Conversion from Terapascals to Pascals

Information about the Terapascal (TPa)

The terapascal is a unit of pressure equal to a thousand gigapascals, written TPa. It is ten million bar. Nothing built by engineers operates at this pressure, and the unit belongs instead to two quite separate corners of science: the stiffness of the strongest materials known, and the interiors of large planets.

Carbon nanostructures put it on the map. A single-walled carbon nanotube has a Young's modulus close to one terapascal, and a sheet of graphene the same, which makes them the stiffest materials ever measured relative to their weight. Diamond, long the benchmark, comes in at 1.2 terapascals. Those three numbers are the reason the unit appears at all in materials science.

Stiffness at this level is not the same as strength. A nanotube resists stretching enormously, but a real fibre made of many of them fails at a far lower stress because the tubes slide past one another. Confusing a terapascal modulus with a terapascal breaking strength is one of the commonest errors in popular accounts of these materials.

Planetary interiors reach genuine terapascal pressures. The centre of Jupiter is estimated at three to four terapascals, and the cores of larger gas giants beyond that. Under such conditions hydrogen behaves as a metal, which is what generates the planet's magnetic field, so the unit describes a state of matter rather than a load on a structure.

Laboratories can now reach it briefly. Laser-driven shock compression and pulsed magnetic techniques drive samples into the terapascal range for nanoseconds at a time, long enough to record how a material's density and structure respond. Those experiments are the only direct evidence available about matter under the conditions inside giant planets.

For scale, one terapascal is ten million times atmospheric pressure and about a thousand times the pressure at the centre of the earth divided by three. The number stops being something a person can feel and becomes a description of what atoms do when they are pushed close enough together to change their chemistry.

One terapascal equals 1,000,000,000,000 pascals, 1000 gigapascals, 10,000,000 bar, or about 145 million pounds per square inch.

Information about the Pascal (Pa)

The pascal is the SI unit of pressure, written Pa. It is defined as one newton of force spread over one square metre, and it is named after Blaise Pascal, whose experiments with barometers in the 1640s established that air has weight and that its pressure falls with altitude. The General Conference on Weights and Measures adopted the name in 1971.

It is a remarkably small unit. Atmospheric pressure at sea level is 101,325 pascals, so the pressure everyone lives under is a six-figure number, and almost every practical use of the unit therefore carries a prefix. That awkwardness is not a design fault but a consequence of coherence: the newton and the square metre were fixed first, and the pascal is whatever falls out of dividing one by the other.

Acoustics is the field where bare pascals are natural. The reference pressure for the decibel scale is twenty micropascals, taken as the quietest sound a healthy young ear can detect, and ordinary conversation is around 0.02 pascals of sound pressure. The threshold of pain sits near 63 pascals, so the entire range of human hearing spans about six orders of magnitude in this unit.

Everyday objects give a sense of the scale. A sheet of office paper lying flat presses on the desk with about 0.8 pascals. A gentle breeze exerts a few pascals on a wall, and the pressure difference that drives ventilation through a building is typically between ten and fifty. Anything a person can feel as force is already in the thousands.

Prefixed forms carry the real work. Weather uses hectopascals, which are numerically identical to the older millibars, so a forecast reading of 1013 needed no relearning. Engineering uses kilopascals for tyre and fluid pressures, and materials science uses megapascals and gigapascals for strength and stiffness, where one megapascal is one newton per square millimetre.

The pascal also appears wherever a stress rather than a pressure is meant, since the two have the same dimensions. Young's modulus, yield strength and shear stress are all quoted in pascals or their multiples, which is why a single unit spans the pressure in a tyre and the stiffness of steel.

One pascal equals 1 newton per square metre, 0.01 millibars, about 0.0000099 atmospheres, or about 0.000145 pounds per square inch.