7592678 iGCSE Cam Physics title.indd 1
15/07/2014 12:37
Contents Getting the best from the book................................4
Section 1 General physics............................................. 8 a) Length and time..................................................... 10 b) Motion....................................................................... 16 c) Mass and weight.................................................... 33 d) Density....................................................................... 39 e) Forces.......................................................................... 48 f) Momentum.............................................................. 76 g) Energy, work and power...................................... 84 h) Pressure...................................................................112 i) Exam-style questions.........................................124
Section 2 Thermal physics....................................... 132 a) Simple kinetic molecular model of matter..................................................................134 b) Thermal properties..............................................147 c) Thermal processes...............................................167 d) Exam-style questions..........................................180
Section 3 Properties of waves................................ 184 a) General wave properties...................................186 b) Light..........................................................................195 c) Sound.......................................................................222 d) Exam-style questions..........................................231
Section 5 Atomic physics......................................... 334 a) The nuclear atom..................................................336 b) Radioactivity..........................................................343 c) Exam-style questions..........................................359
Doing well in your examinations........ 363 Introduction................................................................363 Overview......................................................................363 Assessment objectives and weightings............363 Examination techniques.........................................364 Answering questions...............................................365
Developing experimental skills........... 367 Introduction................................................................367 1. Using and organising techniques, apparatus and materials....................................367 2. Observing, measuring and recording...........369 3. Handling experimental observations and data...................................................................372 4. Planning and evaluating investigations.......374
Glossary................................................................... 380 Answers.................................................................... 385 Index......................................................................... 394
Section 4 Electricity and magnetism.................... 234
3
Contents
a) Simple phenomena of magnetism................236 b) Electrical quantities.............................................246 c) Electric circuits......................................................272 d) Dangers of electricity.........................................297 e) Electromagnetic effects.....................................305 f) Exam-style questions..........................................328
92678_P002_003.indd 3
7/14/14 3:17 PM
This section covers concepts that will be important throughout your course. First, you will look at how to measure quantities such as length and time. Then you will look at speed, velocity and acceleration before considering mass, weight and density. You will then consider forces and their different effects, before looking at energy, work, power and pressure.
STARTING POINTS 1. What would you use to measure: a) the width of this book; b) the length of the school playing field; c) the amount of milk needed to make a dessert? 2. How could you find the time taken to: a) finish your physics homework; b) run 100 metres? 3. What do we mean when we say a car is travelling at 30Â kilometres per hour? 4. If an object is stationary, What must be true about the forces acting on a stationary object? 5. In physics, what do we mean when we say an object is accelerating? 6. How are mass and density related? 7. List four different forms of energy.
CONTENTS a) Length and time b) Motion c) Mass and weight d) Density e) Forces f) Momentum g) Energy, work and power h) Pressure i) Exam-style questions
92678_P008_131.indd 8
7/8/14 10:40 AM
1
General physics
∆∆In this topic you will learn about the forces at work on this parachutist.
92678_P008_131.indd 9
7/8/14 10:41 AM
Length and time INTRODUCTION
Making measurements is very important in physics. Without numerical measurements, physicists would have to rely on descriptions, which could lead to inaccurate comparisons. Imagine trying to build a house if the only descriptions were ‘big’ and ‘small’.
∆∆Fig. 1.1 Using a micrometer.
You also need to make sure that you are consistent in your use of units. For example, the Mars Climate Orbiter mission failed in 1999 because not all of the scientists were using the same units.
KNOWLEDGE CHECK ✓✓Know how to use a rule to measure lengths to the nearest millimetre. ✓✓Know how to use a stopwatch to measure time to the nearest second. ✓✓Know how to use a measuring cylinder to measure volume.
MAKING MEASUREMENTS When making measurements, physicists use different instruments, such as rules to measure lengths, measuring cylinders to measure volume and clocks to measure time. A physicist always takes care to make the measurements as accurate as possible. If she is using a rule, she will place the rule along the object to be measured, and read off the scale the positions of the beginning and the end of the object. The length is the difference between these two readings. When the rule is nearer to her eye than the object being measured, the reading will appear to change as she moves her eye. The correct reading is obtained when her eye is directly above the point being measured.
10
General physics
LEARNING OBJECTIVES ✓✓Be able to use and describe the use of rules and measuring cylinders to calculate a length or volume. ✓✓Be able to use and describe the use of clocks and devices for measuring an interval of time. ✓✓Be able to obtain an average value for a small distance and for a short interval of time by measuring multiples (including the period of a pendulum). ✓✓EXTENDED Understand that a micrometer screw gauge is used to measure very small distances.
92678_P008_131.indd 10
7/8/14 10:41 AM
correct position for eye
incorrect position for eye
∆∆Fig. 1.2 Making accurate measurements.
To improve accuracy further, she may take several readings and use the average of these readings as a better result.
11
To use a measuring cylinder, she will first make sure that the cylinder is standing on a level table. Then she will make sure that her eye is at the same level as the liquid inside the cylinder. The surface of most liquids will bend up or down near the walls of the measuring cylinder. This bent shape is known as a meniscus. However, most of the surface is flat, and measurements are made to this flat surface.
Length and time
∆∆Fig. 1.3 The Maglev train runs for 30 km between Shanghai and Pudong Airport, and completes the journey in 7 minutes, reaching a top speed of 430 km/h. The train uses magnets to hover 10 mm above the track. The track must be placed within a few millimetres of the planned route, requiring great accuracy in all measurements.
92678_P008_131.indd 11
7/8/14 10:41 AM
REMEMBER
Warning: Some measuring cylinders have unusual scales and one division may represent an unexpected quantity, perhaps 2 cm3 or 0.5 cm3. Check carefully.
In this book, volumes will usually be measured in cm3 (or perhaps in m3). In other places, such as on some measuring cylinders, you will see the millilitre. A volume of 1 ml is the same as a volume of 1 cm3. 1000 cm3 = 1000 ml = 1 l (or 1 dm3 to avoid confusion between the number 1 and the letter l) For measuring large volumes we also use the cubic metre. 1 m3 = 1000 dm3 = 1 000 000 cm3 Times are measured by using a stopwatch or stopclock. Hand-operated stopwatches have an accuracy that is limited by the delay between your eye seeing the moment to start, your brain issuing the command to start the watch and your finger pressing the start button. The total delay is typically around 0.2 s. This delay is known as your ‘reaction time’, and it increases the danger of some tasks, such as driving a car. When measuring time accurately is critical, such as in athletics, the clock has to be started and stopped automatically by the athlete breaking a light beam that shines across the track. If you are measuring the time of an oscillation, such as the swing of a pendulum, it is very easy to improve the accuracy of the measurement by timing a number of swings, perhaps 10 or 20. It is important to count correctly. Let the swing go, count zero and start the stopwatch as the pendulum crosses a mark at the bottom of the swing (we call this the fiducial mark). The next time the pendulum crosses the fiducial mark going in the same direction count one, and so on. In this way the count will be correct. After measuring the time for 20 swings, say, divide the total time by 20 to give the period of one oscillation of the pendulum.
You may need to use specialised measuring equipment. For example, the micrometer is used to measure very small distances, such as the diameter of a piece of wire. The micrometer (Fig. 1.4) is designed so that the gap between the jaws changes by 0.5 mm for every complete turn of the thimble. By measuring the exact position of the thimble when an object is being held, the thickness of the object can be measured very accurately.
12
General physics
EXTENDED
92678_P008_131.indd 12
7/8/14 10:41 AM
A physicist will always check that the jaws of the micrometer are clean, and will then check that the reading is 0 mm when the jaws are closed gently. Most micrometers allow the zero reading to be reset, but this may need to be done by a trained person. Most micrometers also have a special ratchet fitted onto the end of the thimble, which slips and emits a clicking sound when sufficient force has been applied. Extra care should be taken not to distort the object being measured if the micrometer does not have one of these. To measure the thickness of an object, open the jaws of the micrometer and close them gently onto the object in question. A scale on the barrel will show by how many complete turns the jaws have been opened, with every two turns indicating another millimetre. The scale around the edge of the thimble is calibrated from 0 to 50. So a reading of 40 indicates that a further 0.40 mm must be added to the thickness. However, be careful: a reading of 5, say, indicates that only 0.05 mm is to be added. In Fig. 1.4, the marks along the top of the line along the barrel show that the jaws have been opened to 5 mm, and the fact that an additional mark has become visible below the line shows that they are opened beyond 5.5 mm. Therefore you know that the answer must be between 5.5 mm and 6.0 mm. Next you look at the scale on the thimble. The reading of 32 shows that you must add 0.32 mm to the reading. So the final answer is 5.82 mm. thimble
∆∆Fig. 1.4 Reading a micrometer.
13
Length and time
END OF EXTENDED
92678_P008_131.indd 13
7/8/14 10:41 AM
End of topic checklist Key terms fiducial mark, meniscus, micrometer
During your study of this topic you should have learned:
❍❍How to use and describe the use of rules and measuring cylinders to calculate a length or a volume.
❍❍How to use and describe the use of clocks and devices for measuring time. ❍❍How to measure and describe how to measure a short interval of time (including the period of a pendulum).
❍❍EXTENDED How to use and describe the use of a mechanical method of
14
General physics
measuring when quantities are small (including the use of a micrometer screw gauge).
92678_P008_131.indd 14
7/8/14 10:41 AM
End of topic questions Note: The marks awarded for these questions indicate the level of detail required in the answers. In the examination, the number of marks awarded for questions like these may be different.
1. Rules that are 30 cm long are often made of wood or plastic that is thicker in the middle and thinner along the edges where the scale is printed. Explain why the user is less likely to make an error if the rule is thinner at the edge, and suggest (3 marks) reasons why the rule is thicker in the middle. 2. A plastic measuring cylinder is filled with water to the 100 cm3 mark. A student measures the column of water in the cylinder with a rule and finds that it is 20 cm high. a) The student pours 10 cm3 of the water out of the cylinder. How high will the (2 marks) column of water be now? b) The student then refills the cylinder back to the 100 cm3 mark by holding it under a dripping tap. She finds that it takes 180 drops of water to do this. What (3 marks) is the volume of one of these drops? c) What is the c ross-sectional area of the cylinder? (Hint: The volume of a cylinder (3 marks) is given by the equation: volume = cross-sectional area × length.) d) From your answer to part c), what is the internal diameter of the measuring (3 marks) cylinder? 3. A student tries to measure the period of a pendulum that is already swinging left and right. At the moment when the pendulum is fully to the left, she counts ‘one’ and starts a stopwatch. She counts successive swings each time that the pendulum returns to the left. When she counts ‘ten’ she stops the stopwatch, and sees that it reads 12.0 s. a) What was her mistake?
(2 marks)
b) What is the period of swing of this pendulum?
(3 marks)
c) In this particular experiment, explain the likely effect of her reaction time on (3 marks) her answer.
15
Length and time
4. EXTENDED What is the distance that has been measured by this micrometer thimble (1 mark) screw gauge?
92678_P008_131.indd 15
7/8/14 10:41 AM