CAPF AC 2022 Q21: Science & Technology – Physics
- (a) pascal (Pa)
- (b) N/m²
- (c) J/m²
- (d) bar
Answer: (c) J/m²
Explanation
Units of pressure
How to think: pressure is force per unit area.
Force is in newtons and area in square metres.
So a pressure unit must reduce to N/m².
| Unit | What it measures |
|---|---|
| pascal (Pa) | Pressure; 1 Pa = 1 N/m² |
| N/m² | Pressure |
| bar | Pressure; 1 bar = 100,000 Pa |
| J/m² | Energy per unit area, as in surface tension or surface energy |
Check J/m²: 1 joule = 1 N m.
So J/m² = N m/m² = N/m, which is not N/m².
Other pressure units: atmosphere (atm), torr and millimetre of mercury.
The unit J/m³ (energy per volume) does equal the pascal.
Answer: (c) J/m².
Why each option is right or wrong
- (a) Incorrect. The pascal is the SI unit of pressure.
- (b) Incorrect. N/m² is the definition of the pascal.
- (c) CORRECT. J/m² equals N/m, a unit of surface tension, not pressure.
- (d) Incorrect. The bar is a pressure unit equal to 100,000 pascal.
Key point
Pressure = force ÷ area. Units: Pa = N/m², bar = 10⁵ Pa, 1 atm = 101,325 Pa = 760 mm of mercury. J/m² = N/m (surface tension). J/m³ = Pa.
Remember
Pressure: Pa, N/m², bar, atm. Not J/m².
Common mistakes
- Thinking bar is not a real unit because it is not SI.
- Confusing J/m² with J/m³, which does equal the pascal.
- Missing the word not and marking the pascal.
Exam tip
Educated guess: the joule is the unit of energy. A unit built on energy and area is unlikely to be a plain pressure unit, while pascal, N/m² and bar appear on every pressure gauge and weather report.
Sources
- NCERT Class IX Science — Gravitation (thrust and pressure)
- NCERT Class XI Physics — Mechanical Properties of Fluids (pressure, surface tension)