Physical Quantities Units And Measurement Pdf
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- Units of Measurement Physics Class 11 Download notes in pdf
- Physical Quantities and SI Units
- 1.3 The Language of Physics: Physical Quantities and Units
- Section Summary
Units of Measurement Physics Class 11 Download notes in pdf
The history of the metre and the kilogram, two of the fundamental units on which the system is based, goes back to the French Revolution. The system itself is based on the concept of seven fundamental base units of quantity, from which all other units of quantity can be derived.
Following the end of the Second World War, it became increasingly apparent that a worldwide system of measurement was needed to replace the numerous and diverse systems of measurements in use at that time. In , the 10 th General Conference on Weights and Measures , acting on the findings of an earlier study, proposed a system based on six base quantities.
The quantities recommended were the metre , kilogram , second , ampere , kelvin and candela. Following the proposals, the conference of the 11 th CGPM introduced the new system to the world. A seventh base unit, the mole , was added following the 14 th CGPM, which took place in An official description of the system called the SI Brochure , first published in and currently as of in its ninth edition, can be downloaded free of charge from the website of the Bureau International des Poids et Mesures BIPM.
The role of the BIPM includes the establishment of standards for the principal physical quantities, and the maintenance of international prototypes. Its work includes metrological research metrology is the science of measurement , making comparisons of international prototypes for verification purposes, and the calibration of standards.
The General Conference currently meets every four years to confirm new standards and resolutions, and to agree on financial, organisational and developmental issues. The value of a physical quantity is usually expressed as the product of a number and a unit. In the past and in some cases up until very recently the unit represented a specific example or prototype of the quantity concerned, which was used as a point of reference. The number represents the ratio of the value of the quantity to the unit.
As of , all of the base units are now defined with reference to seven "defining" physical constants that include fundamental constants of nature such as the Planck constant and the speed of light. The most recent changes occurred with the publication of the ninth edition of the SI brochure in Four base units - the kilogram , ampere , kelvin and mole - were redefined using physical constants.
The second , metre , and candela , already defined using physical constants, were subject to corrections. As a case in point, the kilogram was previously defined with reference to a prototype.
The prototype in question was a platinum-iridium cylinder held under tightly controlled conditions in a vault at the BIPM, identical copies of which are kept under identical conditions located throughout the world. A quantity of two kilograms 2 kg would have been defined as exactly twice the mass of the prototype or one of its copies. Now, however, according to the version of the SI Brochure:. It is defined by taking the fixed numerical value of the Planck constant h to be 6.
Also according to the edition of the SI Brochure, the seven defining physical constants used to define the SI units:. There are seven base quantities used in the International System of Units. The seven base quantities and their corresponding units are:. These base quantities are assumed to be independent of one another. In other words, no base quantity needs to be defined in terms of any other base quantity or quantities.
Note however that although the base quantities themselves are considered to be independent, their respective base units are in some cases dependent on one another. The table below summarises the base quantities and their units. You may have noticed that an anomaly arises with respect to the kilogram the unit of mass.
The kilogram is the only SI base unit whose name and symbol include a prefix. You should be aware that multiples and submultiples of this unit are formed by attaching the appropriate prefix name to the unit name gram , and the appropriate prefix symbol to the unit symbol g.
As stated earlier, each of the derived units of quantity identified by the International System of Units is defined as the product of powers of base units. Each base quantity is considered as having its own dimension , which is represented using an upper-case character printed in a sans serif roman font. Derived quantities are considered to have dimensions that can be expressed as products of powers of the dimensions of the base quantities from which they are derived.
The dimension of any quantity Q is thus written as:. The superscripted characters are the first seven lower-case characters from the Greek alphabet alpha , beta , lambda , delta , epsilon , zeta and eta , and represent integer values called the dimensional exponents.
The dimensional exponents values can be positive, negative or zero. The dimension of a derived quantity essentially conveys the same information about the relationship between derived quantities and the base quantities from which they are derived as the SI unit symbol for the derived quantity.
In some cases, all of the dimensional exponents are zero as is the case, for example, where a quantity is defined as the ratio of two quantities of the same kind. Such quantities are said to be dimensionless , or of dimension one. The coherent derived unit for such a quantity as the ratio of two identical units is the number one. The same principle applies to quantities that cannot be expressed in terms of base units, such as number of molecules , which is essentially simply the result of a count.
These quantities are also regarded as being dimensionless, or of dimension one. Most dimensionless quantities are simply expressed as numbers. Exceptions include the radian and the steradian , used to express values of plane angles and solid angles respectively.
Another notable exception is the decibel , which is described above. The derived units of quantity identified by the International System of Units are all defined as products of powers of base units.
A derived quantity can therefore be expressed in terms of one or more base quantity in the form of an algebraic expression. Derived units that are products of powers of base units that include no numerical factor other than one are said to be coherent derived units.
This means that they are derived purely using products or quotients of integer powers of base quantities, and that no numerical factor other than one is involved. The seven base units and twenty-two coherent derived units of the SI form a coherent set of twenty-nine uinits which is referred to as the set of coherent SI units. All other SI units are combinations of some of these twenty-nine units. The word "coherent" in this context means that equations between the numerical values of quantities are in exactly the same form as the corresponding equations between the quantities themselves.
The twenty-two coherent derived units have special names and symbols. Often, the name chosen acknowledges the contribution of a particular scientist. The unit of force the newton is named after Sir Isaac Newton , one of the greatest contributors in the field of classical mechanics. The unit of pressure the pascal is named after Blaise Pascal for his work in the fields of hydrodynamics and hydrostatics. The table below lists the coherent derived units. Note that aach unit named in the table below has its own symbol, but can be defined in terms of other derived units or in terms of the SI base units, as shown in the last two columns.
Note that the units for the plane angle and the solid angle the radian and steradian respectively are both derived as the quotient of two identical SI base units. They are thus said to have the unit one 1. They are described as dimensionless units or units of dimension one the concept of dimension was described above.
Note that a temperature difference of one degree Celsius has exactly the same value as a temperature difference of one kelvin. The Celsius temperature scale tends to be used for day-to-day non-scientific purposes such as reporting the weather, or for specifying the temperature at which foodstuffs and medicines should be stored.
In this kind of context it is somewhat more meaningful to a member of the public than the Kelvin temperature scale. The units in the coherent set can be combined to express the units of other derived quantities.
Since this allows a potentially unlimited number of combinations, it is not possible to list them all here. The table below lists some examples of derived quantities, together with the corresponding coherent derived units expressed in terms of base units. The example coherent SI derived units shown in the table below are based on a combination of derived units with special names and the SI base units.
The names and symbols for these units reflects the hybrid nature of these units. As with the units in the previous table, each unit has its own symbol but can be defined in terms of the SI base units, as shown in the final column. The value of being able to use both special and hybrid symbols in equations can be appreciated when we look at the length of some of the base unit expressions. The units detailed in the final table are accepted for use with the International System of Units for a variety of reasons.
Many are still in use, some are required for the interpretation of scientific texts of historical importance, and some are used in specialised areas such as medicine.
The hectare , for example, is still commonly used to express land area. The use of the equivalent SI units is preferred for modern scientific texts. Wherever reference is made to non-SI units, they should be cross referenced with their equivalent SI units. For the units shown in the following table, the equivalent definition in terms of SI units is also shown. Most of the units listed that are in widespread daily use, and likely to be so for the foreseeable future.
Note that for most purposes, it is recommended that fractional values for plane angles expressed in degrees should be expressed using decimal fractions rather than minutes and seconds.
Exceptions include navigation and surveying due to the fact that one minute of latitude on the Earth's surface corresponds to approximately one nautical mile , and astronomy. In the field of astronomy, very small angles are significant due to the enormous distances involved. It is therefore convenient for astronomers to use a unit of measurement that can represent very small differences in angle in a meaningful way. Very small angles can be represented in terms of arcseconds , microarcseconds and picoarcseconds.
There are a number of widely accepted conventions for the expression of quantities in hand-written or printed documents and texts. These conventions have been in place with relatively little modification since the General Conference on Weights and Measures first introduced the System of International Units in They are primarily intended to ensure a uniform approach to the presentation of hand written or printed information, and to ensure the readability of scientific journals, textbooks, academic papers, data sheets, reports, and other related documents.
The presentational requirements will vary to some extent according to the norms of the language in which the work is written. We are concerned here only with the conventions as they apply to the English language.
The following list represents some of the more important requirements. Multiples and submultiples of SI units are signified by attaching the appropriate prefix to the unit symbol.
Prefixes are printed as roman upright characters prepended to the unit symbol with no intervening space. Most unit multiple prefixes are upper case characters the exceptions are deca da , hecto h and kilo k. All unit submultiple prefixes are lower case characters.
Prefix names are always printed in lower case characters, except where they appear at the beginning of a sentence, and prefixed units appear as single words e. All multiples and submultiples are integer powers of ten. Beyond one hundred or one hundredth multiples and submultiples are integer powers of one thousand , although they are still expressed as powers of ten.
Physical Quantities and SI Units
Figure 1. The distance from Earth to the Moon may seem immense, but it is just a tiny fraction of the distances from Earth to other celestial bodies. We define a physical quantity either by specifying how it is measured or by stating how it is calculated from other measurements. For example, we define distance and time by specifying methods for measuring them, whereas we define average speed by stating that it is calculated as distance traveled divided by time of travel. Measurements of physical quantities are expressed in terms of units , which are standardized values. For example, the length of a race, which is a physical quantity, can be expressed in units of meters for sprinters or kilometers for distance runners. Without standardized units, it would be extremely difficult for scientists to express and compare measured values in a meaningful way.
Physicists, like other scientists, make observations and ask basic questions. For example, how big is an object? How much mass does it have? How far did it travel? To answer these questions, they make measurements with various instruments e. The measurements of physical quantities are expressed in terms of units, which are standardized values.
The distance D of sun from the earth is 1. Also explore over 10 similar quizzes in this category. Given on this page is a free online quiz which includes mcqs questions and answers related to the topic of physical quantities and units. There are seven base SI units for mass, length, time, electric current, temperature, luminous intensity, and amount of substance from which the other units may be derived. Measurement and Units 2.
1.3 The Language of Physics: Physical Quantities and Units
The range of objects and phenomena studied in physics is immense. Giving numerical values for physical quantities and equations for physical principles allows us to understand nature much more deeply than does qualitative description alone. To comprehend these vast ranges, we must also have accepted units in which to express them. And we shall find that even in the potentially mundane discussion of meters, kilograms, and seconds a profound simplicity of nature appears—all physical quantities can be expressed as combinations of only four fundamental physical quantities: length, mass, time, and electric current. We define a physical quantity either by specifying how it is measured or by stating how it is calculated from other measurements.
The history of the metre and the kilogram, two of the fundamental units on which the system is based, goes back to the French Revolution. The system itself is based on the concept of seven fundamental base units of quantity, from which all other units of quantity can be derived. Following the end of the Second World War, it became increasingly apparent that a worldwide system of measurement was needed to replace the numerous and diverse systems of measurements in use at that time. In , the 10 th General Conference on Weights and Measures , acting on the findings of an earlier study, proposed a system based on six base quantities. The quantities recommended were the metre , kilogram , second , ampere , kelvin and candela.
This is a list of physical quantities. The first table lists the base quantities used in the International System of Units to define the physical dimension of physical quantities for dimensional analysis.
Physics is a quantitative science, based on measurement of physical quantities. Certain physical quantities have been chosen as fundamental or base quantities. The fundamental quantities that are chosen are Length, Mass, Time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. Each base quantity is defined in terms of a certain basic arbitrarily chosenbut properly standardised reference standard called unit such as metre,kilogram,second,ampere,kelvin,mole,and candela. The units for the fundamental base quantities are called fundamental or base units and two supplementary units in relation to quantities plane angle and solid angle radian, Ste radian..
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is a physical quantity, length. They are completely different types of physical quantities measured by different references and units. Suppose you are measuring.
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