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Standard Measurement Units are often used to measure various properties of objects and things. They provide everyone with the same understanding of these properties. Length, mass, time, electric current, thermodynamic temperature etc measure in metre, kilogram, second, ampere, kelvin respectively according to SI system of measurements.
There are different types of SI metric system. They are as follows:
- International System of Units or SI, the modern form of the metric system.
- The Imperial system, also known as British Imperial, is a system used in the United Kingdom.
- the United States customary units are used in the US. This unit system is similar to the Imperial system as for some measurements it uses the same units
There were different methods to carry out measurements when no fixed measurements were present at times for right or feasible measures which were also known as Non-standard ways of measurements.
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| Table of Contents |
Key Terms: Unit, Measurement, Length, Mass, Temperature, SI Unit, Electric Current, Luminous Intensity, Energy
What is SI Unit?
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A standard unit (SI) is the most widely used unit of measurement. It provides everyone with the same understanding of an object's size and other properties.
- Standard Units of Measurement are rooted values and those values cannot be changed or varied.
- It is essential that measurements have uniformity or conformity between anyone that uses these measurements for everyone to have the same sense of measurement.
For example: In the United States the measurement system used is called the US Standard Measurement System, the Customary Measurement System, or the Imperial Measurement System. All other countries except the United States, the measurement system used is the Metric Measurement System or otherwise known as the International Measurement System (SI).
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Units and Measurements Video Lecture
Importance of SI System
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The SI system was first established in 1960 by the Eleventh General Conference on weights and measures. The SI was first invented by Blaise Pascal, who contributed to the field of Mathematics and scientific theories and laws.
- SI system is a basic tool for calculating or measuring any substance whether in solid, liquid, or gaseous state.
- The use of SI units is crucial for science because it improves cross-border communication and makes it convenient to share information.
- There are various systems and units for measuring distinct parameters such as length, area, mass, volume, etc.
Example:
- SI unit of velocity = meter per second (m/s)
- SI unit of heat = joule.
- SI unit of momentum = kilogram meter per second (kg.m/s)
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What is International System of Units?
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The International system of units is the most popular form of measuring metric system. This system is composed of seven fundamental components. This system specifies twenty prefixes to the unit symbols and name to state multiples and fractions of each unit.
- The International System of Units, also known as the SI system specifies seven such quantities or fundamental units.
- SI is based on the meter-kilogram second system of units (MKS).
- SI is applied to all physical measurements.
- There are seven quantities in the SI units system. They are as follows: Mass, Length, Time, Temperature, Current, Luminous Intensity, Amount of substance
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| Relevant Concepts | ||
|---|---|---|
| Unit of Time | Unit of Work | Motion |
| Unit of Weight | Unit of Distance | Equation Motion |
Seven SI System Base Units
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There are seven base SI units and are mentioned below:
| Base quantity | SI Unit | Details |
|---|---|---|
| Time | Second (s) | The second was basically accounted for from a standard day of 24 hours. As known, that earth takes 23 hours, 56 minutes, and 4.1 seconds to complete one full rotation. |
| Hence, a second is now stated as the duration of approximately 9 billion periods of radiation corresponding to the period of transition between the two levels of the ground state of the caesium -133 atom. | ||
| Length | Metre (m) | The metre was stated as 1/10,000,000 of the distance from the Earth’s equator to the North pole measured on the circumference through Paris. |
| In today’s terms, it is calculated as the distance travelled by light in a vacuum over a time interval of 1/299,792,458 of a second. | ||
| Mass | Kilogram (kg) | The kilogram was calculated as a mass of a litre. That is, one thousandth of a cubic metre. |
| Electric current | Ampere (A) | The ampere can be defined as a measure of the amount of electric charge that passes a point in an electric circuit per unit time (6.241x 1018 electrons). |
| Thermodynamic temperature | Kelvin (K) | The kelvin so far known as the unit of Thermodynamic temperature scale. |
| This scale starts at 0 K. The ascending size of the kelvin is similar to that of the degree on the Celsius scale or Centigrade scale. | ||
| The kelvin is the fraction 1/273.16 of the Thermodynamic temperature of the triple point of water (0.01o C). | ||
| Amount of Substance | Mole (mol) | The mole relates to molecular or atomic mass to a constant number of particles. |
| It is simply explained as the amount of substance that consists of as many elementary entities as there are atoms present in 0.012 kg of C-12. | ||
| Luminous Intensity | Candela (cd) | In early days candles were the most common source of light and hence the name candela was coined. |
| Now, in modern days with the availability of fluorescent light sources the candela is known as Luminous Intensity. | ||
| Luminous Intensity radiates monochromatic radiation of frequency and has a radiant intensity of 1/683 watts per steradian. |
Seven Base SI Units of Measurements
What is Derived Units?
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There are other quantities which are called derived units which are stated as seven base quantities by a system of quantity equations. The SI derived units are obtained from these equations. Tabulated below are the details:
| Derived Quantity | Name | Symbol | Expression in terms of other SI units | Expression in terms of base SI units |
|---|---|---|---|---|
| Celsius Temperature | Degree Celsius | oC | - | K |
| Luminous Flux | Lumen | lm | cd.sr | M2.m-2.cd = cd |
| Energy, Work | Joule | J | N-m | m2.kg. s-2 |
| Force | Newton | N | - | m.kg. s-2 |
| Frequency | Hertz | Hz | - | s-1 |
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Things to Remember
- Standard Measurement Units are often used to measure various properties of objects and things.
- The ampere can be defined as a measure of the amount of electric charge that passes a point in an electric circuit per unit time (6.241x 1018 electrons).
- The SI system was first coined by Blaise Pascal.
- The International System of units (SI) is also known as the Metric System.
- The SI is a whole of 7 base units that state the 22 derived units with special names, symbols, and significance.
- The SI system plays a vital role in international commerce and is widely used in technological research.
- Luminous Intensity radiates monochromatic radiation of frequency and has a radiant intensity of 1/683 watts per steradian.
Previous Years’ Questions
- A screw gauge has least count of 0.01 mm and there are 50 divisions … [NEET – 2020]
- Dimensions of stress are...[NEET – 2020]
- Lumen is the unit of… [UPSEE – 2019]
- A quantity z, to be estimated has a dependency on the variables… [TS EAMCET – 20219]
- A student measured the diameter of a small steel ball using… [NEET – 2018]
- A physical quantity of the dimensions of length that can be formed… [NEET – 2017]
- Planck’s constant (ℎ), speed of light in vacuum (c) and Newton”s gravitational constant… [NEET – 2016]
- Which of the following is the smallest unit… [UPSEE – 2010]
- Body weighs 22.42 g and has a measured volume of 4.7… [WBJEE – 2010]
- A student measures the distance traversed in free fall… [NEET – 2010]
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Sample Questions
Ques. What is meant by the Standard Unit of measurement? Can a quantity have dimensions, but no unit associated with it? (3 Marks)
Ans. A standard unit can be defined as the most used unit of measurement. These units are standardised, meaning that everyone can understand the basic unit of size, weight, and other attributes of an object.
- If the quantity has a dimension, then units must associate with it.
- The dimension of a physical quantity refers to the nature of the quantity associated.
- As in, the physical dimensions are expressed in the form of length, mass, and time as L, M, and T respectively.
Ques. List down all the seven Standard Unit of measurements with a typical symbol, name. (5 Marks)
Ans. All the seven Standard Unit of measurements are as follows:
| Quantity | Name | Symbol |
| Length | Meter | m |
| Mass | Kilogram | kg |
| Time | Second | s |
| Electric current | Ampere | A |
| Thermodynamic temperature | Kelvin | K |
| Amount of matter | Mole | mol |
| Luminous intensity | Candela | cd |
Ques. State 5 examples of derived units. (5 Marks)
Ans. Below tabulated are a few examples of derived units:
| Derived Quantity | Name | Symbol | Expression in terms of other SI units | Expression in terms of base SI units |
|---|---|---|---|---|
| Plane Angle | Radian | rad | - | m.m-1=1 |
| Solid Angle | Steradian | sr | - | m2.m-2=1 |
| Frequency | hertz | Hz | - | s-1 |
| Power | watt | W | J/s | m2.kg.s-3 |
| Electromotive Force | volt | V | W/A | m2.kg.s-3.A-1 |
Ques. What is the work done when the force acting on the body and the displacement produced in the body are at right angles to each other? What are supplementary units? (3 Marks)
Ans. The work done when the force acting on the body and the displacement produced in the body are at right angles to each other is Zero.
- It is also possible that some force is acting on a body but still the work done is zero only when the force acts at an angle of 90o with the displacement.
- Supplementary units are dimensionless physical quantities that are used along with fundamental units.
Ques. When a planet X is at 820.7 million kilometres from earth its angular diameter is measured to be 35.72 arc. Calculate the diameter of planet X. (3 Marks)
Ans. Given,
Distance between planet X and earth, r = 820.7×106 km.
The angular diameter θ is given to be,
Θ = 35.72
Θ = 35.72/ (60×60) × π / 180 radian
Diameter = ?
We have the relation,
L = r x θ
⇒ l = 820.7 × 106 (35.7260 × 60 × π/180)
∴ l = 2.81 × 105 km.
Ques. Find the dimensions of Latent heat and Specific heat. (3 Marks)
Ans. As we know that,
Latent heat = Q (heat energy) / m (mass)
Dimensions of Latent heat = M L2 T-2 / M
= [M0L2T-2].
Dimensions of Specific heat = Q (heat energy) / m (mass) Δ T (temperature)
⇒[S] = M L2 T-2 / Mx K
∴[S]= [M0 L2 T-2 K-1].
Ques. The mass of a box measured by a grocer's balance is 2.400 kg. Two gold pieces of masses 20.18 g and 20.15 g are added to the box. What is: (3 Marks)
1) The Total Mass of the Box?
2) The Difference in the Masses of the Pieces to Correct Significant Figures?
Ans. We are given:
1). The total mass of the box is calculated as,
Mass of grocer’s box = 2.400 kg
Mass of gold piece I = 20.18 g = 0.02018 kg
Mass of gold piece II = 20.15 g = 0.02015 kg
Total mass of the box = 2.4 + 0.02018 + 0.02015 = 2.44033 kg.
2). The difference in the masses of the pieces is 20.18 − 20.15 = 0.03g.
Ques. What is a unit? (1 Mark)
Ans. A unit is a quantity of invariable magnitude used in measuring other magnitudes of quantities of the same nature.
Ques. Mention two advantages of three advantages of SI Unit system. (3 Marks)
Ans. Advantages of SI unit system are as follows:
- It is a metric system,
- It is closely related to CGS and MKS systems of units,
- Uses decimal system, hence is more user friendly.
Ques. What is meant by dervied physical quantity/unit? (2 marks)
Ans. All those physical quantities, which can be derived from the combination of two or more fundamental quantities or can be expressed in terms of basic physical quantities, are called derived physical quantities.
The units of all other physical quantities, which can be obtained from fundamental units, are called derived units.
For example, units of velocity, density and force are m/s, kg/m3, kg m/s2 respectively and they are examples of derived units.
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