# Physical Quantities

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## Physical Quantities

The study of Physics is based on experiments, which allowed us to test and validate theories. The results of those experiments are obtained by accurate measurements using physical quantities.

A physical quantity is a quantity that can be measured. All physical quantities consist of a numerical magnitude and a unit.

• The numerical magnitude denotes the size of the physical quantity.
• The unit denotes the physical quantity it is expressing.

There are seven base quantities (E.g. Mass, length). From the seven base quantities, you can obtain all the other physical quantities.

Physical quantities had many different types of units in the past. In the modern times, the units have been standardised and are named the SI units, from the French “Le Systeme International d’Unites”.

## Worked Examples

### Example 1

What types of natural phenomena could serve as time standards?

Atomic clocks are based on electromagnetic waves which atoms emit. Also, pulsars are highly regular astronomical clocks.

### Example 2

Suppose the three fundamental standards of the metric system were length, density, and time rather than length, mass, and time. The standard of density in this system is to be defined as that of water. What considerations about water would you need to address to make sure that the standard of density is as accurate as possible?

Density varies with temperature and pressure. It would be necessary to measure both mass and volume very accurately in order to use the density of water as a standard.

## For Further Reading: Why Do We Need a Standardised System of Units (e.g. SI Units)?

Middle Ages and Renaissance: During the Middle Ages, various regions in Europe maintained their unique systems of measurement, leading to a lack of standardization and hindered scientific progress. Scholars often used units based on local customs, such as the English inch or the Spanish vara, making it challenging to share and compare scientific findings. This lack of uniformity persisted into the Renaissance, a period marked by a renewed interest in scientific inquiry. Figures like Galileo Galilei and Johannes Kepler recognized the need for a universal system of measurement but struggled to implement widespread change due to entrenched regional traditions.

The Metric System and French Revolution: The call for a standardized measurement system gained momentum during the French Revolution, with the revolutionary government recognizing the importance of uniformity for economic and scientific progress. In 1795, the metric system was formally adopted in France, featuring units based on powers of ten for simplicity and ease of conversion. This marked a significant departure from the hodgepodge of units prevalent in the past. The metric system quickly spread beyond French borders, becoming a symbol of scientific enlightenment and rationality.

Other Pre-SI Systems: While the metric system gained prominence, other pre-SI systems persisted in specific scientific disciplines. The centimeter-gram-second (cgs) system, for instance, gained traction in physics due to its applicability in electromagnetism and mechanics. Meanwhile, the Gaussian system found favor in the field of electromagnetism, with its focus on electric and magnetic units. These systems coexisted with the metric system, showcasing the complexity of scientific endeavors in a world still adapting to standardized units.

Practical Units in Daily Life: Beyond the realm of academia, traditional units retained their significance in everyday life. Measurements like inches, feet, pounds, and gallons were deeply ingrained in various cultures and professions. While scientists aimed for precision and standardization, these traditional units continued to be used in construction, trade, and daily interactions. The persistence of these units highlighted the enduring influence of historical practices and the challenges faced in fully transitioning to a universally accepted system.

Challenges with Diverse Units: The coexistence of diverse measurement systems presented numerous challenges to scientists and engineers. Conversion errors were commonplace, leading to inaccuracies in calculations and experimental results. Scientific literature suffered from a lack of clarity, as researchers had to navigate between various units when studying works from different regions. The need for a common language of measurement became increasingly evident, driving the quest for a global standard that would culminate in the formation of the International System of Units (SI).

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