Introduction to Units and Measurement

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Questions and Answers

What is the result of a measurement of a physical quantity expressed by?

A number accompanied by a unit.

Which of the following are the base units in the CGS system?

Centimetre, gram, second (correct)

Metre, kilogram, second

Foot, pound, second

What does SI stand for in measurement?

Système Internationale (correct)

Systematic Integration

Standard International

The SI units are complex and difficult to convert.

False Signup and view all the answers

What are fundamental or base units?

Units for fundamental quantities that serve as the basis for derived quantities. Signup and view all the answers

What is the unit for length in the MKS system?

Metre Signup and view all the answers

The unit for a plane angle is called a _____

radian Signup and view all the answers

The unit for a solid angle is called a _____

steradian Signup and view all the answers

What is the role of the General Conference on Weights and Measures?

To develop and recommend standards for measurement, including the SI units. Signup and view all the answers

What is the purpose of the Système Internationale d’ Unites (SI)?

To provide a standard system of units for international usage Signup and view all the answers

What are the units associated with plane angle and solid angle in the SI system?

Radian and steradian Signup and view all the answers

Which of the following is NOT a base unit in the SI system?

Gram Signup and view all the answers

How is the metre defined in the SI system?

As the length of the path traveled by light in vacuum during a specific time Signup and view all the answers

What additional units are defined in the SI system apart from the seven base units?

Radian and steradian Signup and view all the answers

Why are conversions within the SI system considered simple and convenient?

Due to the use of the decimal system Signup and view all the answers

What is the base unit of mass in the SI system?

Kilogram Signup and view all the answers

What does a radian measure?

The ratio of an arc length to the radius of a circle Signup and view all the answers

What distinguishes fundamental units from derived units?

Fundamental units are based on measurable physical quantities, while derived units are combinations of these. Signup and view all the answers

Which of the following statements is true regarding the CGS, FPS, and MKS systems?

The base units for length, mass, and time differ across these systems. Signup and view all the answers

What is the purpose of establishing a unit of measurement?

To facilitate international communication and comparison Signup and view all the answers

What are the base units for length, mass, and time in the MKS system?

Metre, kilogram, second Signup and view all the answers

What can be said about derived units in relation to fundamental units?

Derived units cannot be expressed without fundamental units. Signup and view all the answers

Why is it beneficial to have a limited number of base units?

It allows for easier conversion and understanding of physical relationships. Signup and view all the answers

What type of systems were used by scientists in earlier times for measurement?

Arbitrary and non-standardized systems Signup and view all the answers

In which measurement system is the gram the unit for mass?

CGS Signup and view all the answers

What method is primarily used to measure large distances that cannot be directly measured with a metre scale?

Parallax method Signup and view all the answers

In the parallax method, what is the basis for measurement?

The distance between the two observation points Signup and view all the answers

What do we observe to measure the parallax angle in the parallax method?

The angle between viewing directions Signup and view all the answers

When using a metre scale, which of the following lengths can it measure directly?

From 0.001 m to 100 m Signup and view all the answers

What is the parallax angle represented by in the context of parallax measurement?

θ Signup and view all the answers

What types of entities can the parallax method be used to measure distances for?

Any type of particle or entity including atoms and stars Signup and view all the answers

Why are derived units necessary in measuring physical quantities?

To align with base units and enhance clarity Signup and view all the answers

How does the concept of parallax apply when observing a pencil against a wall?

The pencil appears to shift position based on the viewpoint Signup and view all the answers

What is the definiton of the second?

The duration of 9,192,631,770 periods of radiation from cesium-133. Signup and view all the answers

What defines an ampere?

A constant current maintaining a force of 2×10–7 newton per meter between conductors 1 meter apart in vacuum. Signup and view all the answers

Which unit is defined by the thermodynamic temperature of water's triple point?

Kelvin Signup and view all the answers

What quantity does a mole refer to?

The number of entities equivalent to 0.012 kilogram of carbon-12. Signup and view all the answers

What is the definition of the candela?

The luminous intensity observed at a frequency of 540×1012 hertz at a radiant intensity of 1/683 watt per steradian. Signup and view all the answers

What does a measurement of 1 kelvin represent?

A fraction of the triple point temperature of water. Signup and view all the answers

What is the significance of the units detailed for general use outside the SI system?

They serve purpose for measurements in specific contexts. Signup and view all the answers

Which of the following is NOT a base unit described in the content?

Pressure Signup and view all the answers

Study Notes

Measurement of a Physical Quantity

The result of a measurement of a physical quantity is expressed by a number and a unit.

Base Units in the CGS System

The base units in the CGS system are the centimeter (cm) for length, the gram (g) for mass, and the second (s) for time.

SI System

SI stands for "Système International d'Unités", which is French for "International System of Units".

SI Units

SI units are not complex or difficult to convert.

Fundamental or Base Units

Fundamental or base units are those units that cannot be expressed in terms of other units.

Unit for Length in the MKS system

The unit for length in the MKS system is the meter (m).

Unit for Plane Angle

The unit for a plane angle is called a radian (rad).

Unit for Solid Angle

The unit for a solid angle is called a steradian (sr).

Role of the General Conference on Weights and Measures

The General Conference on Weights and Measures (CGPM) is responsible for establishing the SI system and its units.
They are responsible for all aspects of the SI system, including defining the base units, making changes to the system, and coordinating international work on measurement.

Introduction

The measurement of any physical quantity involves a comparison with a reference standard called a unit.
The result of such a measurement is represented by a number and the specific unit used.
Only a limited number of fundamental units (like length, mass, and time) are needed to describe all physical quantities because they are interrelated.
The units for these base quantities are known as base units.
Other derived quantities have units derived from combinations of base units, which are called derived units.
A system of units is a collection of base and derived units.

The International System of Units (SI)

The SI is the internationally accepted system of units for scientific, technical, industrial, and commercial purposes.
The SI system employs seven base units, and the symbols for these units are listed in Table 2.1.
The SI system is based on a decimal system which simplifies conversions within the system.
Two additional dimensionless units in the SI are:
- Plane angle (dθ): measured in radians (rad) and is defined as the ratio of the arc length (ds) to the radius (r).
- Solid angle (dΩ): measured in steradians (sr) and is defined as the ratio of the intercepted area (dA) of a spherical surface to the square of its radius (r).

Measurement of Length

Direct methods are used for measuring lengths ranging from 10^-3 m to 10² m using a metre scale.
The parallax method is used to measure large distances like the distance to a planet or star.

Measurement of Large Distances (Parallax Method)

The parallax method relies on the apparent shift in the position of an object when viewed from two distinct locations.
The key concepts are:
- Basis: the distance between the two points of observation (e.g., the distance between two observatories on Earth).
- Parallax angle (θ): the angle between the two directions along which the object is viewed from the two locations.
The larger the basis, the larger the parallax angle, and the smaller the distance to the object.
The parallax method is used by astronomers to measure the distances to stars.

Measurement of Small Distances

The size of an atom is of the order of 10^-10 m, far too small for a meter scale.
Techniques like electron microscopes or X-ray diffraction methods are used to measure such small distances.

Other Units

While the SI is the primary system, some units are still retained for general usage despite not being part of the SI.
Common prefixes for multiples and sub-multiples of SI units are listed in Appendix A2.
Symbols for physical quantities, chemical elements, and nuclides are explained in Appendix A7.
Symbols for SI units and other units are detailed in Appendix A8.

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Description

This quiz covers the fundamental concepts of units and measurement, including the International System of Units (SI). Understand the differences between base and derived units, as well as the significance of standardization in measurement. Test your knowledge of the seven base units of the SI system.