⚡ Overview

Everything around us carries electric charge—either positive or negative. When equal amounts of the two are together, their effects cancel. If a charged object is much smaller than the gap between it and other charges, we treat it as a point charge. :contentReference[oaicite:0]{index=0}

➕➖ Additivity of Charge

  • Charges add just like ordinary numbers (keep the sign!). For point charges \(q_1, q_2, …, q_n\), the total is \(q_\text{total}=q_1+q_2+…+q_n\). :contentReference[oaicite:1]{index=1}
  • Mass is always positive, but charge can be positive or negative—so signs matter. :contentReference[oaicite:2]{index=2}

🔄 Conservation of Charge

  • Charging by rubbing merely moves electrons; no new charge appears or vanishes. :contentReference[oaicite:3]{index=3}
  • Inside a closed system the algebraic sum of all charges stays constant—even when particles transform (e.g., neutron → proton + electron). :contentReference[oaicite:4]{index=4}

🔢 Quantisation of Charge

  • Every free charge is an integer multiple of the elementary charge: \(q = n\,e,\; n = 0, \pm1, \pm2, …\). :contentReference[oaicite:5]{index=5}
  • \(e = 1.602192 \times 10^{-19}\,\text{C}\). :contentReference[oaicite:6]{index=6}
  • Because \(e\) is tiny, everyday charges (mC or µC) feel continuous, but at atomic scales charge comes in discrete “lumps.” :contentReference[oaicite:7]{index=7}

📏 Units & Practical Scales

  • 1 coulomb is the charge that passes a point in 1 s when the current is 1 A.
  • Handy smaller units: \(1\text{ mC}=10^{-3}\text{ C}\), \(1\text{ µC}=10^{-6}\text{ C}\). :contentReference[oaicite:8]{index=8}

🧪 Worked Examples

Example 1 — Draining Electrons ⏳
If \(10^9\) electrons leave a body every second, only \(1.6\times10^{-10}\,\text{C s}^{-1}\) departs. Collecting \(1\,\text{C}\) would take about \(6.25\times10^{9}\,\text{s}\) (≈ 198 years!). :contentReference[oaicite:9]{index=9}

Example 2 — Charge in a Cup of Water ☕
A 250 g cup of water holds roughly \(1.34\times10^{7}\,\text{C}\) of positive charge and the same negative charge, so the net is still zero. :contentReference[oaicite:10]{index=10}

🎯 High-Yield Points for NEET

  1. Charges add algebraically—mind the signs.
  2. Total charge of an isolated system never changes (conservation).
  3. Quantisation: \(q = n\,e\) with \(e = 1.6\times10^{-19}\,\text{C}\).
  4. Useful units: C, mC, µC and their scale.
  5. Grainy nature of charge matters only at microscopic scales.