Which of the following are units of the pressure ? (A) bar (B) Pa (C) torr (D) atm Choose the correct option :

Correct — option (4), All (A), (B), (C) and (D).

Pressure is force per unit area. Its SI unit is the pascal (Pa), one newton per square metre, and other units remain in use beside it. Each of the four listed measures pressure.

(A) bar: 1 bar = 100 000 Pa (10⁵ Pa).
(B) Pa: the pascal itself, the SI unit.
(C) torr: 1 torr = 101 325/760 Pa, about 133.322 Pa.
(D) atm: the standard atmosphere, 1 atm = 101 325 Pa.

All four qualify, so the answer is All (A), (B), (C) and (D).

Exact conversions tie them together: 1 atm = 760 torr, and 1 atm = 1.01325 bar.

The idea to remember: bar, pascal, torr and atmosphere all measure pressure; the pascal is the SI unit, and 1 atm = 101 325 Pa = 760 torr.

Why the others are wrong

• (1)
  Only (B) and (D) — This keeps the pascal and the atmosphere but drops bar and torr, and both of those are pressure units too.

  1 bar is 100 000 Pa, and its thousandth, the millibar, equals one hectopascal. 1 torr is 1/760 of a standard atmosphere, about 133.3 Pa.

• (2)
  Only (B), (C) and (D) — This leaves out bar, which is also a pressure unit: 1 bar = 100 000 Pa exactly, a little less than one standard atmosphere (101 325 Pa).

  The millibar, one-thousandth of a bar, equals one hectopascal (100 Pa). In those units standard sea-level pressure is 1013.25 hPa.

• (3)
  Only (A), (B) and (D) — This leaves out torr, which is also a pressure unit. 1 torr is 1/760 of a standard atmosphere, about 133.3 Pa.

  It is named after Evangelista Torricelli, who invented the mercury barometer, and it is practically equal to the millimetre of mercury (mmHg) in which blood pressure is read.

Pressure is the force acting perpendicular to a surface divided by the area it acts on. The same force spread over a larger area gives a lower pressure.

The SI unit is the pascal: 1 Pa = 1 N/m². It is a small unit, so multiples such as the kilopascal (kPa) and hectopascal (hPa) are used.

Other units sit beside it. The bar is a round 10⁵ Pa; the standard atmosphere is defined as 101 325 Pa; and the torr is defined as 1/760 of an atmosphere, practically the pressure of a 1 mm column of mercury.

Key facts

• Pressure = force ÷ area; the SI unit is the pascal, and 1 Pa = 1 N/m².
• 1 bar = 100 000 Pa (10⁵ Pa); 1 millibar = 1 hectopascal = 100 Pa.
• 1 standard atmosphere (atm) = 101 325 Pa = 1.01325 bar = 760 torr.
• 1 torr = 101 325/760 Pa, about 133.322 Pa; the unit is named after Evangelista Torricelli.
• 1 mmHg is about 133.322 Pa, practically equal to 1 torr; blood pressure is read in mmHg.

Common traps

• Bar and atmosphere are close but not equal: 1 atm = 1.01325 bar. A statement that treats them as identical is wrong.
• Torr and mmHg are practically equal, both about 133.322 Pa, but they are defined differently; the torr is exactly 1/760 of an atmosphere.
• A unit with 'pascal' in its name can measure something else: the pascal-second (Pa·s) is the SI unit of dynamic viscosity, not pressure.

UnlockIAS will link similar questions from RAS Pre 2023 here once that paper is published on this site.

• practice — not a real PYQ

  Arrange the following units of pressure in increasing order of size : A. atm B. Pa C. torr D. bar Choose the correct option :

  • (a)B, C, D, A
  • (b)B, D, C, A
  • (c)C, B, D, A
  • (d)B, C, A, D
  Answer(1) — 1 Pa < 1 torr (≈133.3 Pa) < 1 bar (100 000 Pa) < 1 atm (101 325 Pa). Option (2) puts bar below torr, option (3) puts torr below Pa, and option (4) puts atm below bar.
• practice — not a real PYQ

  Which one of the following is NOT a unit of pressure ?

  • (a)mmHg
  • (b)hPa
  • (c)Pa·s
  • (d)N/m²
  Answer(3) — The pascal-second is the SI unit of dynamic viscosity. Option (1) mmHg, option (2) hectopascal and option (4) newton per square metre all measure pressure.