Physical quantities are divided into two groups: base quantities and derived quantities. These are measured through various methods, and each has its own unit. Since these units were determined through international agreement, they’re called international units or SI units.
Base Quantities
Base quantities can be measured directly and expressed on their own. This set of quantities, which forms the foundation for defining all other physical quantities, consists of seven quantities in total.
| Quantity | Symbol | Unit | Measuring instrument |
|---|---|---|---|
| Mass | m | kilogram (kg) | Equal-arm balance |
| Luminous intensity | I | Candela (cd) | Photometer |
| Temperature | T | Kelvin (K) | Thermometer |
| Electric current | i | ampere (A) | Ammeter |
| Amount of substance | n | mole (mol) | Calculated |
| Length | l | meter (m) | Tape measure |
| Time | t | second (s) | Stopwatch |
If you look at the initial letters of these quantities from top to bottom in the original Turkish list (Kütle, Işık şiddeti, Sıcaklık, Akım şiddeti, Madde miktarı, Uzunluk, Zaman), you’ll notice they spell out a Turkish memory acrostic, “KISAMUZ.”
Explanation: An acrostic is when the first letters of the lines of a poem, read from top to bottom, spell out a meaningful word.
Derived Quantities
Derived quantities are expressed by relating base quantities to one another, or by relating other derived quantities to one another.
| Quantity | Symbol | Unit |
|---|---|---|
| Area | A | (m2) |
| Volume | V | (m3) |
| Density | d | (kg/m3) |
| Speed | v | (m/s) |
| Acceleration | a | (m/s2) |
| Force | F | Newton (N) |
| Work / Energy / Heat | W / E / Q | Joule (J) |
| Power | P | Watt (W) |
| Electric charge | q | Coulomb (C) |
| Quantity | Symbol | Unit |
|---|---|---|
| Electric field | E | (N/C) |
| Electric potential | V | Volt (V) |
| Resistance | R | Ohm (Ω) |
| Capacitance | C | Farad (F) |
| Momentum | p | (kg·m/s) |
| Torque | τ | (N·m) |
| Pressure | P | Pascal or (N/m2) |
| Magnetic field | B | Tesla (T) |
Number of SI Base Quantities
The International System of Units (SI) defines a total of 7 base quantities, such as length, mass, and time.
Example of a Derived Quantity
Quantities such as speed, acceleration, and force are derived through mathematical operations on the base quantities.
Unit Consistency
Using units consistently in physics calculations makes the distinction between base and derived quantities important for the accuracy of results.
Difference Between a Base Quantity and a Derived Quantity
The difference between a base quantity and a derived quantity: a base quantity can be expressed on its own. A derived quantity, on the other hand, is found by relating base quantities or other derived quantities to one another (through multiplication, division, etc).
If a quantity is found by relating two quantities to each other, it’s derived.
For example: Density = Mass/Volume (kg/m3)
Another example: Speed = Distance/Time (m/s)
As another example, Weight is also a derived quantity. Weight = Mass x Gravitational acceleration
Base Quantities and Derived Quantities Mini Test
Related Questions
All other quantities in physics (such as speed, force, pressure) can be obtained by relating or multiplying these seven base quantities in various ways. This means a small number of base units is enough to define an infinite number of derived quantities.
The mole represents the number of particles (atoms, molecules) in a substance, and this number is too large to observe directly. For this reason, the mole is found by calculation from other measurable quantities, such as mass.
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