Perfect Guide. Notes. 2nd Edition. Ryan Bong

Transcription

1 Perfect Guide `O Level PHYSICS PHYSICS Notes 2nd Edition Ryan Bong

2 72 Preface PERFECT GUIDE TO O LEVEL PHYSICS NOTES (2nd Edition) is a study aid for the G.C.E. O Level Physics examination. With comprehensive notes, this study guide promises to build knowledge and facilitate understanding for effective learning. This book can be used in conjunction with the textbook, Physics Matters (2nd edition). Textbook links are provided for convenient referencing and further reading. Features in this book Key Terms and Concepts Highlights important terms and concepts Important Facts Concise notes with diagrams aid understanding IMPORTANT FACTS 9 Kinetic Model of Matter KEY TERMS AND CONCEPTS Describe the molecular structure of solids, liquids and gases Describe and compare the properties of solids, liquids and gases Relate the properties of solids, liquids and gases to the intermolecular forces of attraction and the distances between the molecules, and to molecular motion 9.1 States of Matter 1. The three common states of matter are solid, liquid and gas. 2. The of solids are closely packed together and vibrate about their fixed positions. The intermolecular forces of attraction between the are very strong. 3. The of liquids are randomly arranged and can move relatively freely. The intermolecular forces of attraction between liquid are weaker compared to those between solid. 4. The of gases are much further apart from each other than those of liquids or solids. They move in straight lines in random directions and collide with each other frequently. The intermolecular forces of attraction between the gas are very weak and often negligible. 5. Solids, liquids and gases have different properties with regard to shape, volume, density and compressibility. 6. Brownian motion is the continuous random motion of small in a fluid or a gas. 7. If temperature T increases, gas move at higher speeds. 8. The pressure p of a gas is caused by the continual collisions between the gas molecules and the walls of its container. 9. If the volume V of a gas is constant, then its pressure p is directly proportional to its temperature T (p T). 10. If the pressure p of a gas is constant, then its volume V is directly proportional to its temperature T (V T). 11. If the temperature T of a gas is constant, then its pressure p is inversely proportional to its volume V (p 1 V ). 1. The kinetic model of matter states that all matter is made up of tiny called atoms or molecules in continuous random motion. 2. In a Brownian motion experiment, smoke is trapped in a glass cell and viewed under a microscope. The smoke are observed to move in a haphazard manner. 2. There are three common states of matter, namely solid, liquid and gas. eye 3. The table below compares the structures of solids, liquids and gases. zig-zag paths of smoke State of matter Solid Liquid Gas microscope glass cover Model 9 Kinetic Model of Matter Arrangement of Motion of view through microscope torchlight smoke glass cell Closely packed Loosely packed with Very far apart together in a regular Studying space for the movement haphazard movement compared of smoke to liquids known as Brownian motion. pattern and solids 3. The haphazard motion of smoke is caused by collisions with invisible air molecules. Vibrate about their Move relatively freely Move freely and This random motion of smoke indicates that air molecules also move in random fixed positions but confined to the randomly directions. container 2013 Marshall Cavendish International (Singapore) Private Limited 85 Textbook Link For further reading Space between Very small Small but larger than Much larger than in in solids liquids Common or solids Error Forces between Very strong attractive Strong attractive Weak (often intermolecular In forces the Brownian intermolecular motion forces experiment, but negligible) In the attractive Brownian motion experiment, we cannot see air we can see weaker air than in colliding solids intermolecular. forces We conclude that air collide with with the smoke. the smoke from the haphazard motion of the smoke. Solid Liquid Gas Situation Students standing 4. If the temperature Students moving in the out glass A cell class increases, of students the speed of the air molecules increases. When in orderly straight these faster-moving of school hall air after molecules playing collide a game with of the tag smoke, the smoke can lines during school be observed assembly. to move about more in the rapidly school too. field. assembly. 5. From this experiment, it can be deduced that air (and generally any gas) consists of numerous Description Students can move The motion of Students move that are in continuous random motion. If we cool a gas, we will get a liquid and only a little around students out of the constantly and rarely their standing subsequently school a hall solid. is similar Therefore, come we can into conclude contact that all matter consists of. This is positions which is the origin to the of the flowing kinetic motion model with of matter. each other. The similar to limited of liquid. distances between motion of solid Link students are relatively. Physics Matters (2nd Edition) Section quite 9.2 large too Marshall Cavendish International (Singapore) Private Limited Explain the pressure of a gas in terms of motion of State, explain and apply the relationships between pressure, volume and temperature of a gas when one quantity is kept constant 9.3 Pressure, Volume and Temperature of a Gas 1. Gas are constantly in motion, colliding with each other and with the walls of the enclosing container. When rebounding from a wall of the container, the gas exert a force on the wall. 88 Common Error Refutes common misconceptions Highlights each learning point from the O Level Physics syllabus Useful hints for exams Example Application of concepts learnt, aided by The relationship p = hρg can also be applied for gases. This relationship is derived from the F definition of p = A so both equations are equivalent. 2. The pressure in a liquid changes according to the depth of the liquid. h 1 h 2 h 3 pond The water pressure in a pond increases with the depth. p 1 = h 1 ρg p 2 = h 2 ρg p 3 = h 3 ρg p 1 < p 2 < p 3 To find the total pressure exerted at a certain depth of a lake, remember to always add atmospheric pressure to the pressure due to water. Example 7.3 The cross-section of a swimming pool is a right-angled triangle as shown in the diagram below. The depth of the swimming pool increases from the right side to the left side. Find the pressure due to water acting on a boy who has swum to the deepest part of the swimming pool. (Density of water = 1000 kg m 3 ; gravitational field strength = 10 m s 2 ) h 4.8 m 5.2 m Before applying the equation, first determine the height of the water column at the deepest end, i.e. the depth of the water at the extreme left of the pool. Solution Let h be the height of the water column. Using Pythagoras, Theorem, h 2 + (4.8 m) 2 = (5.2 m) 2 h 2 = ( ) m 2 = 4.0 m 2 h = 2.0 m Therefore, the height h of the water column is 2.0 m. Implications of Newton s Laws of Motions Newton s First Law tells us how we know if a force exists. We cannot see force directly but we can deduce that it exists if its effects are observed. Newton s Second Law tells us how to quantify the magnitude of forces and determine their directions. Newton s Third Law tells us forces must exist in pairs and a force can only appear due to interactions between two bodies. Substitution of values into F = ma In the formula F = ma, (a) F represents the resultant force acting on the object; (b) m represents the mass of the object; (c) a represents the acceleration of the object. A common mistake is the inclusion of forces acting on different objects while calculating the resultant force F. Zero resultant force does not imply that the object is stationary. If the resultant force acting on the object is zero, then F = ma 0 = ma a = 0 m s 2 (since m 0) (a) We can conclude that the acceleration of the object a is zero. However, this does not mean that the velocity of the object is also zero. (b) Zero acceleration only implies that velocity is constant. Therefore, an object acted upon by balanced forces can be moving at constant velocity and still have zero acceleration. Friction on the moving wheel of a car The direction of friction acting on the wheel of a moving car can be found as shown in the diagram below. direction of motion of car wheel spins clockwise direction of tendency towards motion at point A A direction of friction acting on the wheel As friction acts in the direction opposite to motion, the direction of friction is towards the right. Therefore, friction acting on the wheels of the car is in the same direction as the motion of the car. A common mistake is to assume that the direction of friction is opposite to the direction of motion of the car. 3 Forces Things to Note Tackles common mistakes, misconceptions and pitfalls. Also highlights important information. 37 iii

3 Contents Theme I: General Physics Chapter 1 Measurement...1 Chapter 2 Kinematics...13 Chapter 3 Forces...26 Chapter 4 Mass, Weight and Density...38 Chapter 5 Turning Effect of Forces...48 Chapter 6 Energy, Work and Power...59 Chapter 7 Pressure...69 Theme II: Thermal Physics Chapter 8 Temperature...79 Chapter 9 Kinetic Model of Matter...85 Chapter 10 Transfer of Thermal Energy...94 Chapter 11 Thermal Properties of Matter Theme III: Light, Waves and Sound Chapter 12 Light Chapter 13 Waves Chapter 14 Electromagnetic Waves Chapter 15 Sound Theme IV: Electricity and Magnetism Chapter 16 Static Electricity Chapter 17 Current Electricity Chapter 18 D.C. Circuits Chapter 19 Practical Electricity Chapter 20 Magnetism Chapter 21 Electromagnetism Chapter 22 Electromagnetic Induction Formula List iv

4 1 Measurement KEY TERMS AND CONCEPTS 1 Measurement 1. A physical quantity consists of a numerical magnitude and a unit. 2. The common base quantities are length, time, mass, temperature, electric current and amount of substance. The SI units of the base quantities are called SI base units. 3. The common prefixes include giga, mega, kilo, deci, centi, milli, micro and nano. 4. A scalar quantity has magnitude only, while a vector quantity has both magnitude and direction. 5. Using vernier calipers: Measured reading = reading on main scale + reading on vernier scale 6. Using micrometer screw gauge: Measured reading = reading on main scale + reading on thimble scale 7. To avoid parallax error, readings must be taken with the eye directly above the markings. 8. For instruments with zero error: Corrected reading = measured reading zero error 9. The period of a simple pendulum is the time taken to complete one full oscillation. IMPORTANT FACTS Recognise that all physical quantities consist of a numerical magnitude and a unit Identify the six base quantities and their units Use common prefixes and their symbols to indicate decimal sub-multiples and multiples of the SI units 1.1 Physical Quantities and SI Units 1. A physical quantity is a quantity that can be measured. It consists of a numerical magnitude and a unit. Example: Length of a book = 0.16 m physical quantity magnitude unit 1

5 2. The International System of Units (Le Système International d'unités) or SI units were developed to standardise the usage of units worldwide. 3. The six common basic physical quantities, or base quantities, and their corresponding units are shown in the table below. Base quantity Common symbol for base quantity SI unit Symbol for SI unit Length l metre m Time t second s Mass m kilogram kg Temperature θ kelvin K Electric current I ampere A Amount of substance n mole mol 4. All other physical quantities are derived from the base quantities. They are called derived quantities and their corresponding units are known as derived units. The table below shows examples of how the units for some common physical quantities are derived. Derived unit How it is derived from base units Name of unit Symbol for SI unit Unit of area unit of length unit of width square metre m 2 Unit of volume Unit of speed unit of length unit of width unit of height unit of length unit of time cubic metre m 3 metre per second m s 1 Common Error speed v = 20 m/s 1 speed v = 20 m s 1 = 20 m/s density ρ = 19 g/cm 3 density ρ = 19 g cm 3 = 19 g/cm 3 5. Prefixes of units are used to simplify the expression of quantities that are very large or very small. The common prefixes and their symbols are shown in the table below. Factor Prefix Symbol 10 9 giga G 10 6 mega M 10 3 kilo k 10 1 deci d 10 2 centi c 10 3 milli m 10 6 micro µ* 10 9 nano n *The symbol µ is pronounced mu. It is the Greek alphabet equivalent of the letter m. 2

6 6. The standard form can also be used to express quantities that are very large or very small. Numbers in the standard form are given in the following format: A 10 n with 1 A < 10, where n is an integer. 10 n is called the order of magnitude of the number. Examples: Length of F1 Singapore Grand Prix circuit = m Speed of light in vacuum = m s 1 Diameter of a hydrogen atom = m 1 Measurement Example 1.1 Which of the following is greater, 150 µm or cm? Convert the two different units to a common unit before making a comparison. Solution 150 µm = m = m = m cm = cm = m = m Therefore, 150 µm is greater than cm. Common Error 1 m = 100 cm, therefore 1 m 2 = 100 cm 2 1 m = 100 cm Therefore 1 m 2 = 1 m 1 m = 100 cm 100 cm = cm 2 While calculating the answer to a complicated question, you can avoid confusion with units by converting all quantities to SI base units. Identify scalar and vector quantities Add vectors using the graphical method 1.2 Scalar and Vector Quantities 1. Physical quantities can be categorised into either scalar or vector quantities. 2. A scalar quantity has only magnitude. Examples: Mass of a boy = 52 kg Time taken by a man to run 100 m = 12 s 3

7 3. A vector quantity has both magnitude and direction. Examples: Velocity of a car = 100 km h 1 towards the east Force exerted on a falling ball in vacuum = 10 N downwards 4. Here is a simple way to determine whether a physical quantity is a scalar or a vector quantity. Associate the physical quantity with a direction and then try to see if the physical quantity makes sense with the associated direction. 5. Scalar quantities can be added by simply adding or subtracting their numerical values. Example: Total time taken = 15 s + 10 s = 25 s 6. Adding two vector quantities will give a single vector that produces the same total effect as the two vectors. 7. The tip-to-tail method can be used to add two vector quantities. The direction of the arrow represents the direction of the vector quantity, while the length of the arrow is proportional to the magnitude of the vector quantity. 8 cm vector 1 5 cm vector 2 + θ = Tip-to-tail method resultant vector vector 1 θ vector 2 Example 1.2 A man walks 300 m towards the north before walking a further 400 m towards the east. How far is he now from the starting point? A suitable scale is chosen to represent the vector quantities in a vector diagram. Thus, the necessary measurement can then be made using the diagram. Solution Using a scale of 1 cm : 100 m, the path of the man can be represented by the diagram below. B 400 m end point C N 300 m 500 m A start point Measured from the diagram, AC = 5 cm. As 1 cm represents 100 m, 5 cm represents 500 m. Therefore, the man is 500 m away from the starting point. Note: This question can also be solved mathematically, i.e. using Pythagoras Theorem. However, if the question requires the drawing of a scaled vector diagram, the graphical method should be used. The mathematical method may be used to check the answer obtained. 4