Sec 4 Physics Static Electricity notes

Press two different insulators together and rub: electrons leap from the surface that holds them loosely to the one that grips them tightly. The loser turns positive (short of electrons); the winner turns negative (electron surplus). A polythene rod rubbed with a dry duster grabs electrons and goes negative; an acetate strip loses electrons and goes positive — with the cloth always taking the opposite sign to the rod. Protons are locked in nuclei: in solid friction charging, only electrons ever move.

Charge is measured in coulombs (C). A single electron carries a tiny −1.6 × 10⁻¹⁹ C, so everyday charged objects carry around nanocoulombs — a deficit or surplus of trillions of electrons that is still a vanishingly small fraction of the atoms present.

Why does a rubbed balloon cling to an uncharged wall? The balloon’s charge induces a slight charge separation in the wall surface: opposite charge drifts nearest the balloon, like charge retreats. The nearer opposite charge is attracted more strongly than the farther like charge is repelled, so the net force is attraction. Charged rods bending thin water streams work the same way — water molecules are neutral but polarisable.

An electric field is a region in which a charge experiences a force. We draw it with field lines that show the direction of the force on a small positive test charge: lines point away from positive charge and into negative charge. Rules for drawing: lines never cross, they start and end on charge (or reach the edges of the diagram), and closer spacing means a stronger field.

Around an isolated positive point charge the lines radiate outward like a sea urchin; around an isolated negative charge they converge inward. For a pair of like charges side by side, the lines bend away from the midpoint, and halfway between them the two pushes cancel — a neutral point with no field. For a + and − pair, the lines run straight across from the positive to the negative, bulging outward at the sides: the classic dipole picture. Between two parallel oppositely charged plates the lines are parallel, evenly spaced and straight — a uniform field.

In a conductor (metals, graphite) outer electrons are free to drift through the material, so charge poured in at one point spreads over the whole surface, and charge can escape down a finger, a wire or the earth. In an insulator (plastic, glass, dry air) electrons are bound to their atoms: charge stays exactly where it was rubbed on. This is why you can charge a plastic rod held in the hand but not a metal one — the hand silently earths the metal.

The gold-leaf electroscope is a charge detector: a metal cap and stem hang inside a case beside (or above) a thin gold leaf. Charge placed on the cap spreads down the stem and onto the leaf; since leaf and stem now carry like charges, the leaf swings away from the stem. More charge = bigger deflection. Bring a charged rod near the cap without touching: charge redistributes (induction) and the leaf moves before any contact — the electroscope can even reveal the sign of nearby charge if the cap is pre-charged.

Bring a negatively charged rod near one end of an insulated conductor: free electrons are repelled to the far end, leaving the near end positive. The conductor is still neutral overall, but its charge has been separated. Now briefly earth the far end: electrons flee to the ground entirely. Break the earth, then remove the rod — the stranded conductor is left with a net positive charge, opposite to the rod. Touching first and removing the rod first are the classic order errors; the earth must be broken while the rod is still in place.

Two touching spheres work the same way: rod near sphere A repels electrons into sphere B; separate the spheres while the rod is present and you own a positive sphere A and a negative sphere B — charging by induction with two outputs. This is exactly how a charged comb lifts neutral paper, and why a charged rod deflects a tap water stream.

Unwanted charging sparks. Fuelling aircraft and tanker trucks: fuel rushing through hoses rubs charge loose; a stray spark at the nozzle ignites vapour, so the vehicle, hose and nozzle are bonded and earthed to let charge leak away harmlessly. Lightning is static on a grand scale: charge separated inside storm clouds induces opposite charge on the ground and on tall buildings; when the field is strong enough the air breaks down and conducts — lightning rods give the current a safe metal path to earth.

Industry also hires static. The electrostatic precipitator cleans factory smoke: high-voltage wires charge the ash and soot particles, which are then attracted to oppositely charged collecting plates and scraped away — cleaner chimneys with no filter to clog. The laser printer and photocopier: the drum is given a charged image of the page, charged toner powder sticks to the charged regions, then transfers and fuses onto paper. All of it is the same physics: charge a thing, and unlike charge comes running.