Physical Quantities, Units & Measurements Grade 8 PDF

BRIGHTWAY INTERNATIONAL SCHOOL PHYSICS/GRADE 8 PHYSICAL QUANTITIES, UNITS AND MEASUREMENTS PHYSICAL QUANTITY A physical quantity can be measured or calculated from other physical property and...

BRIGHTWAY INTERNATIONAL SCHOOL PHYSICS/GRADE 8 PHYSICAL QUANTITIES, UNITS AND MEASUREMENTS PHYSICAL QUANTITY A physical quantity can be measured or calculated from other physical property and expressed as the product of a number and a unit. Physical quantities are broadly classified into Basic quantities and Derived quantities Basic quantities are the fundamental quantities which cannot be expressed in terms of any other physical quantities Eg: Length, mass, time, temperature, current, amount of a substance and Luminous intensity. BASIC QUANTITIES SI UNIT SYMBOL Length metre m Mass kilogram kg Time second s Current ampere A Temperature Kelvin K Amount of substance mole mol Luminous intensity candela cd Derived quantities are the quantities which can be expressed in terms of any other physical quantities Eg: Area, volume, density, speed etc The SI (System International) is a set of metric units now used in many countries. The SI system is a decimal system in which units are divided or multiplied by 10 to give smaller or larger units. Example 400 = 4 x 10 x 10 = 4 x 102 40 = 4 x 10 = 4 x 101 4 = 4x1 = 4 x 100 0.4 = 4/10 = 4 x 10-1 0.04 = 4/100 = 4 x 10-2 The smaller figures 1, 2, 3 etc., are called powers of ten and it gives the number of times the number has to be multiplied by 10 if it is greater than 1 or divided by 10 if it is smaller than 1. This way of writing numbers is called the standard form. Page 1 Notes/Grade8/T1/SEM1/2023 BRIGHTWAY INTERNATIONAL SCHOOL PHYSICS/GRADE 8 PREFIX We can make a unit bigger or smaller by putting an extra symbol in front is called prefix. PREFIX MULTIPLE SYMBOL Nano 10-9 n Micro 10-6  Milli 10-3 m Centi 10-2 c Kilo 103 k Mega 106 M Giga 10 9 G Length The SI unit of length is metre (m) Smaller units of length are 1 centimetre(cm) = 10-2metre 1 millimetre(mm) = 10-3metre 1 micrometre (  m) = 10-6metre 1 nanometre(nm) = 10-9metre Larger units of length are 1 kilometre (km) = 103metre 1 Megametre (Mm) = 106metre Length measurements are made using a ruler, measuring tape, vernier calipers, micrometer screw gauge… Correct way to measure with ruler is to keep the eye right over the Mark on the scale or thickness of the ruler may cause errors. Page 2 Notes/Grade8/T1/SEM1/2023 BRIGHTWAY INTERNATIONAL SCHOOL PHYSICS/GRADE 8 The line of sight should be perpendicular to the reading in order to avoid Parallax error in measurement Measuring instruments for length Length to be Example Instrument to be used Accuracy measured Very short Diameter of a metal Micrometer screw gauge 0.01mm length wire Short length Diameter of a Vernier calipers 0.01cm measuring cylinder Medium length Image distance of a Metre rule 0.1cm lens Long length Length of physics Measuring tape 0.1cm laboratory Length measuring devices Page 3 Notes/Grade8/T1/SEM1/2023 BRIGHTWAY INTERNATIONAL SCHOOL PHYSICS/GRADE 8 AREA Area of a rectangle = length x breadth Area of a triangle = ½ x base x height Area of a circle =  x (radius)2 SI unit of area is square-metre (m2) 1cm2 = 10-4m2 VOLUME It is the amount of space occupied. SI unit of volume is cubic-metre (m3) 1cm3 = 10-6m3 Volume of a regularly shaped solid Volume of rectangular block = length x breadth x height Volume of a sphere = 4/3  x (radius)3 Volume of a cylinder =  x (radius)2 x height Volume of a liquid Volume of liquid is expressed in litres (l) 1 litre (l) = 1000ml 1millilitre(ml) = 1cm3 1 litre (l) = 1000cm3 Pour the liquid into the measuring cylinder and the level on the scale gives the volume The measuring cylinder should be kept upright on a horizontal smooth surface. Eye level should be perpendicular to the lower meniscus so as to avoid Parallax error Page 4 Notes/Grade8/T1/SEM1/2023 BRIGHTWAY INTERNATIONAL SCHOOL PHYSICS/GRADE 8 Volume of an irregularly shaped solid Pour the liquid into the measuring cylinder. Note the scale reading corresponding to the lower meniscus. Immerse the solid into the measuring cylinder. The liquid level rises. Note the new scale reading as the final reading or the final volume. Volume of the irregularly shaped solid = Final volume – Initial volume Time SI unit is second (s) 1 day = 24 hours (h) 1 hour (h) = 60 minutes (min) 1 minute (min) = 60 seconds (s) 1 hour(h) = 3600 seconds (s) 1 millisecond (ms) = 10-3s 1 microsecond(  s) = 10-6s 1 nanosecond(ns) = 10-9s Time can be measured using stop watch and stop clock. Press the reset button. If the pointer returns to zero then it has no zero error otherwise it has zero error. If the pointer is to left of the zero, add zero error with the reading and if the pointer is to the right of the zero then subtract zero error from the reading. The error in the measurement of time is human reaction error. Page 5 Notes/Grade8/T1/SEM1/2023 BRIGHTWAY INTERNATIONAL SCHOOL PHYSICS/GRADE 8 Simple Pendulum The period of a pendulum is the time it takes to complete one oscillation. An oscillation is defined as the full motion. For example, starting at one side, the pendulum swings to the other side and back. The time required for that motion is the period. The inverse of the period is the frequency. Frequency is the number of cycles in a length of time. A simple pendulum is one with its mass concentrated at the end, such as a weight hanging on a thin string. Note: Changing the mass of a simple pendulum will not change its period. Changing the length of the pendulum does affect the period, with shorter pendulums having shorter periods. Why more number of oscillation (20) is taken to find the time period of a simple pendulum by using a stopwatch? The time for one oscillation is too small to be measured accurately by using a stopwatch due to human reaction error. This can be reduced by taking more number of oscillations and it will improve the accuracy Page 6 Notes/Grade8/T1/SEM1/2023 BRIGHTWAY INTERNATIONAL SCHOOL PHYSICS/GRADE 8 Mass It is the measure of the amount of matter in it. SI unit of mass is kilogram(kg) 1 tonne(t) = 1000kilogram(kg) 1 kilogram(kg) = 1000gram(g) 1 gram(g) = 1000milligram(mg) 1milligram(mg) = 10-6kg Mass of an object is found using various kinds of Balance. 1. Beam balance: An unknown mass is placed in one pan and balanced against the known masses in the other pan. The beam balances when the gravitational pull on the material in each pan is the same. Each pan then contains the same mass of the material. 2. Top-pan balance: A direct reading is obtained from the position on the scale of a pointer joined to the lever system or the reading can be directly read in a top-pan balance. Page 7 Notes/Grade8/T1/SEM1/2023