touching creates a conducting path. Free electrons move until the potential is the same on both sides, so the neutral conductor always picks up some charge (of the same sign as the charged one). It only stays neutral if there is no real contact, such as in charging by induction where you keep them separate.
For two identical spheres with charges q1 and q2, what is the final charge on each?
Each becomes (q1 + q2)/2, the simple average. Watch the signs: if q1 = +8q and q2 = -2q, the total is +6q, so each ends with +3q. Do not average the magnitudes — average the signed charges, then split.
⚠️ The NEET trap ✗ Assuming a charged sphere and an uncharged sphere always end with equal charge, or averaging the magnitudes and ignoring the sign of the charges. ✓ Charge flows until V is equal. For identical spheres each gets the signed average (q1+q2)/2; for different-sized conductors charge splits in proportion to radius (Q ∝ r), not equally. 🧠 Contact equalises potential, not charge — equal charge only for identical conductors.
Real NEET questions
2025
Two identical charged conducting spheres A and B have their centres separated by a certain distance. Charge on each sphere is q and the force of repulsion between them is F. A third identical uncharged conducting sphere is brought in contact with A first, then with B, and finally removed. The new force of repulsion between A and B is:
A · A. F/2
B · B. 3F/8 ✓
C · C. 3F/5
D · D. 2F/3
Solution: Initially F = kq²/d². Sphere C (uncharged) touches A: q splits equally between two identical spheres, so A = q/2 and C = q/2. Then C (q/2) touches B (q): total = 3q/2 shared equally, so each = 3q/4, giving B = 3q/4. Final charges A = q/2, B = 3q/4. New force F' = k(q/2)(3q/4)/d² = (3/8)kq²/d² = 3F/8. Answer B.
2019
Two metal spheres, one of radius R and the other of radius 2R, have the same surface charge density σ. They are brought in contact and then separated. What are the new surface charge densities?
A · A. σ₁ = 5σ/6, σ₂ = 5σ/2
B · B. σ₁ = 5σ/2, σ₂ = 5σ/6
C · C. σ₁ = 5σ/2, σ₂ = 5σ/3
D · D. σ₁ = 5σ/3, σ₂ = 5σ/6 ✓
Solution: Initial charges: Q1 = σ·4πR², Q2 = σ·4π(2R)² = 4σ·4πR². Total Q = 5σ·4πR². On contact both reach a common potential V = kq/r, so charge splits in proportion to radius: Q1':Q2' = R:2R = 1:2. So Q1' = (5σ/3)·4πR², Q2' = (10σ/3)·4πR². New densities: σ₁ = Q1'/(4πR²) = 5σ/3, and σ₂ = Q2'/(4π(2R)²) = (10σ/3)/4 = 5σ/6. Answer D (note: because they are NOT identical, charge does not split equally — it splits by radius).
Solved Electric Charges And Fields NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. The total charge before contact equals the total after; the contact only redistributes it. That is why for two identical spheres each gets exactly half the total — nothing is created or lost.
What is the rule for two identical spheres touching?
Each ends with the signed average of the two charges: q_new = (q1 + q2)/2. If one is uncharged, the charged one loses half to the neutral one.
What decides the final charge for spheres of different sizes?
They reach a common potential V = kQ/r, so charge splits in proportion to radius: Q ∝ r. The bigger sphere takes more charge. Equal split is only the special case of equal radii.
Does touching make the potentials equal or the charges equal?
It makes the potentials equal. Charges become equal only when the conductors are identical; otherwise potentials are equal but charges differ.
