# How are physical quantities measured?

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A **physical quantities** is a physical property of a material or system that can be quantified by measurement. A physical quantity can be expressed as a value, which is the algebraic multiplication of a numerical value and a unit. Wikipedia

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Physical quantities are the quantitative characteristics of physical bodies and phenomena. Examples of physical quantities: body length, time interval, body speed, body temperature, distance traveled by the body.

**Different physical quantities** are measured in different units. So temperature is measured in degrees Celsius (°C), time is in seconds (s), distance is in meters (m), speed is in meters per second traveled (m / s), area is in square meters (m ^{2} ), volume – in cubic meters (m ^{3} ). However, even the same physical quantity can be measured in different ways. For example, time can be measured in seconds, minutes, hours, and other units. Length – in millimeters (mm), centimeters (cm), kilometers (km), etc.

To avoid ambiguity and misunderstanding, scientists agreed to use the International System of Units SI in physics. For example, the unit of length along it is 1 m. Various measuring instruments are used to measure physical quantities. For example, a ruler can measure the length of small objects, measuring cups and test tubes – small volumes of liquid, time is measured in hours.

Measuring instruments have a scale – divisions that have a certain numerical value. The price of division of the measuring device is the difference in the values of the physical quantity between adjacent divisions on the scale. For example, on a ruler, large divisions indicate centimeters. Between two adjacent large divisions there are smaller ones. The difference between two adjacent small divisions is 1 millimeter. For example, some division means 5 cm 3 mm, and the one next to the right will mean 5 cm 4 mm. So, the price of dividing the ruler is 1 mm.

Usually, using measuring instruments, it is impossible to measure a physical quantity very accurately. For example, if, when measuring the length of an object, its end did not exactly coincide with any division of the scale, but is somewhere between two divisions. Therefore, they talk about the measurement error, which is equated to the division price of the device.

Very small and very large physical quantities cannot be measured directly with instruments. How to measure the volume of ocean or even river water? How to find out how many kilometers from the Earth to the Sun? How many millimeters is the diameter of an atom? In the case of such measurements, scientists come up with various methods and indirectly measure the physical quantity.

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