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Science Class 11 - Units And-Measurements Notes

Comprehensive study notes for Class 11 - Units And-Measurements olympiad preparation

Units and Measurements

Welcome to the chapter on Units and Measurements for Class 11. In this chapter, you will learn about physical quantities, the need for measurement, different systems of units, and how to express and convert measurements. By the end of this chapter, you will understand the basics of measurement in science and how to use units correctly in calculations.

Introduction

Measurement is fundamental to science. It allows us to compare, quantify, and communicate physical quantities like length, mass, time, temperature, and more. To measure, we use standard units.

Physical Quantities and Units

  • Physical Quantity: Any quantity that can be measured, such as length, mass, time, temperature, etc.
  • Unit: A standard amount used to express a physical quantity (e.g., meter for length, kilogram for mass).

Systems of Units

  • CGS System: Uses centimeter, gram, and second as base units.
  • MKS System: Uses meter, kilogram, and second as base units.
  • SI System (International System of Units): The modern and most widely used system. It has seven base units: meter (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (cd).

Fundamental and Derived Quantities

  • Fundamental Quantities: Basic quantities that are independent (e.g., length, mass, time).
  • Derived Quantities: Quantities derived from fundamental quantities (e.g., speed, area, volume, force).

Measurement of Length, Mass, and Time

  • Length: Measured in meters (m). Instruments: meter scale, vernier caliper, screw gauge.
  • Mass: Measured in kilograms (kg). Instruments: beam balance, electronic balance.
  • Time: Measured in seconds (s). Instruments: clock, stopwatch.

Accuracy, Precision, and Errors

  • Accuracy: How close a measurement is to the true value.
  • Precision: How close repeated measurements are to each other.
  • Error: The difference between the measured value and the true value.

Significant Figures

Significant figures are the digits in a measurement that are known with certainty plus one last digit that is estimated. They show the precision of a measurement.

Dimensional Analysis

Dimensional analysis is a method to check the correctness of equations and to convert units from one system to another using the dimensions of physical quantities.

Practice Questions

  1. Name the seven base units in the SI system.
  2. What is the difference between accuracy and precision?
  3. Convert 5 km to meters.
  4. How many significant figures are there in 0.00450?
  5. If the measured length of a rod is 20.2 cm and the true length is 20.0 cm, what is the error?

Challenge Yourself

  • List three derived quantities and their SI units.
  • Explain why SI units are preferred in scientific work.

Did You Know?

  • The meter was originally defined as one ten-millionth of the distance from the equator to the North Pole!
  • Atomic clocks are the most accurate time-keeping devices in the world.

Glossary

  • Unit: Standard quantity used for measurement.
  • SI: International System of Units.
  • Significant Figures: Digits that carry meaning in a measurement.
  • Dimensional Analysis: Checking equations or converting units using dimensions.

Answers to Practice Questions

  1. Meter (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), candela (cd).
  2. Accuracy is closeness to the true value; precision is closeness of repeated measurements to each other.
  3. 5 km = 5000 meters.
  4. Three significant figures.
  5. Error = 20.2 cm - 20.0 cm = 0.2 cm.

Understanding units and measurements is the first step to mastering science!

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