Home › UPPCS › Physics › Gap Sheets › SI Units and Measurement
UPPCS Physics Gap Sheet: SI Units and Measurement.
UPPCS has asked 29 of these. 6 have not been asked yet. Lock the first list, then read the second: it holds what UPPCS has not used yet.
First, the concept
A physical quantity needs a number and a unit. SI has seven base units; derived units describe combinations of base quantities. A unit of energy cannot be used as a unit of power.
Some useful units are outside SI. Learn the quantity first, then its unit and any special use. A dimensionless ratio such as Mach number is different from a unit such as knot.
✅ Already asked (29)
Base units
- Length — metre · The SI base unit of length.
- Mass — kilogram · The SI base unit of mass; weight is a force.
- Time — second · The SI base unit of time.
- Current — ampere · The SI base unit of electric current.
- Temperature — kelvin · The SI base unit of thermodynamic temperature; no degree sign is used.
- Amount of substance — mole · One mole contains exactly 6.02214076 × 10²³ specified entities.
Derived units
- Force — newton · Weight is measured in newtons, not kilograms.
- Pressure — pascal · Pressure is force acting per unit area.
- Energy and work — joule · Heat, work and mechanical energy share this SI unit.
- Power — watt · Power tells how quickly energy is transferred.
- Frequency — hertz · One hertz means one cycle per second.
- Potential difference — volt · It measures energy transferred per unit charge.
- Capacitance — farad · Capacitance describes charge storage for a given potential difference.
- Radioactivity — becquerel · One becquerel is one nuclear decay per second; curie is a non-SI activity unit.
Special units and scales
- Light year and parsec · Both measure astronomical distance, not time.
- Electron volt · A unit of energy used for particles.
- Kilowatt-hour · An energy unit; one electricity billing unit is one kilowatt-hour.
- Knot and nautical mile · Knot measures speed; a nautical mile measures distance and equals 1852 metres.
- Mach number · The ratio of an object's speed to the local speed of sound; it has no unit.
- Decibel · A logarithmic sound-level measure; SI sound intensity is measured in watts per square metre.
- Angstrom and nanometre · One angstrom is 0.1 nanometre; both measure very small lengths.
- Earthquake magnitude and intensity · Richter describes magnitude; Mercalli intensity describes observed effects at a place.
More unit distinctions
- Inductance — henry · The SI unit of inductance; inductance resists changes in current.
- Luminous flux — lumen · Describes total visible-light output with weighting for human vision.
- Illuminance — lux · Describes luminous flux falling per unit surface area.
- Oil barrel · A standard petroleum barrel is about 159 litres.
- Cusec and cumec · Measure volume flow rate: cubic feet per second and cubic metres per second respectively.
- Femtometre · Also called a fermi; used for nuclear dimensions.
- Plane angle — radian · Radian is the SI derived unit of plane angle; steradian is used for solid angle.
❌ Not asked yet (6)
Items marked 'asked in other exams' have come in UPSC or other state exams, not yet in UPPCS.
🔒 6 items not asked yet
Base units: 1 · Derived units: 4 · More unit distinctions: 1
The UPPCS Pass opens every Repeat List and Gap Sheet, in English and Hindi.
Unlock the UPPCS Pass · ₹149Already bought? Enter your key
An All Access key works too.
⚠️ Kelvin measures temperature; joule measures heat energy. Decibel measures a sound level, not the SI sound intensity itself.
'Asked' and 'not asked' are checked against every UPPCS question on this site. 'Not asked yet' does not mean it will surely come.