Illustrated Physics
physics, drawn
Concept

Transmission line — where it appears

A pair of conductors long enough that a signal takes appreciable time to travel along them, so voltage and current have to be treated as waves. A step moves at nearly the speed of light, with a ratio of voltage to current fixed by the geometry as the line's characteristic impedance.

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

The field round a thin wire. The electric field against distance from the axis, on a logarithmic axis, for a wire inside a coaxial cylinder of radius 10 cm at 60 kV, for wire radii of 0.2, 1 and 5 mm; the dashed line is 30 kV/cm, the field at which air breaks down across a gap of a few centimetres, and each curve starts at its wire's surface. The field falls as 1/ρ, so it is concentrated at a thin wire: 483 kV/cm at the surface of the 0.2 mm wire, 130 at the 1 mm wire and 40.1 at the 5 mm one. The field exceeds 30 kV/cm only within 3.2 mm of the axis for the thinnest wire, 4.3 mm for the middle one and 6.7 mm for the thickest; across the remaining nine centimetres the air is far below breakdown, so ionisation can start at the wire and has nowhere to go.

The glow a thin wire wears before it sparks

Raise the voltage on a thin wire inside a metal tube and, long before anything sparks across, the wire starts to glow faintly violet and to hiss. The air right next to it is ionising, and the air a millimetre further out is not. The field round a wire falls away so fast that an avalanche of ionisation can start at the surface and find nowhere to go. That partial breakdown, corona, costs power lines energy in the rain, sets the size of the wires they are made from, cleans the smoke from chimneys and counts particles in detectors — and the field it needs is not a property of air but of the wire's radius.

electromagnetism · Dielectrics The current climbs a stair, one round trip a step. Current through the lamp against time for rails with a light-second out to each joined end. It starts at V/(2Z₀+R) = 7.1 mA as soon as the fields have crossed the metre between battery and lamp, stays flat until the reflected fronts return two seconds later, and then rises by steps of about 14 mA at each return. It reaches half of the final 400 mA after about 38 seconds. A time-stepped line gives the same stair as the algebra.

The lamp that lights before the loop is round

Put a battery and a lamp a metre apart, and connect them with wires that run a light-second out and a light-second back. Close the switch. The lamp begins to carry a current after 3.3 nanoseconds, not two seconds. It is a small current, set by the impedance of the wires, and it climbs to the full value in steps. What reaches the lamp first is a field that never went round.

electromagnetism · Retardation

Named alongside it

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

Characteristic impedanceCoaxial lineCorona dischargeDielectric breakdownElectric fieldElectrostatic precipitatorIonisationPoynting vectorRetardationSpeed of lightTownsend avalanche

All concepts