Electrostatics — Coulomb's Law, Capacitance and Why a Lightning Conductor Is Pointed

Charge That Sits Still

Rub a plastic comb through dry hair and it lifts bits of paper. Pull off a sweater and you hear a crackle. Step out of a car and a spark jumps to your finger. All three are the same physics: charge that has been separated and is now sitting still.

Charge at rest is electrostatics; charge in motion is current electricity. Every formula in the current chapter starts here.

Benjamin Franklin named the two kinds positive and negative. Like charges repel, unlike attract — and repulsion is the only sure test of charge, because a charged body attracts an uncharged one as well.

The SI unit is the coulomb (C). One electron carries 1.6 × 10⁻¹⁹ C, so 1 C is about 6.25 × 10¹⁸ electrons — an enormous quantity. Charge is quantised: Q = ne, always a whole-number multiple of e.

Rubbing never creates charge. It only moves electrons from one body to the other, so the two charges produced are always equal and opposite.

1. The distance between two point charges is doubled. The force between them becomes —

2. The SI unit of capacitance is the —

3. Excess charge given to a solid metal sphere resides —

4. Two capacitors of 2 μF and 3 μF are joined in PARALLEL. The equivalent capacitance is —

5. A lightning conductor is made pointed at the top because —

6. The charge on a body is measured as 4.8 × 10⁻¹⁹ C. The number of electrons it has gained is —

7. Two charges are moved from air into water, whose dielectric constant is 80. The force between them becomes —

8. Which observation proves with certainty that a body carries charge?