9th Class Physics Chapter 1 Notes PDF — Physical Quantities and Measurement

<p><em>Parhai — Free Punjab Board Notes</em></p> <p>This chapter opens the door to physics. You will learn what physics actually studies, how it touches your everyday life, how scientists measure the world using base and derived quantities, and how to use real laboratory instruments such as the vernier caliper, the screw gauge, balances, the stopwatch and the measuring cylinder. The chapter also teaches scientific notation and significant figures, which are the language of precise measurement.</p> <h2>Key Concepts</h2> <h3>What Is Physics?</h3> <p>Science is the knowledge we gain by carefully observing nature and testing our ideas through experiments. The word science itself comes from the Latin word <em>scientia</em>, which simply means knowledge. Physics is the branch of science that studies matter, energy, and the relationship between them. In earlier centuries, the study of the physical world was part of a single subject called natural philosophy. As knowledge grew, it split into two streams: the physical sciences, which study non-living things, and the biological sciences, which study living things.</p> <h3>Physics in Our Daily Life</h3> <p>Almost every device around you works on a principle of physics. A car runs on the laws of mechanics, while a refrigerator works on the principles of thermodynamics. Pulleys make it easy to lift heavy loads. Electricity gives us not only light and heat but also the mechanical energy that spins fans and motors. Air conditioners, washing machines, vacuum cleaners and microwave ovens are all applications of physics. Radio, television, telephones and computers are the result of physics applied to communication. Even the mobile phone in your pocket is a bundle of physics — it connects you to people across the world, stores your photographs, carries your messages, receives radio broadcasts and works as a calculator.</p> <h3>Branches of Physics</h3> <ul> <li><strong>Mechanics:</strong> studies the motion of objects, its causes and the conditions of rest.</li> <li><strong>Thermodynamics (Heat):</strong> deals with the nature of heat, how it transfers from one body to another, and its effects.</li> <li><strong>Sound:</strong> studies the physical aspects of sound waves — how they are produced, their properties and their uses.</li> <li><strong>Optics (Light):</strong> studies light, its properties, and the working of optical instruments.</li> <li><strong>Electricity and Magnetism:</strong> studies electric charges at rest and in motion, their effects, and how they connect with magnetism.</li> <li><strong>Atomic Physics:</strong> studies the structure and properties of atoms.</li> <li><strong>Nuclear Physics:</strong> studies nuclei and the particles inside them.</li> <li><strong>Plasma Physics:</strong> studies the production and properties of plasma, the fourth state of matter.</li> <li><strong>Geophysics:</strong> studies the internal structure of the Earth.</li> </ul> <h3>Physical Quantities</h3> <p>Every quantity that can be measured is called a physical quantity — length, mass, time and temperature are familiar examples. A physical quantity always has two parts: a numerical magnitude and a unit. Saying a student's height is 104 cm gives both the number (104) and the unit (centimetre). Stating only 104 or only cm would be meaningless. Physical quantities fall into two families:</p> <ul> <li><strong>Base quantities:</strong> the seven fundamental quantities on which all others are built — length, mass, time, electric current, temperature, luminous intensity (intensity of light) and amount of substance.</li> <li><strong>Derived quantities:</strong> quantities expressed in terms of base quantities — for example area, volume, speed, force, work, energy, power, electric charge and electric potential.</li> </ul> <h3>SI Base Units</h3> <p>Each base quantity has its own SI base unit:</p> <table> <tr><th>Base Quantity</th><th>SI Unit</th><th>Symbol</th></tr> <tr><td>Length</td><td>metre</td><td>m</td></tr> <tr><td>Mass</td><td>kilogram</td><td>kg</td></tr> <tr><td>Time</td><td>second</td><td>s</td></tr> <tr><td>Electric current</td><td>ampere</td><td>A</td></tr> <tr><td>Temperature</td><td>kelvin</td><td>K</td></tr> <tr><td>Luminous intensity</td><td>candela</td><td>cd</td></tr> <tr><td>Amount of substance</td><td>mole</td><td>mol</td></tr> </table> <h3>Derived Units</h3> <p>Units used for derived quantities are called derived units. They are formed by multiplying or dividing base units. The unit of area (m<sup>2</sup>) and the unit of volume (m<sup>3</sup>) are built on the metre, the base unit of length. Speed is defined as distance covered per unit time, so its unit is metre per second (m/s). In the same way, the units of density, force, pressure and power are all derived from base units. Some common ones:</p> <table> <tr><th>Derived Quantity</th><th>SI Unit</th><th>Symbol</th></tr> <tr><td>Area</td><td>square metre</td><td>m<sup>2</sup></td></tr> <tr><td>Volume</td><td>cubic metre</td><td>m<sup>3</sup></td></tr> <tr><td>Speed / velocity</td><td>metre per second</td><td>m/s</td></tr> <tr><td>Acceleration</td><td>metre per second squared</td><td>m/s<sup>2</sup></td></tr> <tr><td>Force</td><td>newton</td><td>N (kg·m/s<sup>2</sup>)</td></tr> <tr><td>Work / energy</td><td>joule</td><td>J (N·m)</td></tr> <tr><td>Power</td><td>watt</td><td>W (J/s)</td></tr> <tr><td>Density</td><td>kilogram per cubic metre</td><td>kg/m<sup>3</sup></td></tr> <tr><td>Pressure</td><td>pascal</td><td>Pa (N/m<sup>2</sup>)</td></tr> <tr><td>Frequency</td><td>hertz</td><td>Hz (1/s)</td></tr> <tr><td>Electric charge</td><td>coulomb</td><td>C</td></tr> </table> <h3>Prefixes and Scientific Notation</h3> <p>Many quantities are extremely large or extremely small — for example 250,000 m or 0.000002 s. Prefixes are words or letters placed before a unit to express such multiples and sub-multiples neatly. For instance, kilo stands for 10<sup>3</sup> (1000), so 20,000 g = 20 × 10<sup>3</sup> g = 20 kg. Common prefixes:</p> <table> <tr><th>Prefix</th><th>Symbol</th><th>Multiplier</th></tr> <tr><td>giga</td><td>G</td><td>10<sup>9</sup></td></tr> <tr><td>mega</td><td>M</td><td>10<sup>6</sup></td></tr> <tr><td>kilo</td><td>k</td><td>10<sup>3</sup></td></tr> <tr><td>deci</td><td>d</td><td>10<sup>−1</sup></td></tr> <tr><td>centi</td><td>c</td><td>10<sup>−2</sup></td></tr> <tr><td>milli</td><td>m</td><td>10<sup>−3</sup></td></tr> <tr><td>micro</td><td>μ</td><td>10<sup>−6</sup></td></tr> <tr><td>nano</td><td>n</td><td>10<sup>−9</sup></td></tr> <tr><td>pico</td><td>p</td><td>10<sup>−12</sup></td></tr> </table> <p>Two rules to remember: prefixes are never doubled, and no prefix is attached to the kilogram (it already contains kilo). So 10<sup>6</sup> micro = 10<sup>3</sup> nano = 1 pico. In standard scientific notation, a number is written as a single digit before the decimal point multiplied by a power of ten, e.g. 6.4 × 10<sup>3</sup> km, 3 × 10<sup>8</sup> m/s, 8.64 × 10<sup>4</sup> s.</p> <h3>Vernier Callipers</h3> <p>A vernier caliper measures small lengths — such as the internal or external diameter of a cylinder — more accurately than an ordinary metre rule. It has two jaws: a fixed jaw carrying the main scale (marked in centimetres and millimetres) and a sliding jaw carrying the vernier scale. The vernier scale has 10 divisions covering 9 mm, so each vernier division is 0.9 mm.</p> <p><strong>Least count:</strong> the difference between one main-scale division and one vernier division. It is also found by dividing the smallest main-scale reading by the number of vernier divisions: 1 mm ÷ 10 = 0.1 mm = 0.01 cm. This value is also called the vernier constant.</p> <p><strong>Zero error:</strong> close the jaws gently. If the vernier zero lines up exactly with the main-scale zero, there is no zero error. If the vernier zero lies to the right of the main-scale zero, the zero error is positive; if it lies to the left, the zero error is negative.</p> <p><strong>Taking a reading:</strong> place the object between the jaws and close them gently. Note the main-scale reading just before the vernier zero, then find the vernier division that coincides with a main-scale division and multiply it by the least count. Add the two, then apply the zero correction (zero correction = −zero error) to get the correct measurement.</p> <h3>Screw Gauge (Micrometer Screw Gauge)</h3> <p>The screw gauge measures very small lengths — such as the diameter of a thin wire — with even greater accuracy. Its parts are: a U-shaped frame, a fixed stud, a hollow cylinder (sleeve) carrying a millimetre scale along an index line, a thimble with a threaded spindle inside it, and a ratchet.</p> <p><strong>Pitch:</strong> when the thimble makes one complete rotation, the spindle advances by 1 mm along the index line. This distance between consecutive threads is called the pitch.</p> <p><strong>Least count:</strong> pitch ÷ number of circular-scale divisions = 1 mm ÷ 100 = 0.01 mm.</p> <p><strong>Zero error:</strong> close the gap between stud and spindle using the ratchet. If the circular-scale zero coincides with the index line, the zero error is zero. If the zero is behind the index line (has not crossed it), the zero error is positive — multiply the uncrossed divisions by the least count. If the zero has crossed the index line, the zero error is negative.</p> <p><strong>Using it:</strong> place the wire between the stud and the spindle, turn the ratchet until the object is held gently, read the main scale and the coinciding circular-scale division, and repeat three times to take an average.</p> <h3>Balances</h3> <ul> <li><strong>Physical balance:</strong> measures mass by comparison. A beam rests on a central fulcrum with a scale pan hanging from each side. The unknown mass goes on the left pan; standard masses are added to the right pan until the pointer rests at zero.</li> <li><strong>Lever balance:</strong> uses a system of levers. The object is placed in one pan and standard masses in the other; the pointer is brought to zero by adjusting the masses.</li> <li><strong>Electronic balance:</strong> switched on and zeroed first, then the object is placed on the pan and the display shows its mass directly. Available in milligram, gram and kilogram ranges.</li> </ul> <p><strong>Precision comparison:</strong> a sensitive beam balance detects changes as small as 0.1 g; a physical balance goes down to 0.01 g (10 mg); an electronic balance reaches 0.001 g (1 mg) — the most sensitive of the three.</p> <h3>Stopwatch</h3> <p>A stopwatch measures the time interval of an event. A mechanical stopwatch has a least count of 0.1 s; a digital stopwatch commonly used in laboratories measures down to 0.01 s. On a mechanical stopwatch the knob winds the spring and also works as start–stop–reset: press once to start, press again to stop, and press a third time to return the needle to zero. On a digital stopwatch the start/stop button begins and ends the timing, while a separate reset button restores zero. Measuring very small time intervals gives more accurate results in experiments.</p> <h3>Measuring Cylinder</h3> <p>A measuring cylinder is a transparent glass or plastic cylinder with a scale along its length marked in millilitres (mL). Cylinders of 100 mL to 2500 mL capacity are common. It measures the volume of liquids and powdered substances, and also finds the volume of a small irregular solid that sinks in liquid, by the displacement method: lower the solid (tied with a thread) into water, note the rise in level, and the difference (V<sub>2</sub> − V<sub>1</sub>) is the solid's volume.</p> <p>Keep the cylinder vertical on a flat surface. Note that water's surface curves downward (concave meniscus) while mercury's curves upward. Always keep your eye level with the meniscus — looking from above makes the level appear higher, and looking from below makes it appear lower.</p> <h3>Significant Figures</h3> <p>Significant figures are all the accurately known digits in a measurement plus the first doubtful (estimated) digit. They reflect the precision of a measured value. Better instruments give more significant figures.</p> <p><strong>Rules:</strong></p> <ol> <li>Non-zero digits are always significant.</li> <li>Zeros between two significant figures are also significant.</li> <li>Final zeros to the right of the decimal point are significant.</li> <li>Zeros on the left of the decimal point, written only to place the decimal point, are not significant.</li> <li>In whole numbers ending in zeros without a decimal point, it is unclear which zeros are significant — rewrite the number in scientific notation to decide.</li> </ol> <p><strong>Accuracy of a measurement</strong> depends on three things: the quality of the measuring instrument, the skill of the observer, and the number of observations taken.</p> <h2>Important Questions with Answers</h2> <h3>Q1. What is the vernier constant?</h3> <p>The difference between one main-scale division and one vernier-scale division is called the vernier constant. For the common vernier caliper it equals 0.1 mm (0.01 cm).</p> <h3>Q2. What is the difference between base quantities and derived quantities? Give three examples of each.</h3> <p>Base quantities are the seven fundamental physical quantities on which all others are expressed — for example length, mass and time. Derived quantities are expressed in terms of base quantities — for example volume, speed and force.</p> <h3>Q3. Pick out the base units from: joule, newton, kilogram, hertz, mole, ampere, metre, kelvin, coulomb, watt.</h3> <p>The base units are: kilogram, mole, ampere, metre and kelvin. The rest (joule, newton, hertz, coulomb, watt) are derived units.</p> <h3>Q4. Which base quantities are involved in (a) speed, (b) volume, (c) force, (d) work?</h3> <p>(a) Speed: length and time. (b) Volume: length. (c) Force: length, mass and time. (d) Work: length, mass and time.</p> <h3>Q5. What is the zero error of a measuring instrument?</h3> <p>If the zeros of the two scales of an instrument do not coincide with each other, the instrument has a zero error, and every reading will be slightly larger or smaller than the true value.</p> <h3>Q6. Why is zero error correction necessary?</h3> <p>Applying the zero correction (which equals −zero error) removes the instrument's built-in bias, so the final measurement is correct.</p> <h3>Q7. What is a stopwatch? What is the least count of the mechanical stopwatch used in laboratories?</h3> <p>A stopwatch is an instrument that measures the time interval of an event. The mechanical stopwatch used in school laboratories has a least count of 0.1 second.</p> <h3>Q8. Why do we need to measure extremely small intervals of time?</h3> <p>Smaller measurable time intervals give more accurate results in experiments such as finding the time period of a pendulum.</p> <h3>Q9. A screw gauge has 50 divisions on its circular scale and a pitch of 0.5 mm. What is its least count?</h3> <p>Least count = pitch ÷ circular-scale divisions = 0.5 mm ÷ 50 = 0.01 mm.</p> <h3>Q10. Which of these have three significant figures: (a) 0.005 m, (b) 4.80 × 10<sup>3</sup> m, (c) 5.05 × 10<sup>−7</sup> kg, (d) 301.0 s?</h3> <p>(b) 4.80 × 10<sup>3</sup> m and (c) 5.05 × 10<sup>−7</sup> kg have three significant figures. (a) has one, and (d) 301.0 has four.</p> <h3>Q11. Find the significant figures in (a) 1007.0 and (b) 0.00450 kg.</h3> <p>(a) 1007.0 has five significant figures. (b) 0.00450 kg has three significant figures (4, 5 and the final 0).</p> <h3>Q12. A chocolate wrapper is 6.7 cm long and 5.4 cm wide. Calculate its area to a reasonable number of significant figures.</h3> <p>Area = 6.7 × 5.4 = 36.18 cm<sup>2</sup>. Since each measurement has two significant figures, the reasonable answer is 36 cm<sup>2</sup>.</p> <h3>Q13. On closing the jaws of a vernier caliper, the vernier zero lies to the right of the main-scale zero, and the 4th vernier division coincides with a main-scale division. Find the zero error and zero correction.</h3> <p>Zero error = 4 × 0.01 cm = +0.04 cm. Zero correction = −0.04 cm.</p> <h2>Multiple Choice Questions</h2> <ol> <li>The number of base units in SI is: (a) 3 (b) 6 (c) <strong>7</strong> (d) 9</li> <li>Which one of the following is NOT a derived unit? (a) pascal (b) <strong>kilogram</strong> (c) newton (d) watt</li> <li>Amount of a substance is measured in: (a) gram (b) kilogram (c) newton (d) <strong>mole</strong></li> <li>An interval of 200 μs is equivalent to: (a) 0.2 s (b) 0.02 s (c) 2 × 10<sup>−3</sup> s (d) <strong>2 × 10<sup>−4</sup> s</strong></li> <li>Which is the smallest quantity? (a) 0.01 g (b) 2 mg (c) 100 μg (d) <strong>5000 ng</strong></li> <li>The most suitable instrument to measure the internal diameter of a test tube is: (a) metre rule (b) <strong>vernier calipers</strong> (c) measuring tape (d) screw gauge</li> <li>A student reported the diameter of a wire as 1.032 cm using vernier calipers. The correct reading should be: (a) 1 cm (b) 1.0 cm (c) <strong>1.03 cm</strong> (d) 1.032 cm</li> <li>A measuring cylinder is used to measure: (a) mass (b) area (c) <strong>volume</strong> (d) density</li> <li>On a screw gauge the main scale shows 3 mm and the 8th circular division coincides with the index line. The thickness is: (a) 3.8 mm (b) <strong>3.08 mm</strong> (c) 3 mm (d) 3.8 cm</li> <li>Significant figures in an expression are: (a) all the digits (b) all the accurately known digits (c) <strong>all the accurately known digits and the first doubtful digit</strong> (d) all the doubtful digits</li> </ol> <h2>Key Formulas and Definitions</h2> <table> <tr><th>Item</th><th>Formula / Definition</th></tr> <tr><td>Least count of vernier calipers</td><td>Smallest main-scale division ÷ vernier divisions = 1 mm ÷ 10 = 0.1 mm = 0.01 cm</td></tr> <tr><td>Vernier constant</td><td>One main-scale division − one vernier division</td></tr> <tr><td>Vernier reading</td><td>Main-scale reading + (coinciding vernier division × least count) + zero correction</td></tr> <tr><td>Pitch of screw gauge</td><td>Distance moved by spindle per thimble rotation = 1 mm</td></tr> <tr><td>Least count of screw gauge</td><td>Pitch ÷ circular-scale divisions = 1 mm ÷ 100 = 0.01 mm</td></tr> <tr><td>Zero correction</td><td>−(zero error)</td></tr> <tr><td>Volume of irregular solid</td><td>V = V<sub>2</sub> − V<sub>1</sub> (rise in liquid level)</td></tr> <tr><td>Area (chocolate wrapper example)</td><td>Length × width, rounded to the significant figures of the least precise measurement</td></tr> <tr><td>Age in seconds</td><td>Years × 365 × 24 × 3600</td></tr> <tr><td>Hair growth rate (1 mm/day)</td><td>10<sup>6</sup> nm ÷ 86400 s ≈ 11.6 nm/s</td></tr> <tr><td>Significant figures</td><td>All accurately known digits plus the first doubtful digit</td></tr> </table> <p>More free notes at Parhai.</p>

9th Class Physics Chapter 1 Notes - Physical Quantities and Measurement

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