Physics
🔒 Log in to trackSI units, conversions and measuring instruments
🔒 Log in to trackThe 7 SI base quantities
| Quantity | SI unit | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Electric current | ampere | A |
| Thermodynamic temperature | kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
Derived units asked again and again
| Quantity | Unit | Quantity | Unit |
|---|---|---|---|
| Force | newton (N) | Electric charge | coulomb (C) |
| Work / energy / heat | joule (J) | Potential difference | volt (V) |
| Power | watt (W) | Resistance | ohm (Ω) |
| Pressure | pascal (Pa) | Capacitance | farad (F) |
| Frequency | hertz (Hz) | Inductance | henry (H) |
| Magnetic field | tesla (T) | Magnetic flux | weber (Wb) |
| Power of a lens | dioptre (D) | Radioactivity | becquerel (Bq) |
| Luminous flux | lumen | Illuminance | lux |
| Sound level | decibel (dB) | Speed of ships | knot (nautical mile per hour) |
Special units and conversions
| Unit | Measures | Value |
|---|---|---|
| Light year | Distance (not time) | about 9.46 × 10¹⁵ m |
| Parsec | Distance | about 3.26 light years |
| Astronomical unit | Distance | Mean Earth–Sun distance, about 1.5 × 10¹¹ m |
| Ångström | Length (wavelength) | 10⁻¹⁰ m |
| Fermi | Nuclear size | 10⁻¹⁵ m |
| Nautical mile | Distance at sea | 1,852 m |
| Horsepower | Power | about 746 W |
| Kilowatt-hour ('unit') | Electrical energy | 3.6 × 10⁶ J |
| Calorie | Heat | about 4.18 J |
| Atmosphere | Pressure | about 1.013 × 10⁵ Pa |
Instruments
| Instrument | Measures / use |
|---|---|
| Barometer | Atmospheric pressure (Torricelli) |
| Hygrometer | Relative humidity |
| Hydrometer | Relative density of liquids |
| Lactometer | Purity (density) of milk |
| Anemometer | Wind speed |
| Altimeter | Altitude (aircraft) |
| Ammeter / Voltmeter | Current (in series) / potential difference (in parallel) |
| Galvanometer | Detects small currents |
| Seismograph | Earthquake waves |
| Sphygmomanometer | Blood pressure |
| Pyrometer | Very high temperatures (furnaces) |
| Fathometer | Ocean depth (echo sounding) |
| Odometer / Speedometer | Distance covered / speed of a vehicle |
| Tachometer | Rotation speed (rpm) |
| Manometer | Pressure of gases |
| Audiometer | Hearing |
Detailed notes
Why units matter
Every measurement has two parts: a number and a unit. "The rod is 5" means nothing; "the rod is 5 metres" is a measurement. To let scientists everywhere compare results, the world uses one agreed system — the SI system (Système International, adopted in 1960). Competitive exams ask more questions on units and instruments than on any other physics area, because they are pure recall.
The seven base quantities
Base (fundamental) quantities do not depend on any other quantity. SI has exactly seven:
| Quantity | SI unit | Symbol |
|---|---|---|
| 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 |
Note: the unit of temperature is kelvin, not "degree kelvin", and not degree Celsius.
Derived units named after scientists
Every other quantity is derived by multiplying or dividing base quantities. Many derived units carry a scientist's name:
- Force — newton (N) = kg·m/s². Work / energy / heat — joule (J). Power — watt (W) = J/s.
- Pressure — pascal (Pa) = N/m². Frequency — hertz (Hz) = 1/s.
- Charge — coulomb (C). Potential difference — volt (V). Resistance — ohm (Ω). Capacitance — farad (F).
- Magnetic flux — weber (Wb). Magnetic field — tesla (T). Inductance — henry (H).
- Radioactivity — becquerel (Bq). Illuminance — lux. Power of a lens — dioptre (D) = 1/metre. Some derived quantities have no special name: momentum is kg·m/s, velocity is m/s, acceleration is m/s², density is kg/m³.
Practical (non-SI) units — the trap list
- Light year = distance light covers in one year ≈ 9.46 × 10¹⁵ m. It measures distance, not time.
- Parsec ≈ 3.26 light years (also distance). Astronomical unit (AU) = mean Earth–Sun distance ≈ 1.5 × 10¹¹ m.
- Angstrom = 10⁻¹⁰ m (wavelengths); fermi = 10⁻¹⁵ m (nuclear sizes).
- Nautical mile = 1852 m; knot = one nautical mile per hour (speed of ships and aircraft).
- Fathom = 6 feet, used for depth of water. Carat = 200 mg for gems; for gold, "carat/karat" means purity (24 carat = pure).
- Horsepower ≈ 746 W. Calorie ≈ 4.186 J. Electron volt ≈ 1.6 × 10⁻¹⁹ J.
- Kilowatt-hour (kWh) = 1 "unit" on your electricity bill = 1000 W × 3600 s = 3.6 × 10⁶ J. It is a unit of energy, not power.
- Bar = 10⁵ Pa; 1 atmosphere ≈ 1.013 × 10⁵ Pa. Decibel (dB) measures loudness level.
Scalars and vectors
A scalar has only size (magnitude): mass, speed, distance, time, work, energy, power, temperature. A vector has size and direction: displacement, velocity, acceleration, force, momentum, weight. Easy test: if "towards the north" makes sense with it, it is a vector.
Quick conversions
- km/h to m/s: multiply by 5/18. So 36 km/h = 10 m/s. m/s to km/h: multiply by 18/5.
- Energy bill: units (kWh) = watts × hours ÷ 1000. A 100 W bulb for 10 hours uses 1 kWh.
Instruments — what they measure
| Instrument | Measures |
|---|---|
| Ammeter / Voltmeter | Current / potential difference |
| Galvanometer | Detects small currents |
| Barometer | Atmospheric pressure |
| Manometer | Pressure of gases |
| Hygrometer | Relative humidity |
| Hydrometer | Density of liquids |
| Lactometer | Purity of milk (its density) |
| Anemometer | Wind speed |
| Altimeter | Altitude (aircraft) |
| Seismograph | Earthquake waves |
| Sphygmomanometer | Blood pressure |
| Pyrometer | Very high temperature |
| Tachometer | Rotational speed (rpm) |
| Odometer / Speedometer | Distance run / speed of a vehicle |
| Fathometer / SONAR | Depth of sea |
| Crescograph | Growth of plants |
Quick revision
- 7 base units: m, kg, s, A, K, mol, cd.
- Light year, parsec, AU = distance. kWh = energy (3.6 × 10⁶ J). Horsepower ≈ 746 W.
- Nautical mile = 1852 m; knot = nautical mile per hour.
- km/h × 5/18 = m/s.
- Hygro = humidity, hydro = liquid density, lacto = milk, anemo = wind, sphygmo = blood pressure.
Types of questions asked
Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.
Type 1: SI unit of a physical quantityvery common3 practice Q
'The SI unit of pressure / charge / magnetic flux / power is …', or 'which pair of quantity and unit is wrongly matched'.
- Decide first whether the quantity is a base quantity (7 units) or a derived one.
- For derived quantities, recall the scientist-named unit: N, J, W, Pa, Hz, C, V, Ω, F, Wb, T, H.
- If there is no named unit (momentum, velocity), build it: momentum = kg·m/s. Why: every unit is fixed by an international agreement, so recall is the only method — and elimination by dimensions helps.
Example: The SI unit of magnetic field (magnetic flux density) is —
Tesla (T). Weber is the unit of magnetic flux; tesla = weber per square metre.
Type 2: Instrument → what it measuresvery common4 practice Q
An instrument name ending in '-meter' or '-graph' is given and its use is asked, or the reverse.
- Break the name into its root: hygro (moisture), hydro (water/liquid), anemo (wind), baro (weight of air = pressure), seismo (shaking), sphygmo (pulse).
- Pick the option that fits the root.
- Watch the pairs hygrometer vs hydrometer, manometer vs sphygmomanometer. Why: most names are Greek roots that describe the job.
Example: An altimeter is used to measure —
Altitude (height above sea level), mainly in aircraft. It works like a barometer: pressure falls as height rises.
Type 3: Practical and astronomical unitscommon3 practice Q
Units like light year, parsec, AU, knot, nautical mile, fathom, horsepower, carat or kWh — 'is a unit of …' or 'equals …'.
- Classify the unit by quantity: light year/parsec/AU/fathom/nautical mile = distance; knot = speed; kWh/calorie/eV = energy; horsepower = power.
- Recall the headline value (nautical mile 1852 m, 1 hp ≈ 746 W, 1 kWh = 3.6 × 10⁶ J).
- Beware the word 'year' and 'hour' in a unit name — they do not make it a unit of time.
Example: A parsec is a unit of —
Distance (astronomical). 1 parsec ≈ 3.26 light years.
Type 4: Base vs derived units; scalar vs vectorcommon2 practice Q
'Which is NOT a fundamental unit', 'which is a vector quantity', 'which pair are both scalars'.
- Base units: metre, kilogram, second, ampere, kelvin, mole, candela — everything else is derived.
- Vector check: does direction change the meaning? Velocity, force, momentum, displacement, acceleration = vectors.
- Speed, distance, mass, work, energy, power, time = scalars.
Example: Which of the following is a scalar quantity? (a) Force (b) Momentum (c) Work (d) Displacement
(c) Work. Work has only magnitude; force, momentum and displacement all need a direction.
Type 5: Unit-conversion and energy-bill numericalscommon2 practice Q
A speed in km/h to convert, or appliance watts × hours to turn into kWh or ₹.
- For speed: multiply km/h by 5/18 to get m/s (divide by 3.6).
- For bills: watts × hours per day × days ÷ 1000 = units; multiply by the rate.
- Check the size: a 1 kW appliance for 1 hour = exactly 1 unit. Why: 1 km = 1000 m and 1 h = 3600 s, so 1000/3600 = 5/18.
Example: Convert 54 km/h to m/s.
54 × 5/18 = 15 m/s.
Formulas
1 'unit' of electricity
a unit of distance
Shortcut tricks
⚡ Base-unit roll call: 'Mighty Kings Seldom Answer Kind Monks Correctly'
Metre, Kilogram, Second, Ampere, Kelvin, Mole, Candela — the 7 SI base units. Anything else (newton, joule, volt, watt…) is derived.
Example: Which of these is NOT an SI base unit: kelvin, candela, newton, mole?
Newton — it is derived (kg m s⁻²).
⚡ 'Meter' by what it measures
Hygro = humidity (hygiene/water), Hydro = liquid density, Lacto = milk, Anemo = wind (Greek anemos), Sphygmo = pulse/BP, Pyro = fire/high temperature, Seismo = earthquake, Alti = altitude.
Where students lose marks
Treating the light year as a unit of time — it is a distance.
Using kWh as a unit of power — it is a unit of energy.
Confusing hygrometer (humidity) with hydrometer (liquid density).
Practice sets — 18 questions
Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.
Topic test · 10 questions
Suggested time 3 min · wrong answers go to your mistake notebook automatically.