In physics, a magnitude is a property or quality of a body that can be observed and measured . The magnitude can be quantified through direct or indirect methods, using measurement tools and devices.
Magnitudes are divided into two types: fundamental and derived quantities. There are only seven fundamental magnitudes, but there are dozens of derivatives. The difference between the two is that the derivatives start from the fundamental ones, being a combination of these to measure a variety of physical and chemical phenomena.
Let’s look at both types of magnitudes in depth.
The seven fundamental quantities:
The fundamental magnitudes, known as basic, are those independent of other magnitudes. Therefore, they do not require the calculation of other magnitudes to be quantified.
The seven fundamental quantities in SI (International System of Units) are:
|Amount of substance
It is the amount of matter that a body or substance has . It is expressed in kilograms, but it is common to use other measurements such as grams, milligrams or tons.
Mass is also a measure of the inertia of the body or substance, presenting an opposition to changing its speed when subjected to a force. The greater the mass, the more force will have to be applied to the body to move it.
Amount of substance
It is the number of elementary particles that a substance has . This can refer to the number of atoms, electrons, ions or other types of particles, as well as molecules and different groupings of particles.
The measurement is based on Avogadro’s number, which is defined as approximately 6.022·10 23 /mol. In other words, one mole is equivalent to about 6.022·10 23 particles.
Length is defined as the distance between two points , quantified in a linear dimension. This applies to any direction, be it width, height or depth.
The unit of length in the international system is the meter. Depending on the context, it is common to measure lengths in millimeters, centimeters, kilometers, inches, feet, or miles.
Temperature measures the amount of kinetic energy present in the particles of a system . The higher the kinetic energy, the more the particles will move, which implies a high temperature. On the other hand, if the particles have little kinetic energy, the temperature will be very low.
Although temperature is measured in Kelvin in the International System, in everyday life degrees Celsius or Fahrenheit are often used.
It is a magnitude that measures the duration of a transformation or phenomenon . The second is the unit in the International System, but also other units, such as the minute, hours or days.
Current intensity or electric current
Current intensity or electric current quantifies the flow of electrons or charges that move through a material . For example, the greater the number of electrons that pass through the material, the greater the intensity.
In general, this measurement is linked to electrical circuits, and is measured in amperes.
Luminous or luminous intensity is the amount of luminous flux emitted per unit of solid angle, in a specific direction. The unit of the International System is the candle.
What are derived quantities
Derived quantities are those that start from two or more fundamental quantities to quantify a property or phenomenon. These can become complex, combining several basic magnitudes.
To obtain the derived magnitudes, the fundamental ones are multiplied or divided. This allows establishing relationships of direct or inverse proportionality between the derivative and the respective basic ones. Let’s look at some examples.
Examples of derived quantities
|Units using basic quantities
|Area or surface
|kg·m 2 /s 3
|kg·m 2 /s 3 ·A
|kg m 2 /s 3 A 2
Magnetic flux density
|cd·m 2 /m 2
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