Illustrated Physics
physics, drawn
Concept

Electrostatic induction — where it appears

The rearrangement of charge on a conductor brought near a charged body, with opposite charge gathering on the near side and like charge on the far side. Because the near side is closer the result is an attraction, even to an uncharged conductor, and it can beat repulsion between like charges at close range.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Water drops that charge two cans to a spark. The voltage between the two cans of a Kelvin water dropper against time, on a logarithmic axis, starting from a stray difference of 1 V, for 10, 20, 40 drops a second in each stream; drops 3 mm across, each with a capacitance of 0.17 pF to its ring, cans of 20 pF. Each drop carries a charge induced by the opposite can's potential into its own can, which raises the opposite can's induced charge on the next drop: the difference grows exponentially, multiplying by e every 6.0 s at 20 drops a second. It passes the 10 kV that will jump a few millimetres of air after 55 s — from one volt to a spark, with nothing but falling water.

The water drops that charge themselves to a spark

Two streams of water drip from one earthed pipe through two metal rings into two metal cans, and each ring is wired to the opposite can. Nothing is charged to begin with, nothing is rubbed, nothing is plugged in. Within a minute a spark cracks between the cans at ten thousand volts. A stray volt between them makes the drops forming in one ring pick up a slight charge, which they carry to the can that makes the other ring's drops pick up the opposite charge — a loop in which every drop strengthens the imbalance that charged it. Kelvin built it in 1867; it is a dynamo made of water and gravity.

electromagnetism · Potential Two like charges that pull together when close. The force between two equal conducting spheres both carrying positive charge, against the gap between them, on a signed logarithmic scale, for charge ratios of 1, ½, ⅕ and 1/20. Equal charges repel at every gap. Unequal ones repel when far apart, as Coulomb's law says, and attract once the gap falls below a distance that grows as the charges become more unequal: 0.043 of a radius for a ratio of ½, 0.40 for ⅕, 1.56 for 1/20. The more strongly charged sphere induces a patch of the opposite sign on the near side of the weaker one, and close up that patch wins.

The like charges that attract at close range

Like charges repel: it is the first rule of electricity anyone learns, and for point charges it never fails. For two charged metal spheres it fails whenever they are close and their charges are unequal. The stronger sphere pulls electrons to the near side of the weaker one, the weaker sphere's own charge is pushed round to its far side, and below a certain gap the induced attraction beats the repulsion. Only one ratio of charges never attracts, and it is the ratio the spheres would share if they touched.

electromagnetism · Conductors

Named alongside it

The objects these essays reach for when they reach for this one.

CapacitanceCapacitance coefficientsConductorCoulombs lawDielectric breakdownElectric potentialExponential growthMethod of imagesPositive feedbackSurface chargeSymmetry breaking

All concepts