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This booklet contains notes for guidance, intended for students who will sit for the Form III National Assessment in Physics. It is a free booklet meant to help Mauritian students in Physics and it is not for sale. Physics National Assessment 2017 (Click on icon to view related Videos) Dushan BOODHENA Table of Contents CHAPTER ONE: MEASUREMENT .................................................................................................... 4 Measurement of length ...................................................................................................................... 4 Common types of errors and their prevention ................................................................................... 4 End error ......................................................................................................................................... 4 Zero error ........................................................................................................................................ 4 Parallax error................................................................................................................................... 5 How to read a Vernier scale? .............................................................................................................. 5 Measurement of volume .................................................................................................................... 7 Volumes of liquids ........................................................................................................................... 8 Measurement of time ......................................................................................................................... 9 Measurement of mass ........................................................................................................................ 9 Measurement of temperature ............................................................................................................ 9 CHAPTER TWO: MOTION ............................................................................................................ 11 Scalar and vector quantities ............................................................................................................. 11 Linear motion (Optional)................................................................................................................... 11 Distance and displacement ............................................................................................................... 11 Speed and velocity ............................................................................................................................ 12 Average speed and average velocity (Optional) ............................................................................... 13 Acceleration ...................................................................................................................................... 13 Distance-time (s-t) graphs (Optional) ............................................................................................... 14 Speed-time (v-t) graphs .................................................................................................................... 15 CHAPTER THREE: ENERGY ........................................................................................................... 19 The SI unit of energy ......................................................................................................................... 19 The various forms of energy ............................................................................................................. 19 Renewable and non-renewable sources of energy (Optional) ......................................................... 19 The need for alternative energy sources (Optional) ......................................................................... 19 Advantages and disadvantages of different energy sources (Optional) ........................................... 20 Biomass ......................................................................................................................................... 20 Hydroelectric energy ..................................................................................................................... 20 Solar energy .................................................................................................................................. 20 Tidal energy................................................................................................................................... 20 Wind energy .................................................................................................................................. 20 © 1995-2017 Dushan [δβ] BOODHENA 1 Chemical energy (fossil fuels) ....................................................................................................... 20 Nuclear energy .............................................................................................................................. 20 Work done ........................................................................................................................................ 20 Kinetic energy ................................................................................................................................... 21 Potential energy ................................................................................................................................ 21 The law of conservation of energy.................................................................................................... 21 Power ................................................................................................................................................ 22 Ways and means to save energy (Optional) ..................................................................................... 23 To save electrical energy ............................................................................................................... 23 To save fuel energy ....................................................................................................................... 23 CHAPTER FOUR: OPTICS ............................................................................................................. 25 Light travels in straight lines ............................................................................................................. 25 Luminous and non-luminous bodies ................................................................................................. 25 The laws of reflection........................................................................................................................ 25 Common applications of reflection of light ...................................................................................... 26 Lateral inversion............................................................................................................................ 26 Rear-view mirror and blind spot ................................................................................................... 27 The periscope ................................................................................................................................ 27 Ray diagram for a point object reflected in a mirror ........................................................................ 28 Refraction of light ............................................................................................................................. 29 Laws of refraction ......................................................................................................................... 29 Common applications of refraction of light ...................................................................................... 30 Real depth and apparent depth .................................................................................................... 30 Lenses............................................................................................................................................ 30 CHAPTER FIVE: ELECTRICITY........................................................................................................ 33 Electrical charge and electric current ............................................................................................... 33 Conductors and Insulators ................................................................................................................ 33 Current .............................................................................................................................................. 33 Potential difference .......................................................................................................................... 34 Electrical resistance and Ohm’s law.................................................................................................. 34 Experiment to determine the resistance of a metallic conductor .................................................... 35 Resistors in series.............................................................................................................................. 36 Resistors in parallel ........................................................................................................................... 37 Direct current (D.C) circuits .............................................................................................................. 39 © 1995-2017 Dushan [δβ] BOODHENA 2 Safe use of electrical energy (Optional) ............................................................................................ 40 Saving electrical energy (Optional) ................................................................................................... 40 INDEX………………………………………………………………………………………………………………………………………….42 © 1995-2017 Dushan [δβ] BOODHENA 3 CHAPTER ONE: MEASUREMENT Physics is the science of matter, motion and energy. Whichever branch of science we study, we have to make accurate measurements to ensure standard and universally accepted values. During the measurement of simple quantities, we: 1. ask different persons to repeat the measurement (sometimes with a different set of the same instrument) to ensure that the values are the same or, at least, very close to each other, 2. repeat the measurement at least four times and then calculate the average value, 3. express the physical quantity as a number, a unit and sometimes a direction, for example, ‘10 m to the North.’ Measurement of length Experiment 1: To measure the length and width of the Physics laboratory using a measuring tape. Experiment 2: To measure the length and width of a book using a metre-rule. Experiment 3: To measure the inside and outside diameters of a boiling tube using Vernier callipers. Length is the distance between two points and is measured in metres (m). Longer distances like javelin or discus throws are measured using measuring tapes. Average distances, like the length of a book, are measured with a metre rule, half-metre rule or ruler. Shorter distances, like the diameter of a coin, are measured with Vernier callipers (also written as ‘calipers’). Common types of errors and their prevention End error An end error arises with metre rules and half-metre rules. This occurs because the zero marks of these instruments are situated at their very ends. When the ends of these rules wear out with time, their zero marks are no longer available. To overcome an end error, we start with the 1 cm mark and then subtract the 1 cm from the scale reading in order to get the correct length of the object. Notice that in an ordinary ruler, there is a gap or dead space before the zero mark. The problem of end error cannot therefore arise with an ordinary ruler although we may have a zero error if we measure from its end. Measured values will be smaller than the true value. Zero error Theoretically, when we close the jaws of a Vernier calliper completely, we should get a reading of 0.00 cm because we are not measuring any length. In practice, however, some Vernier callipers will still give a small reading when they are completely closed. We say that there is a zero error on this instrument. To overcome any zero error arising from a Vernier calliper, we subtract the zero error from the measured length, in order to get the true length. © 1995-2017 Dushan [δβ] BOODHENA 4 Parallax error Metre rules and half-metre rules are thick. Because of this thickness, if we do not place one eye perpendicular to its scale, we will read a value which is too high or too low. We say that parallax error has occurred. To overcome parallax error we should read with only one eye placed directly above the scale. How to read a Vernier scale? Step 1: Where is the zero of the Vernier pointing on the main scale? If it points between two numbers (3.0 and 3.1 in the example below), always choose the smaller number as the main scale reading. Step 2: Number the Vernier scale readings as 0, 5 and 10. Which line on the Vernier scale is matching exactly with any line on the main scale? This is the Vernier scale reading which gives the second decimal place. Step 3: Add the main scale reading in step 1 to the Vernier scale reading in step 2. 3.0 2 0 3.1 4 45 10 3.0 4 cm Vernier reading= measured length = main scale reading + Vernier scale reading © 1995-2017 Dushan [δβ] BOODHENA 5 Details of the commonly used instruments are given below: Instrument Usual Range Measuring tape 0—100 m Metre rule Smallest Division 1 cm 0—1 m 0.1 cm or 1 mm Half-metre rule 0—50 cm 0.1 cm or 1 mm Vernier callipers 0—12.5 cm 0.01 cm or 0.1 mm Uses Special Precautions Place eye directly above scale (to avoid parallax error). Avoid using the 0 mm mark to avoid end error. To measure the Place eye directly length and width above scale. of a table Avoid using the 0 mm mark. To measure the Place eye directly length and width above scale. of a book Avoid using the 0 mm mark. To measure the Unlock the locking diameter of a coin screw while or the measuring. inside/outside Subtract any zero diameter of a error on the pipe instrument. To measure Javelin or discus throws Some useful conversions of length are: 1 km = 1000 m 1 m = 100 cm 1 cm = 10 mm © 1995-2017 Dushan [δβ] BOODHENA 6 Measurement of volume Volume is the space occupied by an object in three dimensions and it is measured in cubic metres (m3) or cubic centimetres (cm3). For regular objects, we use appropriate formulae to calculate their volumes. SHAPE FORMULA TO CALCULATE ITS VOLUME Cube Length, l Volume = length x length x length V = l3 Cuboid height, h breadth, b Volume = length x breadth x height V = l x b x h length, l Sphere 𝟒 radius, r Volume = 𝟑 x π x radius x radius x radius V = 𝟒 𝟑 π r3 © 1995-2017 Dushan [δβ] BOODHENA 7 Experiment 4: To determine the volume of a stone using the displacement method. To measure the volume of an irregular solid such as a stone, we use the displacement method described below: thread measuring cylinder final volume initial volume A measuring cylinder is filled with a known volume of water. Using a thread, the stone whose volume is to be determined is carefully immersed into the measuring cylinder. The final volume is then noted. The volume of the irregular object is given by the formula: volume of irregular object = (final volume—initial volume) Precautions: 1. The measuring cylinder must be placed on a flat, horizontal surface, free from mechanical vibrations. 2. The readings should be taken by placing one eye on the same level as the liquid meniscus. 3. A very thin thread should be used to immerse the irregular solid. 4. The solid should not dissolve in the liquid, nor should there be air bubbles on the surface of the solid. Before reading the volume, we must place the eye on the same level as the lowest part of the meniscus. Volumes of liquids The volume of a liquid is measured directly by placing it in a measuring cylinder. If the liquid is mercury, we must place one eye on the same level as the top of the meniscus before taking the reading. For all other liquids, we must place one eye on the same level as the bottom of the meniscus before taking the reading. Some useful conversions of volume are: 1 m3 = 1,000,000 cm3 or ml 1 cm3 = 1000 mm3 1 litre or l = 100 cl = 1000 ml or cm3 © 1995-2017 Dushan [δβ] BOODHENA 8 Measurement of time Experiment 5: To measure the time it takes for a feather to fall to the ground using a stopwatch (both analogue and digital). Time is a measure of the duration of an event and it is measured in seconds (s). Long intervals of time are measured using a clock. Shorter intervals of time are measured using an analogue or digital stopwatch. Analogue stopwatches may have a zero error on them which need to be subtracted. Some useful conversions of time are as follows: 1 day = 24 h 1 h = 60 minutes 1 minute = 60 seconds Measurement of mass Experiment 6: To measure the mass of a stone. The mass of an object tells us how much matter it contains and it is measured in kilograms (kg). It can be measured using an electronic balance or a beam balance. An electronic balance reads and displays the mass of an object directly but it must be placed on a flat horizontal surface free from mechanical vibrations. A beam balance uses a set of standard masses to compare the mass of the object. Some useful conversions of mass are as follows: 1 kg = 1000 g 1 g = 1000 mg Measurement of temperature Experiment 7: To measure the room temperature with a laboratory thermometer. Temperature is the degree of hotness and it is measured in degrees celsius (0C) with a thermometer. The S.I unit of temperature is the kelvin (K). We add 273 to a temperature in 0C to convert it into a temperature in K. © 1995-2017 Dushan [δβ] BOODHENA 9 worn out ends of metre and half-metre rules K = 0C + 273 start with another mark and subtract its value Measured in kelvins (K) wrong position of eye when measuring value of zero error determined with nothing measured defect or feature in instrument 1 m = 100 cm place eye perpendicular to scale before measuring subtract zero error from instrument occurs because of occurs because of 1 km = 1000 m 1 cm = 10 mm occurs because of solution solution Range: 0 to 100 m solution End error Zero error measured with Parallax error Meaured with a laboratory thermometer Meaured in metres (m) accuracy Measurement of temperature 1 day = 24 h 1 h = 60 minutes Measuring tape unit Smallest division reads 1 cm To measure distance for javelin or discus throws where used? Longer distances Common types of errors Measured in seconds (s) Metre rule Measurement of length 1 minute = 60 s measured with Range: 0 to 1 m unit Average distances Measurement of time Long intervals measured with a clock accuracy Measurement where used? To measure length and width of a table Shorter intervals measured with an analogue/digital stopwatch Shorter distances measured with Measurement of volume Subtract any zero error for an analogue stopwatch Measured in kilograms (kg) Measurement of mass unit 1 kg = 1000 g Measured with an electronic balance Volume of irregular solid = (final volume initial volume) calculation Place on a flat, horizontal surface, free from vibrations Place measuring cylinder on a flat, horizontal surface Read at the bottom of meniscus Range: 0 to 12.5 cm Smallest division reads 0.1 mm To measure inside/ outside diameters of a test-tube For liquids, use a measuring cylinder Place eye on same horizontal level as liquid for most liquids precaution Vernier callipers where used? unit For irregular solids use the displacement method Place eye at same horizontal level as liquid Standard masses have to be used for comparison accuracy Measured in cubic metres m3 Compared on a beam balance 1 g = 1000 mg Smallest division reads 1 mm Read at the bottom of meniscus for most liquids Read at the top of meniscus for mercury For regular objects use formuale 1 m3 = 1,000,000 cm3 or ml Volume of cube = length x length x length 1 cm3 = 1000 mm3 Volume of cuboid = length x breadth x height Volume of sphere = 4/3 x π x radius x radius x radius 1 litre or l = 100 cl = 1000 ml or cm3 © 1995-2017 Dushan [δβ] BOODHENA 10 CHAPTER TWO: MOTION Scalar and vector quantities A scalar quantity is one which has magnitude or size only, but no direction. Examples of scalar quantities are mass, length, time, distance, speed, etc. When scalar quantities are added, we simply add their magnitudes. A vector quantity is one which has both magnitude and direction. Examples of vector quantities are displacement, velocity, acceleration, force, etc. When vector quantities are added, we have to consider both their magnitudes and their directions. Worked Example: Calculate the resultant force in the following: (a) 2N 5N Resultant force = 5 + 2 = 7 N to the right← (b) 5N 2N Resultant force = 5—2 = 3 N to the right← Linear motion Linear motion refers to motion in a straight line. When considering linear, unidirectional motion we may use the terms distance and displacement interchangeably. We may also interchange the terms speed and velocity when we talk about linear, unidirectional motion. When the direction of the moving object changes, then distance and displacement are two different things. We say that the object is undergoing non-linear motion. The speed and velocity of a moving object will have different values in non-linear motion. Distance and displacement Distance is the length of the path between two points, regardless of the direction. Displacement is the distance and direction between two points. Both distance and displacement are measured in metres (m). © 1995-2017 Dushan [δβ] BOODHENA 11 Distance is classified as a scalar quantity because it has magnitude or size only but no direction. For example, when we say that the driving distance between town A and town B is 5000 m, this means that the distance by road between these two points is 5000 m. It is not necessarily the distance measured in a straight line! Displacement on the other hand, is classified as a vector quantity because it has both magnitude and direction. For example, when we say that the displacement of town B from town A is 3000 m due North, this means that if we draw a straight, imaginary line pointing North from town A, we will find town B 3000 m away along this same line. Speed and velocity The speed of a body is its distance travelled per unit time. It is a scalar quantity. When stating a speed we must state its magnitude and its unit. 𝐬𝐩𝐞𝐞𝐝, 𝐢𝐧 𝐦/𝐬 = 𝐝𝐢𝐬𝐭𝐚𝐧𝐜𝐞 𝐭𝐫𝐚𝐯𝐞𝐥𝐥𝐞𝐝, 𝐢𝐧 𝐦 𝐭𝐢𝐦𝐞 𝐭𝐚𝐤𝐞𝐧, 𝐢𝐧 𝐬 The velocity of a body is its displacement per unit time. We can also define velocity as the distance travelled per unit time in a specified direction. It is a vector quantity. When stating a velocity, we must state its magnitude, its unit and its direction. 𝐯= 𝐬 𝐭 where, v = velocity, in m/s, s = displacement, in m, t = time taken, in s Both speed and velocity are measured in metres per second (m/s or m s-1). If the direction of a moving object is changing, its speed may be constant but since its direction is changing its velocity will not remain the same. Worked example: N B 5000 m 3000 m A © 1995-2017 Dushan [δβ] BOODHENA 12 The distance by road from town A to town B is 5000 m. B is situated 3000 m to the North of A. If a car takes 1000 s to travel from town A to town B by road, calculate: (a) its speed, distance travelled 5000 speed = = = 5 m/s ← time taken 1000 (b) its velocity s 3000 velocity, v = = = 3 m/s due North ← t 1000 Note: We can also state the velocity as ‘3 m/s along AB’. Average speed and average velocity An object does not always move at a constant speed or constant velocity all the time. At times it stops, at other times it moves with different speeds. For example, a car may have to stop at the traffic lights but the driver can catch up by moving faster! The speed or velocity of a body at a precise instant is called its instantaneous speed or instantaneous velocity. For a car the instantaneous speed can be read on its ‘speedometer’. For the whole trip or journey of a moving object, we prefer to use the terms ‘average speed’ or ‘average velocity.’ Average speed is the ratio of the total distance travelled to the total time taken: 𝐚𝐯𝐞𝐫𝐚𝐠𝐞 𝐬𝐩𝐞𝐞𝐝 𝐢𝐧 𝐦/𝐬 = 𝐭𝐨𝐭𝐚𝐥 𝐝𝐢𝐬𝐭𝐚𝐧𝐜𝐞 𝐭𝐫𝐚𝐯𝐞𝐥𝐥𝐞𝐝, 𝐢𝐧 𝐦 𝐭𝐨𝐭𝐚𝐥 𝐭𝐢𝐦𝐞 𝐭𝐚𝐤𝐞𝐧, 𝐢𝐧 𝐬 Average velocity is the ratio of the total displacement to the total time taken: 𝐚𝐯𝐞𝐫𝐚𝐠𝐞 𝐯𝐞𝐥𝐨𝐜𝐢𝐭𝐲 = 𝐭𝐨𝐭𝐚𝐥 𝐝𝐢𝐬𝐩𝐥𝐚𝐜𝐞𝐦𝐞𝐧𝐭 𝐭𝐨𝐭𝐚𝐥 𝐭𝐢𝐦𝐞 𝐭𝐚𝐤𝐞𝐧 For an object which is undergoing uniform acceleration: 𝐮+𝐯 < 𝐯 >= 𝟐 where <v> = average velocity, in m/s, u = initial velocity, in m/s, v = final velocity, in m/s Acceleration Acceleration is the rate of change of velocity per unit time. Acceleration is measured in metres per second per second, simply written as m/s2 or m s-2. 𝐯−𝐮 𝐚= 𝐭 where, a=acceleration, in m/s2 or m s-2, u=initial velocity, in m/s, v=final velocity, in m/s, t=time taken, in s © 1995-2017 Dushan [δβ] BOODHENA 13 Acceleration gives us an idea of how fast a moving object is changing its velocity with time. For example, if an object is dropped on Earth, its downward velocity will increase by about 10 m/s after each second. We say that the acceleration due to gravity (also known as the acceleration of free fall) is about 10 m/s per s, or simply 10 m/s2, directed vertically downwards. A positive value of acceleration means that the object is speeding up: we say that the object is undergoing acceleration. A negative value of acceleration means that the object is slowing down: we say that the object is undergoing retardation. Distance-time (s-t) graphs (Optional) A distance-time graph is a graph of distance (y-axis) against time (x-axis). It tells us the position of an object at different times during its journey. For linear, unidirectional motion, the gradient of a distance time graph gives the speed of the object: 𝐬𝐩𝐞𝐞𝐝 = 𝐠𝐫𝐚𝐝𝐢𝐞𝐧𝐭 𝐨𝐟 𝐝𝐢𝐬𝐭𝐚𝐧𝐜𝐞 − 𝐭𝐢𝐦𝐞 𝐠𝐫𝐚𝐩𝐡 = 𝐫𝐢𝐬𝐞 𝐫𝐮𝐧 Below is the graph of an object which is not moving. The object is said to be ‘at rest’ and the graph is a horizontal line placed at any height above, below or even on the x-axis: s/m t/s The graph shown below is obtained for an object moving with uniform or constant speed. Its gradient is the same anywhere along the line: s/m t/s © 1995-2017 Dushan [δβ] BOODHENA 14 Worked Example: Below is a distance-time graph for a person in a room. distance in m 2 1.5 1 0.5 rise 0 1 run 2 3 4 5 6 7 8 9 time in s Describe her journey. She walks for 2 seconds and then stops for 2 more seconds. She then walks faster for one second before stopping for a further 4 seconds. Calculate her initial speed. initial speed = gradient of graph = rise 0.5 = = 0.25 m/s ← run 2 Speed-time (v-t) graphs A speed-time graph is a graph of speed (y-axis) against time (x-axis). For an object which is undergoing linear, unidirectional motion, the gradient of its speed-time graph gives its acceleration. acceleration, in m/𝐬𝟐 = gradient= rise run For an object which is undergoing linear, unidirectional motion, the area under its speed-time graph gives its distance travelled. area under v-t graph=displacement, in m Below is the graph for an object at rest. Both its speed and its acceleration are zero: v/ms-1 t/s © 1995-2017 Dushan [δβ] BOODHENA 15 The graph shown below is obtained for an object moving with uniform or constant speed. There is no change in speed and so its acceleration is zero: v/ms-1 t/s Below is the graph for an object moving with uniform or constant acceleration. After each second, a constant speed is being added to its previous speed: v/ms-1 t/s The graph shown below is for an object moving with uniform or constant deceleration or retardation. After each second, a constant speed is being subtracted from its previous speed: v/ms-1 t/s © 1995-2017 Dushan [δβ] BOODHENA 16 Worked Example: A car starts from rest and accelerates uniformly for 5 s and reaches a speed of 20 m s-1. It continues to move with this speed for 10 s till it comes to rest with uniform deceleration for a further 5 s. (a) Sketch a speed-time graph to illustrate the motion of the car. speed in m/s 30 20 rise (negative) 10 0 5 10 15 run 20 time in s (b) Calculate the deceleration of the car. rise −20 Acceleration = gradient = = = −4 run 5 ∴Deceleration = 4 m/s2← (c) What is the total distance travelled by the car? Total distance travelled=area under v-t graph (trapezium) 1 2 1 (10 + 2 = (sum of parallel sides)x(perpendicular distance between them) = 20) x 20 = 300 m← (not m2 !!!) (d) Calculate the average speed of the entire journey. total distance travelled 300 Average speed = = = 15 m/s ← total time taken 20 © 1995-2017 Dushan [δβ] BOODHENA 17 Length of path between two points, regardless of direction Scalar quantity Measured in metres (m), e.g 22 m definition has magnitude or size only has both magnitude and direction Distance and direction between two points unit Vector quantity measured in (m), e.g 22 m to the North definition unit Distance Displacement Scalar quantity Measured in m/s, e.g 65 m/s upwards Vector quantity Measured in m/s, e.g 65 m/s unit Scalar quantity average speed total dis tan ce travelled vector quantity unit formula Motion Speed Velocity formula average velocity total time formula total time taken formula definition speed total displacement dis tance travelled definition velocity displacement time taken time taken Speed-time graph Distance-time graph distance travelled per unit time Acceleration formula definition formula speed=gradient rise run Graph of distance (yaxis) against time (xaxis) displacement per unit time Rate of change of velocity per unit time v u a t unit Measured in m/s2, e.g acceleration due to gravity=10m/s2 vertically downwards Vector quantity formula formula displacement=area under v-t graph Graph of speed (yaxis) against time (xaxis) acceleration=gradient rise run © 1995-2017 Dushan [δβ] BOODHENA 18 CHAPTER THREE: ENERGY The SI unit of energy Energy is the capacity to do work. It is measured in a unit called joule (J). In Physics, work is said to be done when an object is made to move or when a moving object is stopped. The various forms of energy The commonly known forms of energy are mechanical energy, heat, light, sound, electrical energy, chemical energy or nuclear energy. But all forms of energy can be classified into two broad types namely potential energy or kinetic energy. Energy can change from one form to another during a process. For example, hydroelectric generation, which is the generation of electrical energy from flowing water, involves interchanges of various forms of energy. We can represent this change using a flow diagram as shown below: Potential energy of water At the top of a dam Kinetic energy of water At bottom of the dam Electrical energy From generator Renewable and non-renewable sources of energy Some energy sources like biomass, hydroelectric energy, solar energy, tidal energy, wind energy are called renewable energy or alternative energy sources. They refer to usable energy derived from replenishable sources. Fossil and nuclear fuels are, on the other hand, non-renewable or finite energy sources. One day they will be completely used up. The need for alternative energy sources Fossil fuels include coal, petroleum, natural gas and heavy oils. They will all be used up in less than two centuries. Burning fossil fuels can also cause air pollution and lead to global warming by the emission of greenhouse gases like carbon dioxide. Global warming refers to the measurable increases in the average temperature of the Earth’s atmosphere, oceans and landmasses. Gases like carbon dioxide, methane or nitrogen dioxide can trap too much of the Sun’s heat and cause global warming. Carbon dioxide and nitrogen dioxide come from the burning of fossil fuels like petroleum, coal and natural gas (methane). Global warming will cause a rise in the sea level. It will also be responsible for destructive climatic conditions. This will eventually cause the extinction of many plant and animal species. This is why there is a need for alternative energy sources for the future of humankind. © 1995-2017 Dushan [δβ] BOODHENA 19 Advantages and disadvantages of different energy sources Biomass The term biomass refers to plant materials and animal waste used especially as a source of fuel. Biomass energy is derived from five distinct energy sources: garbage, wood, waste, landfill gases, and alcohol fuels. They are renewable sources of energy but are thought to contribute to global warming by the emission of carbon dioxide gas. Hydroelectric energy It refers to electricity produced from generators driven by water turbines that convert the potential energy of falling or fast-flowing water to mechanical energy. Although the electricity produced is cheap, it costs a lot to build such power stations. Solar energy Solar energy refers to radiation from the Sun capable of producing heat, causing chemical reactions, or generating electricity. Although it is a cheap source of energy and does not cause pollution, it is available only during the day. Tidal energy Tidal energy refers to any form of renewable energy in which tidal action in the oceans is converted to electrical energy. Although it is cheap it costs a lot to build them. But it can disrupt estuarine ecosystems during their construction and operation. Wind energy Wind energy is a form of energy conversion in which turbines convert the kinetic energy of wind into mechanical or electrical energy. Although it is cheap, it is only available intermittently. Chemical energy (fossil fuels) Chemical energy is easy to obtain simply by burning fossil fuels but they cause pollution and lead to global warming. Fossil fuels are also non-renewable sources of energy. Nuclear energy Nuclear energy, also called atomic energy refers to energy that is released in significant amounts in processes that affect atomic nuclei, the dense cores of atoms. Large amounts of energy are produced from small amounts of nuclear fuels but the danger of nuclear accidents is ever present. Nuclear fuels are also non-renewable sources of energy. Work done Work is said to be done when a force produces movement. Work done is a measure of the energy that we need to move an object or to stop a moving object and, like all other forms of energy, it is also measured in joules. 𝐖=𝐅𝐬 where, W = work done , in J, F = force, in N, s = displacement in direction of force, in m © 1995-2017 Dushan [δβ] BOODHENA 20 Worked Example: What is the amount of energy required to push an object horizontally over a distance of 100 m with a force of 40 N? Energy required = work done, W = F s = 40 x 100 = 4000 J or 4 kJ ← Kinetic energy The kinetic energy is the energy that a body has because it is moving. EK = 1 m v2 2 where EK = kinetic energy, in J, m = mass of the body, in kg, v = final velocity of the body, in m/s Thus, wind energy is itself a form of kinetic energy. Worked example: Calculate the kinetic energy possessed by a shooting star of mass 0.001 kg moving at a speed of 100 m/s. 1 1 Kinetic energy, EK = mv 2 = x 0.001 x (100 x 100) = 5 J ← 2 2 Potential energy The potential energy is the energy that a body has because of its position or state. 𝐄𝐏 = 𝐦 𝐠 𝐡 where EP = gravitational potential energy, in J, m = mass of the body, in kg, g = gravitational field strength=10 N kg-1, h = height above a reference level, in m Thus, chemical or fuel energy is a form of potential energy as it involves the regrouping of atoms. Nuclear energy is also potential energy at the subatomic level. A spring or elastic band also has ‘elastic potential energy’ when wound or stretched. Worked example: A climber ascends a mountain top of height 1200 m. If the climber has a mass of 60 kg, deduce (a) the gravitational potential energy he acquires, Gravitational potential energy, EP = m g h = 60 x 10 x 1200 = 720 000 J or 720 kJ ← (b) where this energy comes from. From the chemical energy stored in his body as food. The law of conservation of energy Energy can neither be created nor destroyed in any process. It can be converted from one form into another or transferred from one body to another, but the total amount remains constant. © 1995-2017 Dushan [δβ] BOODHENA 21 Worked Example: A ball of mass 12 kg is released from a height of 10 m above the ground (position A). A B 10 m C (a) What type of energy does the ball have at B? Both potential and kinetic energies. (b) Calculate the velocity with which the ball hits the ground at C. Neglecting energy losses, EP of ball at A=EK of ball at C 1 mgh= mv 2 2 2 v =2gh velocity with which ball hits the ground, v=√2gh=√2x10x10=14 ms-1 (c) Suggest why the velocity you have calculated in (b) is usually different from its actual value, and state the energy changes that take place. Most of the potential energy of the ball is converted into kinetic energy, while some energy is lost, mainly as heat, in overcoming frictional forces. Power Power is the rate of doing work or energy converted. 𝐏= 𝐖 𝐄 𝐨𝐫 𝐭 𝐭 where P = power, in joules per second (J s-1) or in watts (W), W = work done, in J, E = energy converted, in J, t = time taken, in s Thus, an electric bulb having a power of 60 W means that each second the bulb converts 60 J of electrical energy into heat and light. © 1995-2017 Dushan [δβ] BOODHENA 22 Worked example: A car is moved through a horizontal distance of 25 m by a force of 800 N in 4 s. Calculate the power developed in doing so. W F s 800 x 25 Power developed, P = = = = 5000 W or 5 kW ← t t 4 Ways and means to save energy We need to save energy so that our non-renewable resources can last longer on the planet and also to combat the problem of climate change which is a consequence of global warming. To save electrical energy 1. Replace electrical heaters by solar heaters. 2. During the day, replace electrical lighting by natural lighting, by opening all window and door curtains. Use energy-efficient light bulbs and fluorescent tubes rather than using filament bulbs. 3. Use energy-efficient refrigerators and other electrical appliances. 4. Turn off unnecessary lighting and appliances as soon as they are no longer needed. To save fuel energy 1. Drive slowly since driving fast burns more fuel for the same distance covered. 2. Keep your car well cleaned and your tyres well inflated to reduce fuel wastage. 3. Use public transport rather than cars. (For a particular journey, one bus will burn much less fuel for 60 passengers, compared to the amount of fuel that would be burnt if each of the 60 passengers was travelling in his private car). 4. Promote alternative forms of transport like the bicycle. 5. Go to a place on foot whenever you can. © 1995-2017 Dushan [δβ] BOODHENA 23 Replace electrical heaters by solar heaters Replace electrical lighting by natural lighting Use energy efficient electrical appliances Work is done when a force produces movement light W Fs sound heat mechanical Turn off unnecessary lighting or appliances electrical chemical Promote alternative forms of transport Measured in joules (J) Can neither be created nor destroyed formula definition of work done nuclear Use public transport conservation of energy Can be broadly classified as potential energy or kinetic energy unit Forms of energy Plant materials and animal wastes used as fuel Capacity to do work Renewable source of energy energy that a body has because of its position or state Saving energy definition Biomass May contribute to global warming EP m g h formula Potential energy Energy Energy produced by running water Electricity produced is cheap Hydroelectric energy energy that a body has because it is moving definition Kinetic energy formula EK Costs a lot to build power station 1 m v2 2 Solar energy definition Power Cheap and clean source of energy Nuclear energy W t Measured in watts (W) Chemical energy Energy released from processes involving atomic nuclei Kinetic energy of winds converted into electricity Cheap source of energy Costs a lot to build the power station P unit Wind energy Cheap source of energy formula Tidal energy Not available at night Energy of tidal action converted into electricity rate of doing work or energy converted Sources of energy Energy from radiation of the Sun Not available at all times Energy obtained by burning fossil fuels Easy to obtain Non-renewable source of energy Non-renewable source of enegy Causes pollution and leads to global warming Potential nuclear accident hazard Large amounts of energy obtained from small amounts of nuclear fuels © 1995-2017 Dushan [δβ] BOODHENA 24 CHAPTER FOUR: OPTICS Light travels in straight lines Light is a form of energy which can be detected by the eyes. It is made up of electromagnetic waves which can travel at a speed of 300,000,000 m/s in a vacuum. Experiment 8: To show that light travels in a straight line. light source cardboard tiny hole observer If the observer is to see the lamp, the holes in the cardboards must all be perfectly aligned. This shows that light travels in a straight line. Luminous and non-luminous bodies A luminous body is one which emits light because it is usually very hot. Examples of luminous bodies are the Sun, a flame, a lamp or a television screen. A non-luminous body is one which does not emit visible light, although it may emit infrared or ultraviolet rays. Non-luminous objects are only visible because they reflect light incident on them. Examples of non-luminous bodies are the Moon, a table or a cinema screen. The laws of reflection A normal is an imaginary line drawn at right angle to the mirror where the incident ray strikes the mirror. Experiment 9: To demonstrate the first law of reflection. The first law of reflection states that the incident ray, the reflected ray and the normal to the surface, all lie on the same plane. plane mirror i r incident ray reflected ray normal The angle of incidence is the angle between the incident ray and the normal. The angle of reflection is the angle between the reflected ray and the normal. Experiment 10: To demonstrate the second law of reflection. © 1995-2017 Dushan [δβ] BOODHENA 25 The second law of reflection states that the angle of incidence is equal to the angle of reflection, that is: 𝐢=𝐫 where i= angle of incidence, in degrees, r=angle of reflection, in degrees. Worked Example: A ray of light is incident on the surface of a mirror at an angle of 300, measured from its surface. (a) What is the angle of incidence? (b) Hence, deduce the angle of reflection. normal i r 300 (a) Angle of incidence, i = 90 — 30 = 600← (b) Angle of reflection, r = i = 600← Common applications of reflection of light Lateral inversion Experiment 11: To demonstrate the characteristics of the image formed in a plane mirror. Some characteristics of the image formed in a plane mirror are: 1. The image is of the same size as the object. 2. The image is virtual, that is, it cannot be formed onto a screen. 3. Image distance from the mirror = object distance from the mirror. 4. The image is upright. 5. The image is laterally inverted (left-to-right inversion). Because the image formed in a plane mirror is laterally inverted you will see the words POLICE or AMBULANCE painted on their vehicles as: ECILOP and ECNALUBMA When these vehicles are behind a driver, the latter can read these two words in his rear-view mirror as if they were correctly written. © 1995-2017 Dushan [δβ] BOODHENA 26 Rear-view mirror and blind spot The truck driver can see cyclist B overtaking but cannot see cyclist A A B The rear-view mirrors of motor vehicles allow the driver to see what is happening at the back. However, if a bicycle is overtaking the vehicle too close to it, the driver may not see the cyclist. This is a potential danger for the latter. This is why rear-view mirrors should be properly adjusted to enable the driver to see not only what is happening behind, but also what is happening close to the sides of the vehicle. The periscope mirror 1 laterally inverts image object image mirror 2 puts the image right A periscope is an assembly of two parallel mirrors, both placed at 450, which enables us to look over high obstacles. Periscopes can be used in submarines to see what is happening at the surface of the sea, without having to float to the surface. There is no lateral inversion when we look through a periscope. © 1995-2017 Dushan [δβ] BOODHENA 27 Ray diagram for a point object reflected in a mirror M I O observer To show the formation of an image in a plane mirror: 1. Using set-squares, drop a perpendicular from the object through the mirror. 2. Using compasses locate the image I, using the fact that image distance is equal to the object distance from the mirror. 3. Draw rays of light from the image to the observer’s eye. The lines should be dotted behind the mirror and continuous in front of it. 4. Draw rays from the object to the mirror and show, using arrows, how it reaches the observer’s eye. To the observer, it appears as if the rays are coming from the image. Worked example: A man sits in an optical chair, looking into a plane mirror which is 2.5 m away from him, and views the image of a chart which faces the mirror and is 0.5 m behind his eyes. How far away from his eyes does the chart appear to be? chart’s image man’s image mirror chart man 0.5 2.5 2.5 0.5 The chart appears to be 2.5 + 2.5 + 0.5 = 5.5 m from his eyes← © 1995-2017 Dushan [δβ] BOODHENA 28 Refraction of light Experiment 12: To demonstrate refraction through a glass block. Refraction is the bending of light as it passes from one transparent material into another. This takes place because light has different speeds in different optical media. normal incident ray angle of incidence, i vacuum/air optically less dense medium optically denser medium glass angle of refraction, r refracted ray Laws of refraction The first law of refraction states that the incident ray, the refracted ray and the normal to the surface, all lie on the same plane. The angle of incidence is the angle between the incident ray and the normal. The angle of refraction is the angle between the refracted ray and the normal. The second law, which is also known as Snell’s law, states that for two optical media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. An optical medium is a transparent medium through which light can pass. Snell' s law, sin i =constant sin r When a ray of light is passing from air to glass or water, it will bend towards the normal. However, if the same ray of light is passing from glass or water to air, it will bend away from the normal. Note: The phenomenon of refraction is not observed when the angle of incidence is zero degree. © 1995-2017 Dushan [δβ] BOODHENA 29 Common applications of refraction of light Real depth and apparent depth pool appears less deep to observer air water apparent depth real depth bottom of pool Each year, many people get drowned in swimming pools or in the sea because they have wrongly estimated the depth of water. This is due to refraction which causes the water to appear less deep than it actually is. If you want to estimate the depth of water in a pool multiply its apparent depth by 1.5. Lenses Experiment 13: to demonstrate the action of a converging and that of a diverging lens. A lens is a piece of transparent material which is used to converge or diverge rays of light by refraction, depending on their applications. A converging lens is thicker in the middle and it will converge a parallel beam of light: converging lens Converging lenses are used in telescopes, cameras, photocopying machines and in slide projectors. There is a converging lens in the human eye too. Converging lenses can also be prescribed as ‘glasses’ to persons suffering from hypermetropia or long-sightedness. Long-sighted persons cannot see nearby objects clearly unless they wear glasses made of converging lenses. © 1995-2017 Dushan [δβ] BOODHENA 30 A diverging lens is thinner in the middle and it will diverge a parallel beam of light: diverging lens Diverging lenses are used as the eyepiece of some terrestrial telescopes but they are more often prescribed as ‘glasses’ for the correction of myopia or short-sightedness. Short-sighted persons cannot see far-away objects clearly unless they wear glasses made of diverging lenses. Worked Example: The diagram below shows a ray of light incident on a glass prism. triangular prism (a) Copy and complete the diagram to show how the ray of light is refracted into the glass. Label the incident ray, the normal, the refracted ray, the angle of incidence (i) and the angle of refraction (r), (b) Draw the ray of light as it emerges out of the other face of the prism. Label this ray as the ‘emergent ray’. normal i refracted ray r incident ray emergent ray triangular prism © 1995-2017 Dushan [δβ] BOODHENA 31 Form of energy detected by the eyes Water appears shallower than it actually is Travels at 300,000,000 m/s in vacuum One which emits light Sun, flame, lamp, television screen Travels in straight lines One which does not emit visible light speed Lenses used as glasses, in telescopes, in microscopes and in prisms definition examples Properties of light definition Visible only because they reflect light incident on them Luminous bodies Moon, table, cinema screen examples Applications of refraction Non-luminous bodies Optics Incident ray, reflected ray and normal, all lie on same plane Laws of reflection first law Ratio of sine of angle of incidence to sine of angle of refraction is a constant, i.e sin i cons tant sin r Laws of refraction First law Incident ray, reflected ray and normal, all lie on same plane Second law second law Applications of reflection Characteristics of image formed in plane mirror angle of incidence = angle of reflection, i.e i=r Image is of the same size as object Vehicles have words written laterally inverted on them so that they appear correctly in rear-view mirror Image is laterally inverted The periscope Rear-view mirror and blind spot Image is virtual Image distance from mirror=object distance from mirror Image is upright © 1995-2017 Dushan [δβ] BOODHENA 32 CHAPTER FIVE: ELECTRICITY Electrical charge and electric current Experiment 14: To distinguish between electrical conductors and electrical insulators. Matter is made up of tiny particles called atoms. In its centre or nucleus, an atom contains two types of tinier particles, called protons and neutrons. Another type of particles, called electrons, moves round this nucleus in circular orbits. Protons are said to be positively charged. Electrons are negatively charged. Neutrons are said to be electrically neutral. Electrical charges are measured in a unit called coulomb (C) and they are responsible for phenomena like lightning or electric currents that will make many electrical appliances run. Conductors and Insulators An electrical conductor is a material that allows electricity to pass through it. Conductors contain a large number of free electrons per unit volume. Examples are metals and graphite. An electrical insulator is a material that does not allow electricity to pass through it. Insulators do not contain enough free electrons per unit volume. Examples are glass, silk and plastics. Current Experiment 15: To measure current using an ammeter. Current is defined as the rate of flow of charge per unit time. 𝐈= 𝐐 𝐭 where, I = current, in ampere (A), Q = charge, in C, t = time taken, in s An electric current flow is similar to water flow through a pipe: just as we can measure the volume of water flowing in one second for the water, we can also measure the number of electric charges passing a point in one second. Electric current is measured with an instrument called an ammeter, which is always connected in series. Worked example: A conducting sphere which has a charge of 120 C is earthed. It takes 60 s for the sphere to discharge completely. (a) Calculate the current flowing. Q 120 Current, I = = =2A← t 60 (b) With which instrument will you measure this current and how will you connect it? With an ammeter connected in series. © 1995-2017 Dushan [δβ] BOODHENA 33 Potential difference Experiment 16: To measure potential difference using a voltmeter. We define potential difference between two points as the energy converted from electrical into other forms when one coulomb of charge passes between the points. V= W Q where V=e.m.f, p.d or voltage, in volts (V), W=work done, in J, Q=charge, in C. Electric charges have to be given energy so that they can move inside wires. It is this same electrical energy that will be converted into other forms of energy, for example, heat and light through a bulb. This energy, that each coulomb of charge has, is called the voltage, potential difference (p.d) or electromotive force (e.m.f). Potential difference is measured using an instrument called a voltmeter, which is always connected in parallel. Water will normally flow down a pipe because the two ends of the pipe are at different heights. The larger the difference in height between its two ends, the faster the water will flow. Similarly, the more energy we give to charges in a wire, the faster they flow in it: the larger the potential difference applied across the wire, the larger will be the current. This was discovered by a scientist known as Ohm. Worked example: A coil of copper is connected across the terminals of a battery of e.m.f 6 V. What is the chemical energy transferred when 30 C of charge passes round the circuit? Using V = W Q Total energy transferred, W = VQ = 6 x 30 = 180 J ← Electrical resistance and Ohm’s law Electrons do not flow smoothly inside a metallic wire. In fact, they often collide against the metal atoms and the latter offer resistance to their motion inside the wire. This opposition to the flow of electrons and hence the current flow is called electrical resistance. If the metal is heated the situation becomes worse for the electrons: now the metal atoms vibrate faster and makes it more difficult for the current to pass through. A temperature rise increases the resistance of a metal. But if the temperature remains fixed, the current flowing through the metal will double if we double the potential difference across its ends. This is known as Ohm’s law which states that the current flowing through a metal conductor is directly proportional to the potential difference across its ends provided the temperature remains constant. © 1995-2017 Dushan [δβ] BOODHENA 34 The resistance of a material is defined as the ratio of the potential difference applied across it to the current flowing through it. 𝐕 = 𝐈𝐑 where V=potential difference, in V, I=current, in A, R= resistance, in ohms (Ω) Experiment to determine the resistance of a metallic conductor Experiment 17: To determine the resistance of a fixed resistor. battery The apparatus is set up as shown. switch Variable Resistor (rheostat) R ammeter A V voltmeter The rheostat is set to maximum resistance so that a low current flows in the circuit. The circuit is briefly closed and the voltmeter reading (V) and the ammeter reading (I) are taken. The resistance of the rheostat is decreased in steps and several sets of potential differences and their corresponding currents are noted. The results are tabulated as shown below: Experiment Number Voltmeter reading V/V Ammeter reading I/A A graph of potential difference (y-axis) against current (x-axis) is plotted as shown below: p.d/V current/A The gradient of the graph gives the resistance of the metallic conductor in Ω. © 1995-2017 Dushan [δβ] BOODHENA 35 Resistors in series Experiment 18: To demonstrate connections in series. Resistors are tiny devices, often cylindrical in shape and with coloured bands on them. They are used to control the current flowing in a circuit or to control the potential difference across them. If your mobile phone operates on 3.6 V and you connect a battery of 4.5 V across it, it will be damaged after some time. This is where resistances come into play: by arranging these resistances in a specific way we can get exactly 3.6 V from a 4.5 V supply, and your mobile phone will last a lot longer! The diagram shown below shows three resistors connected in series to a battery. direction of current A R1 R2 R3 V1 V2 V3 We can connect the ammeter anywhere round the circuit because the current is the same. The p.d, however, does not remain the same. In fact, p.d across the whole circuit=sum of p.d’s in the series circuit 𝐕 = 𝐕𝟏 + 𝐕𝟐 + 𝐕𝟑 But since the same current is flowing, the combined or effective resistance of all the resistors is given by, 𝐑 = 𝐑𝟏 + 𝐑𝟐 + 𝐑𝟑 We can therefore connect resistors in series if we need a higher combined resistance. © 1995-2017 Dushan [δβ] BOODHENA 36 Resistors in parallel Experiment 19: To demonstrate connections in parallel. The diagram shown below shows three resistors connected in parallel to a battery. V I A I1 I2 I I3 R1 R2 R3 The p.d across the battery is equal to the p.d across each of the resistors: 𝐕 = 𝐕𝐑 𝟏= 𝐕𝐑𝟐 = 𝐕𝐑 𝟑 Connections in the home are made in parallel so that the voltage remains the same. The current, however, is not the same through each resistor: main current=sum of currents through each resistor 𝐈 = 𝐈𝟏 + 𝐈𝟐 + 𝐈𝟑 But since the p.d does not change, the combined or effective resistance of the resistors in parallel is given by: 1 1 1 1 = + + R R1 R2 R3 We can therefore connect resistors in parallel if we need a lower combined resistance. Worked example: A number of 8 Ω resistors are available. Draw diagrams to show how you could connect a suitable number of resistors to give an effective resistance of (a) 24 Ω X 8Ω 8Ω 8Ω Y Effective resistance across XY, RXY=8+8+8=24 Ω← © 1995-2017 Dushan [δβ] BOODHENA 37 (b) 4 Ω 8Ω X Y 8Ω 1 1 1 2 = + = R XY 8 8 8 8 Effective resistance across XY, R XY = 2 =4Ω← (c) 18 Ω Connections in series can give the 16 Ω while the remaining 2 Ω can be obtained by a parallel connection. 8Ω 8Ω X 8Ω 8Ω Y 8Ω 8Ω 1 8 1 8 1 8 1 8 Effective resistance across XY, R XY = 8 + 8 + ( + + + )−1=18 Ω← Note: We can connect cells in series or in parallel. Connecting cells in series makes a battery of greater e.m.f, but connecting them in parallel allows a larger current to flow in the circuit, although it provides the same e.m.f. 3.0 V battery made of two 1.5 V cells in series Two 1.5 V cells in parallel © 1995-2017 Dushan [δβ] BOODHENA 38 Direct current (D.C) circuits A direct current or d.c is a current which flows in one direction only, as opposed to an alternating current (a.c) which constantly changes its direction of flow. Direct currents can be obtained from cells, batteries or d.c power sources. A simple direct current or d.c circuit is an electrical circuit that consists of a combination of batteries and resistors. Worked example: The circuit shown below consists of two 1.5 V cells connected to two identical lamps, each one labelled 3.0 V. Each lamp has a resistance of 15 Ω at normal brightness. 1.5 V 1.5 V — Dushan® + — Dushan® + lamp A copper wires lamp B (a) How are the cells connected? In series. (b) (i) How are the lamps connected? In parallel. (ii) What are the advantages of connecting the lamps in this way? Both lamps will light up at normal brightness. If one lamp burns out, the other lamp will continue to light up. (c) (i) Use a circuit diagram to represent the circuit shown above. 3.0 V battery — + 0.4 A Lamp A Lamp B 0.2 A 0.2 A (ii) Calculate the current flowing through lamp A. Voltage of the battery= 1.5 + 1.5 = 3.0 V Since the lamps are connected in parallel, voltage across each lamp = 3.0 V. © 1995-2017 Dushan [δβ] BOODHENA 39 Using V = I R, V 3.0 = = 0.2 A ← R 15 (iii) What is the current flowing through the battery? Explain your reasoning. A current of 0.2 A flows through lamp A and a current of 0.2 A flows through lamp B. Both currents add up to (0.2 + 0.2) or 0.4 A and flow through the battery. ∴Current through the battery (also called ‘main current’) = 0.4 A← Current through lamp 𝐀, I = Note: Current always flows from the positive terminal to the negative terminal of a battery. Safe use of electrical energy Experiment 20: Wiring the three-pinned plug of an appliance to protect the appliance and its user. DANGERS HAZARDS PREVENTIVE MEASURES Damaged insulation Live wire can come into contact with the casing of an electrical appliance, making it live. Overheating of cables may melt the insulation causing a short circuit, which may start a fire. Wet insulators will no longer be good insulators, because impure water is a good conductor. Earthe the metal casing and put a suitable fuse and a switch in the live wire. Too many equipment connected to one plug Damp conditions Use a suitable fuse to prevent an excessively large current from flowing through the cable. Appliance must be earthed. Saving electrical energy Much of our electricity is being generated by burning fossil fuels which are non-renewable and are also responsible for global warming. We therefore need to save electricity if we want to live in a healthy environment and if we want to avoid an energy crisis. To save electrical energy: 1. Replace electrical heaters by solar heaters. 2. During the day, replace electrical lighting by natural lighting, by opening all window and door curtains. Use energy-efficient light bulbs and fluorescent tubes rather than using filament bulbs. 3. Use energy-efficient refrigerators and other electrical appliances. 4. Turn off unnecessary lighting and appliances as soon as they are no longer needed. © 1995-2017 Dushan [δβ] BOODHENA 40 measured in amperes (A) with an ammeter connected in series rate of flow of charge per unit time Allow electricity to pass through them definition Metals and graphite formula energy converted from electrical into other forms when one coulomb of charge passes between the points measured in volts (V) with a voltmeter connected in parallel unit examples definition Conductors Do not allow electricity to pass through them Glass, silk and plastics formula unit Electric current examples Potential difference Insulators current is directly proportional to voltage for ohmic conductors Ohm s law Electricity Earthe the metal casing of an appliance Electrical resistance and ohm's law Safe use of electrical energy formula V=IR unit Put a switch in the live wire Resistors in series Resistors in parallel Same current Put a suitable fuse in the live wire application conclusion Avoid damp conditions in electrical circuits resistance measured in ohms (Ω) formula formula Same voltage formula conclusion formula Connections in the home are made in parallel To decrease resistance connect resistors in parallel To increase resistance connect resistors in series © 1995-2017 Dushan [δβ] BOODHENA 41 A acceleration .................................................................. 13 air pollution .................................................................. 19 alternating current ........................................................ 39 alternative forms of transport ...................................... 23 ammeter ....................................................................... 33 analogue stopwatch........................................................ 9 angle of incidence ......................................................... 25 angle of reflection ......................................................... 25 apparent depth ............................................................. 30 average speed ............................................................... 13 end error ......................................................................... 4 energy ........................................................................... 19 energy-efficient light bulbs ..................................... 23, 40 F first law of reflection ..................................................... 25 first law of refraction .................................................... 29 flow diagrams................................................................ 19 forms of energy ............................................................. 19 frictional forces ............................................................. 22 fuse ............................................................................... 40 B G beam balance.................................................................. 9 biomass ......................................................................... 20 blind spot ...................................................................... 27 glasses ........................................................................... 31 global warming ............................................................. 19 H C cameras ........................................................................ 30 cells in series ................................................................. 38 chemical energy ...................................................... 20, 21 clock ................................................................................ 9 conductors .................................................................... 33 connections in parallel. ................................................. 37 connections in series .................................................... 36 converging lens ............................................................. 30 cube ................................................................................ 7 cuboid ............................................................................. 7 current .......................................................................... 33 half-metre rule ................................................................ 6 human eye .................................................................... 30 hydroelectric energy ..................................................... 20 hypermetropia .............................................................. 30 I instantaneous speed ..................................................... 13 Insulators ...................................................................... 33 K kinetic energy................................................................ 21 D dead space ...................................................................... 4 digital stopwatch ............................................................ 9 direct current ................................................................ 39 displacement .......................................................... 11, 12 displacement method ..................................................... 8 distance................................................................... 11, 12 distance-time graph ...................................................... 14 diverging lens ................................................................ 31 L Lateral inversion ........................................................... 26 law of conservation of energy....................................... 21 length .............................................................................. 4 lenses ............................................................................ 30 light ............................................................................... 25 linear motion ................................................................ 11 long-sightedness ........................................................... 30 luminous body .............................................................. 25 E earthing ........................................................................ 40 elastic potential energy ................................................ 21 electrical charge ............................................................ 33 electronic balance ........................................................... 9 electrons ....................................................................... 33 M mass ................................................................................ 9 measuring tapes.......................................................... 4, 6 mercury ........................................................................... 8 metre rule ................................................................... 4, 6 © 1995-2017 Dushan [δβ] BOODHENA 42 myopia .......................................................................... 31 N negative accelerations .................................................. 14 neutrons ....................................................................... 33 non-luminous body ....................................................... 25 non-renewable energy sources .................................... 19 normal .......................................................................... 25 nuclear energy .............................................................. 20 O scalar quantity .............................................................. 11 second law of reflection ................................................ 26 second law of refraction ............................................... 29 short-sightedness .......................................................... 31 slide projectors ............................................................. 30 solar energy .................................................................. 20 solar heaters ........................................................... 23, 40 speed ............................................................................ 12 speed of light ................................................................ 25 speed-time graph .......................................................... 15 sphere ............................................................................. 7 submarines.................................................................... 27 T ohm’s law...................................................................... 34 P parallax error .................................................................. 5 periscopes ..................................................................... 27 photocopying machines ................................................ 30 potential difference ...................................................... 34 potential energy ........................................................... 21 power............................................................................ 22 protons ......................................................................... 33 R ray diagrams ................................................................. 28 real depth ..................................................................... 30 refraction ...................................................................... 29 renewable energy sources ............................................ 19 repeating measurements................................................ 4 resistance ...................................................................... 35 resistance (measurement) ............................................ 35 rheostat ........................................................................ 35 telescopes ..................................................................... 30 temperature .................................................................... 9 terrestrial telescopes .................................................... 31 thermometer .................................................................. 9 tidal energy ................................................................... 20 time ................................................................................. 9 V vector quantity.............................................................. 11 velocity .......................................................................... 12 vernier callipers........................................................... 4, 6 vernier scale .................................................................... 5 virtual images ................................................................ 26 volume ............................................................................ 7 W wind energy .................................................................. 20 work done ..................................................................... 20 S Z saving energy ................................................................ 23 saving fuel ..................................................................... 23 zero error ........................................................................ 4 © 1995-2017 Dushan [δβ] BOODHENA 43