Definition
A physical quantity can be measured — e.g. length, mass, time.
Base quantities are the fundamental ones the SI system is built on: length (metre, m), mass (kilogram, kg), time (second, s), electric current (ampere, A), temperature (kelvin, K), amount of substance (mole, mol), luminous intensity (candela, cd).
Derived quantities are built from base quantities by multiplication/division — e.g. speed , density .
Example 1 — base vs derived
Is density a base or derived quantity? Why?
- Density is calculated from mass and volume, both already-defined quantities — it isn't independently fundamental.
Answer: Derived (mass volume)
Rule
Common prefixes: kilo (), centi (), milli (), micro (), nano ().
Example 2 — unit conversion
Convert kg into grams.
- kg g g.
- .
Answer: g
Example 3 — density calculation
Find the density of mercury: mass g, volume cm.
- Density .
Answer: g/cm
Definition
Least count is the smallest measurement an instrument can read accurately.
Vernier calipers (least count cm) measure small lengths, internal/external diameters and depths.
A micrometer screw gauge (least count mm) measures very small thicknesses, like a sheet of paper or a wire's diameter.
Remember
The standard metre rod is made of platinum-iridium alloy because it barely expands or corrodes, keeping the standard accurate over time.
No measurement is perfectly exact. Random errors scatter unpredictably around the true value (reduced by repeating and averaging); systematic errors shift every reading the same way (fixed by correcting or recalibrating the instrument).
Quick Recap
- ●Base quantities: length, mass, time, current, temperature, amount of substance, luminous intensity
- ●Derived quantities are combinations of base quantities, e.g. density mass volume
- ●Least count = the smallest reading an instrument can measure; Vernier calipers cm, screw gauge mm
- ●Random error: scatters, reduced by averaging repeated readings. Systematic error: shifts every reading the same way
