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Description

Study Guide for Physics Mike Folland

Hodder Murray A MEMBER OF THE HODDER HEADLINE GROUP

Hodder Headline’s policy is to use papers that are natural,

renewable and recyclable products and made from wood grown in sustainable forests

The logging and manufacturing processes are expected to conform to the environmental regulations of the country of origin

Orders: please contact Bookprint Ltd,

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Oxon OX14 4SB

Telephone: (44) 01235 827720

Fax: (44) 01235 400454

Lines are open 9

Monday to Saturday,

with a 24-hour message answering service

Visit our website at www

© Mike Folland 2005 First published in 2005 by Hodder Murray,

an imprint of Hodder Education,

a member of the Hodder Headline Group 338 Euston Road London NW1 3BH Impression number 10 9 8 7 6 5 4 3 2 1 Year 2010 2009 2008 2007 2006 2005 All rights reserved

Apart from any use permitted under UK copyright law,

no part of this publication may be reproduced or transmitted in any form or by any means,

including photocopying and recording,

or held within any information storage and retrieval system,

without permission in writing from the publisher or under licence from the Copyright Licensing Agency Limited

Further details of such licences (for reprographic reproduction) may be obtained from the Copyright Licensing Agency Limited,

London W1T 4LP

Cover photo TEK Image/Science Photo Library Illustrations by Mike Humphries Typeset in Bembo 12/14pt by Pantek Arts Ltd,

Maidstone,

Kent Printed and bound in Great Britain by CPI Bath A catalogue record for this title is available from the British Library ISBN-10: 0 7195 7903 1 ISBN-13: 978 0 7195 7903 5

Contents Introduction

Topic 1

General Physics Measuring length,

velocity and acceleration Mass and weight Experiments to measure density Forces and change of size and shape Forces and change of motion Turning effect and equilibrium Centre of mass and stability Scalars and vectors Energy Work and power Pressure

Topic 2

Thermal Physics Kinetic theory of molecules Solids Liquids Everyday uses and consequences of thermal expansion Gases Measurement of temperature Changes of state Gas pressure Thermal capacity Transfer of thermal energy

Topic 3

Waves The nature of waves Water waves Light Electromagnetic spectrum Sound

Topic 4

Electricity and Magnetism Magnetism Electromagnetism Electric charge Electric current Electromotive force and potential difference Resistance Electrical energy and power Electric circuits Digital electronics Dangers of electricity The motor effect Electromagnetic induction Cathode rays

Contents

Topic 5

Atomic Physics Atomic model Radioactivity

87 87 90

Preparing for the examination

Useful equations

Answers

Certain sections of this book are shaded

This indicates Supplement material and should be studied by students following the Extended curriculum

Introduction Structure of the book This text has primarily been written to support students in the study of Physics to IGCSE

The five topics covered in this book correspond to the five topics in the IGCSE examination syllabus

The syllabus has two components,

the latter defines the Extended curriculum

The Core curriculum is graded C to G and the Extended curriculum can be graded A* to G

To differentiate between these components,

sections of this book covering the Supplement are shaded

Wherever possible,

this practice has been extended to the questions,

some of which may involve material from both the Core and Supplement

Each topic starts with a list of Key objectives which specifies the skills and knowledge you will need to have acquired during your study of this topic

The list can also serve as a checklist of your progress in this topic

Each topic also includes a number of essential terms which are defined in a table of Key definitions

The main sections of each topic are the Key ideas,

and it is essential that you both learn and understand these concepts

This is the Physics that you will need for the IGCSE examination

Common misconceptions and errors are listed to help you avoid the common mistakes made by students (see page viii)

In each topic there are questions

Sample questions are in the style of IGCSE questions

These are followed by an answer given by an imaginary student to illustrate how answers of different quality would be awarded marks

This is then followed by examiner’s comments and the correct answer

There are many Try this questions to check you have learnt the Physics

You should use these for practice and to assess your understanding and recall of the topic

You will find the answers at the back of the book

Preparing for the examination is primarily directed at those taking the external IGCSE examination,

but much of the advice offered is also relevant to internal examinations

How to use this book Who is this book for

? The purpose of this book is to aid any IGCSE Physics student to achieve the best possible grade in the exam

Ideally,

you will have good teaching and access to the textbook IGCSE Physics,

written specifically for this course by Tom Duncan and Heather Kennett

This Study Guide will give you support during the course as well as advice on revision and preparation for the exam itself

However,

not all students are in such a fortunate position

For those with only a little qualified teaching and no textbook,

this Study Guide provides all the essentials you will need to prepare yourself for the exam

This book is designed to help teachers with the course content by giving key ideas in summarised form,

Introduction

questions with comments from an experienced examiner,

as well as numerous tips and misconceptions to avoid

The book is also designed to help students when they work on their own

The material is presented in a clear,

concise form and there are questions for you to try,

many with examiner’s comments on the expected answers

When should this book be used

? One main aim of this book is to help students revise and prepare for the exam at the end of the course

However,

to get the most from the book and the whole course,

it should be used on an ongoing basis throughout the course

The Key ideas,

tips and things to avoid are best learnt as early as possible

The examples will be helpful at any stage

The book will be especially helpful to students preparing for end-of-topic tests and internal exams held at various stages during the course by their school or college

If you want to get the maximum value from this book,

it is strongly advised that you attempt to answer all the questions on paper and not in the book

Then you can repeat the exercises at intervals throughout the course

How do we learn

? We remember very little of what we have only heard

We remember a little more of what we have heard and seen

We only really learn things properly when we actually do things with the material – active learning

Not only is this more interesting and motivating for students,

learn and remember things much better this way

Active learning has always been promoted by good teachers whose lessons are not just ‘talk and chalk’

In a practical subject like Physics,

there should be demonstrations with the active involvement of students who may take readings,

discuss results and suggest further lines of enquiry

Practical work,

should ideally involve the students in the planning of the investigation

In any case,

the students are actively involved in handling the apparatus and taking readings,

and they usually go on to make calculations,

draw graphs and deduce conclusions

Study on your own should also be organised so that you are active

Simply reading through a textbook (or even this Study Guide

!) and hoping you will learn just does not work

The Preparing for the examination section on pages 96–100 will give advice in detail on how to learn and revise for tests and exams

Before you start to work with this book,

you should ideally have a separate notebook or folder in which to record your work

You might wish to make the occasional comment in your normal exercise book but there will not be space for the amount of extra work that should be done,

and some schools may not allow these extra comments

Even though your separate notebook may not be seen by anyone else,

you should take the same pride in keeping it as if it were to be marked by the strictest teacher

Introduction

As your course proceeds,

it will be useful to compare the notes in your exercise book with the topics presented here

There will be differences but this does not mean that either your notes or this book is wrong

In fact,

you will learn a lot from carefully comparing the two different versions and you will usually find that the differences are alternative ways of presenting the same information

You should make a note of such points with comments about the differences and similarities

How to answer different types of questions ● Calculations

Always show your working

The first reason for this is that if you write down all your working,

you are far more likely to work in the logical way needed to reach correct answers

students often make errors early on in a multi-part question and examiners try hard not to penalise more than once for the same error

If you continue working from an early wrong answer without making a further mistake,

you will normally not lose any more marks as long as the examiner can see clearly what you have done

Do not expect the examiner to try and guess whether you worked on correctly

Train yourself to set out work logically

We are not all naturally neat and tidy in how we work but most people can improve with a little effort

Neatness certainly helps logical working and it is important to develop this habit thoughout the course

You may think you can suddenly work logically in the exam but it will not just happen if you have not practised it

Ordinary school or college exercises,

and even work from this book which is just for you,

should be logically presented with all the steps of your working

Show the units

Sometimes the question will make it easy for you and give the units

Sometimes units will be asked for

Marks are often given for units,

so develop the habit of thinking about the units and writing them down even when not asked for

Scientific quantities are meaningless without units

It makes a lot of difference whether the speed limit on a road is 30 m/s,

Drawing graphs

The axes should be labelled with units

The scales should fill more than half the space available in each direction

Think carefully about whether the origin should or should not be included

Do not use scales with awkward multiples like 3 or 7 – there is no need to fill every last bit of the available space

Points should be plotted carefully to an accuracy of 0

This means using a sharp pencil to mark the point with a small cross or a circled dot

If the line of best fit is a straight line,

carefully judge its position and draw one thin line with a ruler

If the line is a curve,

smooth line through the points

It must not be distorted to pass through every point

Introduction

Reading off graphs

you should work to an accuracy of 0

Draw vertical and horizontal lines to the axes to show your working

Logical thinking

The important thing is not to abandon logic just because words are involved,

Many questions require step-by-step descriptions and/or deductions

You must be just as logical with words as you would be in working out a question with numbers

How much do I write

? The space available is a rough guide but not a fixed rule

If you need much less space than that provided,

think carefully about whether you have missed out something important

If you need a lot more space,

you are probably writing about something not in the question or wasting time on extra detail for which the mark scheme will give no more marks

Examination terms explained Define

A precise statement is needed

What do you understand by

Give the definition and some additional explanation

Give a concise answer

Describe

Give clear,

positive statements about the situation/objects involved

drawing a diagram is allowed and sometimes very helpful

Explain

You must give reasons and/or underlying theory

Predict

You are not supposed to know the answer from memory but deduce it,

usually from information in the question

Suggest

This implies that there is more than one acceptable answer or that candidates are expected to arrive at the answer using their general knowledge of Physics

Common misconceptions and errors Common misconception ✘ A body in free fall is weightless

✓ A body in free fall may feel weightless,

but weight is the force of the Earth’s gravitational field,

■ ✘ Shows that this statement is wrong ✓ Shows that this is the correct idea ■ Indicates the end of the Common misconceptions and errors section

TOPIC 1 General Physics Key objectives

● To know the relationships between distance,

velocity and acceleration and relate them to moving objects in a variety of situations ● To know that mass is a measure of the amount of matter in an object and that weight is a force related to mass ● To know what is meant by density and how it can be determined ● To know the effects of forces and be able to apply this knowledge to practical situations ● To be able to explain the distinction between scalars and vectors ● To know and be able to explain the relationship between energy,

work and power ● To know how pressure is related to force and area

Key definitions

Key ideas correct

Definition

Velocity

Speed in a specified direction

Acceleration

change of velocity time taken for change

Density

A force exerted by gravity

The turning effect of a force

A quantity with magnitude (size) only

A quantity with magnitude (size) and direction

Force × distance moved in direction of force

Pressure

Force exerted on a unit area

Measuring length,

Measure length by looking perpendicularly to the ruler to avoid parallax (see Figure 1

Figure 1

TOPIC 1

Examiner’s tip

䉴 You must be able to explain how to use a micrometer or vernier scale to measure a small distance

Measure the volume of a liquid by looking level with the bottom of the meniscus (see Figure 1

look level with the top of the meniscus

Common error Figure 1

✘ Measuring volume of liquid from the top of the meniscus

Use the start and stop buttons on a stop watch or a clock to measure time interval

Measure a short repeated time interval by timing a number of cycles and then dividing the total time by the number of cycles

Examiner’s tip

䉴 Time at least ten cycles and repeat the measurement

Common error ✘ Doing the division the wrong way round,

that is number of cycles ■ calculating the repeated time interval as ––––––––––––––– total time

The answers are given on p

He forgets to zero the timer which reads 0

He starts the stop watch at the end of the first swing of the pendulum and stops the watch at the end of the tenth swing

The final reading on the timer is 5

Work out a) the number of swings he has timed,

b) the time taken for these swings,

velocity and acceleration –––––– Use the formula: speed = distance time Sample question A runner completes an 800 m race in 2 min 30 s'after completing the first lap of 400 m in 1 min 10 s

Find her speed for the last 400 m

but used the time for the whole race instead of the time for the last 400 m

Correct answer Time = 2 min 30 s'– 1 min 10 s'= 1 min 20 s'= 80 s'400 = 5

General Physics

Velocity is speed in a specified direction

of velocity Use the formula: acceleration = change ––––––––––––––– time

Examiner’s tips

䉴 The formula only applies to constant acceleration

䉴 You must be able to recognise linear motion for which acceleration is constant

䉴 You must be able to recognise motion for which acceleration is not constant

● Distance–time graph object 1 – straight line so speed is constant distance

object 2 – horizontal line so speed is zero

Figure 1

object 1 – straight line so acceleration is constant speed

object 2 – horizontal line so speed is constant

object 3 – horizontal line along x-axis so object at rest time

Figure 1

The area under a speed–time graph is the distance covered

Acceleration occurs when speed changes

Beware of the above simplification in the Core syllabus

Supplement students must know the difference between speed and velocity,

and that acceleration is rate of change of velocity

A body in free fall near the Earth has constant acceleration,

TOPIC 1

Figure 1

In the atmosphere there is air resistance

At point A in Figure 1

the speed is slow so there is negligible air resistance and the body has free fall acceleration

At point B,

the speed is higher and there is some air resistance,

so acceleration is less than free fall

At point C,

the body has high speed and high air resistance,

Therefore,

there is no acceleration – this constant speed is called terminal velocity

Common misconception ✘ A body in free fall is weightless

✓ A body in free fall may feel weightless,

but weight is the force of the Earth’s gravitational field,

Sample question A car is moving in traffic and its motion is shown in Figure 1

Figure 1

a) Choose from the following terms to describe the motion in Parts A,

B and C: acceleration,

steady speed [3 marks] b) Work out the total distance covered

[5 marks] c) Work out the acceleration in Part C

Part B: deceleration,

Part C: steady speed [1 mark] b) Distance = speed × time = 18 × 45 = 810 m [0 marks] 18 change of velocity 2 c) Acceleration = –––––––––––––––– = –– 15 = 1

General Physics

Examiner’s comments

a) The answers to Parts B and C are the wrong way round

b) The formula used is distance = average speed × time,

but this is not appropriate as the average speed is unknown

The student should have worked out the area under the graph,

which equals the distance covered

c) The calculation is correct but the student should have specified a negative acceleration

Correct answer

Part B: steady speed,

Part C: deceleration [3 marks] b) Distance = area under graph [1 mark] Part A area = –12 × 18 × 10 = 90 m [1 mark] Part B area = 18 × 20 = 360 m [1 mark] Part C area = –12 × 18 × 15 = 135 m [1 mark] Distance = total area = 90 + 360 + 135 = 585 m [1 mark] change of velocity = –18 2 ––– c) Acceleration = –––––––––––––––– 15 = –1

The answers are given on p

It continues at a constant speed of 15 m/s for 8 s

a) Show this information on a speed–time graph

b) Use the graph to find the total distance covered

Mass and weight Examiner’s tips

䉴 You must be clear about the difference between mass and weight

䉴 Mass measures the amount of matter in an object

䉴 Weight is the force of gravity acting on an object

Figure 1

The greater the mass,

the more a body resists a change of motion

A balance actually compares two weights

As mass determines weight,

the balance also compares masses

In Figure 1

mass 1 = mass 2 because weight 1 = weight 2

Experiments to measure density Examiner’s tips

mass 䉴 Use the formula: density = –––––––– volume

For a regularly-shaped solid,

measure the dimensions with a ruler and work out the volume

Find the mass on a balance

For a liquid,

measure the volume in a measuring cylinder

Find the mass of an empty beaker,

pour the liquid into the beaker,

and find the total mass of the beaker and liquid

Work out the mass of the liquid by subtraction

TOPIC 1

Sample question The mass of an empty measuring cylinder is 185 g

When the measuring cylinder contains 400 cm3 of a liquid,

Find the density of the liquid

but used the total mass instead of working out and using the mass of the liquid itself

Correct answer Mass of liquid = 465 – 185 = 280 g ––– = 0

For an irregularly-shaped solid,

submerge the object in liquid in a large measuring cylinder

The volume of the solid is the increase in the reading (see Figure 1

Alternatively,

The volume of the solid is the volume of liquid displaced

Figure 1

The answers are given on p

More liquid is poured into the cylinder up to the 140 cm3 mark and the top pan balance now reads 246 g

A solid is gently lowered into the cylinder

the liquid rises to the 200 cm3 mark and the top pan balance reading to 411 g

Work out a) the density of the liquid,

Forces and change of size and shape ●

Forces can change the size and shape of a body

Examiner’s tips

䉴 You must be able to describe an experiment to measure the extension of a spring,

piece of rubber or other object with increasing load

䉴 You must be able to plot an extension–load graph with the results of such an experiment

Identify the elastic and plastic regions as well as the limit of proportionality of extension–load graphs (see Figure 1

General Physics

Figure 1

State Hooke’s Law for the elastic region: extension is proportional to load – the graph is a straight line

Recall,

understand and use the formula for Hooke’s Law: F = kx

Forces and change of motion ●

A resultant force gives an acceleration to an object

If the object is stationary,

If the object is moving,

it will gain or lose speed depending on the direction of the force

acceleration stationary object

acceleration object moving this way

acceleration object moving this way

Figure 1

Common misconception ✘ If no forward force acts on a moving body,

✓ If a friction force acts on a moving body and there is no forward force,

there is a resultant force backwards on the body and it will slow down

✓ If no resultant force acts on a moving body,

it will continue moving with the same speed

TOPIC 1 ●

Recall and use the formula: F = ma

Examiner’s tips

䉴 Acceleration a is in the direction of the resultant force

When the force is perpendicular to motion,

the object follows a circular path

Some examples of this are shown below

Circular motion

Planet in orbit

Gravitational force towards the Sun

Planet moves around the Sun

Car turning a corner

Friction force

Car drives around the corner

Ball on a length of string

String tension

Ball whirls around in a circle

Examiner’s tips

䉴 You must be able to find the resultant of two or more forces acting in the same line

䉴 You must state the direction of the resultant force

Sample question An empty lift weighs 2000 N

Four people enter the lift and their total weight is 3000 N

After the button is pressed to move the lift,

the tension in the cable pulling up from the top of the lift is 4000 N

a) Work out the resultant force on the lift

[2 marks] b) State how the lift moves

[2 marks] c) Work out the resultant acceleration (take the weight of 1 kg to be 10 N)

a) Resultant force = 3000 + 2000 – 4000 = 1000 N [1 mark] b) The lift will move down

[1 mark] –––– c) Mass of lift and people = 5000 9

81 = 509

Examiner’s comment

a) The student correctly worked out the size of the force but did not state the direction downwards

b) The words ‘move down’ are too vague

c) The student’s answer is correct in itself but the correctly remembered exact value for g was used,

not the approximate value quoted,

! [1 mark is lost for disregard of instructions]

Correct answer

a) Resultant force = 3000 + 2000 – 4000 = 1000 N downwards

[2 marks] b) The lift will accelerate downwards

[2 marks] 5000 c) Mass of lift and people = –––– 10 = 500 kg F 2 – = 1000 –––– [4 marks] Acceleration = m 500 = 2 m/s downwards

The answers are given on p

The air resistance is 500 N

a) Work out the resultant force on the rocket

b) Describe how this resultant force changes the motion of the rocket

General Physics

Turning effect and equilibrium ●

The moment of a force is its turning effect

Examiner’s tip

䉴 You must be able to give some everyday examples of moments or turning effects,

Figure 1

A beam balances if the anticlockwise moment of the force on the left of the pivot equals the clockwise moment of the force on the right

Moment = force × perpendicular distance from pivot

Examiner’s tip

䉴 You must be able to perform and describe an experiment to show that there is no net moment on a body in equilibrium,

balance a beam or rule on a pivot with one or more weights on each side of the pivot,

work out the total clockwise moment and total anticlockwise moment and show that they are equal

Sample question A student carries out an experiment to balance a regular 4 m long plank at its mid-point

A mass of 4 kg is placed 80 cm to the left of the pivot and a mass of 3

Explain,

Figure 1

Student’s answer 4 × 80 = 3

[2 marks]

Examiner’s comments The student’s calculation and conclusion are entirely correct but the instruction in italic to work out the moments was ignored

Correct answer Anticlockwise moment = 40 × 0

An object is in equilibrium if there is no resultant turning effect and no resultant force

The answer is given on p

A child of weight 400 N sits 1

A child of weight 300 N sits 1

A parent holds the end of the see-saw on the same side as the lighter child

Work out the magnitude and direction of the force the parent must exert to hold the see-saw level

TOPIC 1

Centre of mass and stability Examiner’s tip

䉴 You must be able to describe and carry out an experiment to find the centre of mass of an irregular,

The lower the centre of mass,

the more stable an object becomes

Sample question A girl is seated safely and steadily in a canoe but when she tries to stand up,

the canoe capsizes (see Figure 1

Explain this in terms of centre of mass and stability

Figure 1

Student’s answer The centre of gravity goes up so she falls over

[1 mark]

Examiner’s comments The centre of mass does rise but the student did not mention stability

the student used the old term ‘centre of gravity’ which is not the correct syllabus term,

although the student would not lose marks for this

Correct answer The centre of mass rises so the canoe becomes unstable and capsizes

Scalars and vectors ● ● ● ● ● ●

A scalar only has size (magnitude)

Examples of scalars: mass,

Scalars are added by normal addition

A vector has direction and size (magnitude)

Examples of vectors: force,

Vectors are added by taking into account their direction

Common misconception ✘ Speed is a vector

✓ Speed is a scalar because it has no direction

Velocity has size and direction so it is a vector

䉴 You must be able to use a graphical technique to represent two vectors to find their resultant

䉴 Learn carefully how to use the parallelogram law or the triangle law

General Physics

Sample question An aircraft flies at 900 km/h heading due south

There is a crosswind of 150 km/h from the west

Graphically,

find the aircraft’s resultant velocity

[4 marks] Student’s answer scale: 1 cm represents 100 km/h

vector of aircraft’s own velocity 900 km / h

Figure 1

the question is extremely well-answered and the graphical work accurate

stating the scale shows excellent work

However,

the student has omitted the direction part of the resultant velocity,

Correct answer The answers shown in Figure 1

[4 marks]

Energy You must understand and be able to give examples of the forms of energy listed below

Energy due to motion (kinetic energy),

Energy due to position (gravitational energy),

water in a mountain lake that can flow downhill to generate electricity,

a raised weight that gains gravitational energy due to being in a higher position,

energy stored in waves and tides that can also be used to generate electricity

TOPIC 1

Chemical energy released in chemical reactions,

eating food to provide energy to muscles,

providing electrical energy from chemical reactions in a battery,

burning fuel in a boiler to provide steam,

which can drive a turbine to generate electricity

Strain energy due to the stretching or bending of materials,

compressing or extending a spring

The term potential energy includes gravitational and strain energy

Nuclear energy released during fission,

Internal energy (heat or thermal energy),

the increase in temperature when an object is heated

Electrical energy that can be produced in power stations and batteries (but cannot be stored on a large scale),

widely-used for electronic devices,

lighting and heating in homes and industry because it is easily transmitted and transformed into other types of energy

Sound energy – longitudinal pressure waves that travel through a compressible material

Geothermal energy from within the Earth,

which can be used to generate electricity and provide heat for homes and factories

Light energy and other forms of electromagnetic radiation that can travel through a vacuum,

Examiner’s tips

䉴 You must be able to give examples of conversion of energy from one form to another

䉴 You must be able to give examples of the transfer of energy from one place to another

䉴 Energy is always conserved

It can be transformed into other forms but cannot be created or destroyed

䉴 You must be able to recall and use the formulae: k

= mgh 䉴 Efficiency is a measure of the proportion of energy input to a device which is output

energy output Efficiency = –––––––––––––– energy input It is not essential to know this formula but many students will find it the easiest way to understand the relationship

When energy is transformed,

some energy is lost to the surroundings as thermal energy,

often due to friction (see Figure 1

Figure 1

General Physics

Sample question A man winds up the spring of the clockwork radio shown in Figure 1

The internal spring then unwinds to provide energy to power the radio

Figure 1

a) State the type of energy stored in his muscles

[1 b) State the type of energy stored in the spring

[1 c) Name the component that converts the energy from the spring into useful energy for the radio

[1 d) Name the type of energy required by the circuits of the radio

[1 e) Name the type of useful energy output by the radio

[1 f) Most of the energy from the spring will eventually be turned into a form of waste energy

Name this type of energy

Examiner’s comments

Correct answer

Chemical energy is stored in his muscles

Potential energy is stored in the spring

A generator converts the energy in the spring

The circuits require electrical energy

The useful output energy is radio waves

The waste energy is friction

a) Correct answer b) The student should have specified the type of potential energy

a spring stores strain potential energy – often simply called strain energy

c) and d) Correct answers e) The radio is a receiver not a transmitter so the output is sound energy

f) Friction is not a type of energy

it is a force which can occur when other types of energy are converted to heat

Chemical energy is stored in his muscles

Strain (potential) energy is stored in the spring

A generator converts the energy in the spring

The circuits require electrical energy

The useful output energy is sound energy

The waste energy is heat,

mark] mark] mark] mark] mark] mark]

TOPIC 1

Common misconception ✘ Energy lost as waste heat has been destroyed

The answers are given on p

Consider the jumper when he has fallen another 10 m and is travelling at 15 m/s

State a) a form of energy that has been lost,

b) two forms of energy that have been gained

c) Work out how much energy is stored in the rope

Take g = 10 m/s2 and ignore air resistance

Work and power ● ● ● ● ● ●

Work is done when a force moves though a distance

The greater the force,

The greater the distance moved,

You must be able to recall and use the equation: ∆W = Fd = ∆E,

where ∆W is work done and ∆E is energy transferred

Power is the rate of doing work

The greater the work done in a given time,

The shorter the time in which a given amount of work is done,

done ––––––––– * Power = work time taken * –t You must be able to recall and use the equation: P = E

Examiner’s tip

䉴 Core students must be able to explain the relationships for work and power

You may find the easiest way to do this is to use the equations marked * although the syllabus does not actually require you to know these equations

Sample question The two cranes shown in Figure 1

Crane A raises a load of 1000 N through a height of 12 m in 10 s

Crane B raises the same load of 1000 N through the same height of 12 m but takes 12 s

Figure 1

General Physics

the work done by the two cranes

[2 marks] c) Calculate the energy transferred and power of each crane

a) Both cranes do the same amount of work because the force and distance moved are the same

[2 marks] b) Crane B has more power because the amount of work done is the same but the time is bigger

[0 marks] c) Energy transferred by each crane = 1000 × 12 = 12 000 [1 mark] Power of A = 12 000 × 10 = 120 000 [0 marks] Power of B = 12 000 × 12 = 144 000 [0 marks]

Examiner’s comments

a) Correct answer b) The student has confused the relationship

the shorter the time taken the greater the power

c) The calculation of energy transferred is correct,

except that the unit ( J) has been omitted

Both power calculations are incorrect because the wrong equation has been used

the unit of power (W) has also been omitted

Correct answer

a) Both cranes do the same amount of work because the force and distance moved are the same

[2 marks] b) Crane A has more power because the amount of work done is the same but less time is taken

[2 marks] c) Energy transferred by each crane = 1000 × 12 = 12 000 J [2 marks] 12 000 Power of A = ––––– [1 mark] 10 = 1200 W 000 ––––– Power of B = 12 12 = 1000 W

[1 mark]

Pressure ● ● ●

Pressure on a surface is the force exerted on a given area

The greater the force on a given area,

The smaller the area on which a given force acts,

Examiner’s tip

䉴 Many Core students may find the easiest way to explain the force relationships for pressure is to use the equation: pressure = –––––– area The syllabus does not actually require you to know this equation

Common error ✘ Carelessly using the word ‘pressure’ instead of ‘force’ in the answer to descriptive questions

■ F ● You must be able to recall and use the equation: p = – A ● The unit of pressure is the pascal (Pa)

A force of 1 N on an area of 1 m2 exerts a pressure of 1 Pa

TOPIC 1

Figure 1

A barometer is used to measure atmospheric pressure in millimetres of mercury,

which is given by the height of the mercury column (Figure 1

Pressure beneath a liquid surface depends on the depth and the density of the liquid

The greater the depth in a given liquid,

At a given depth,

the greater the density of the liquid,

You must be able to recall and use the equation: p = tgh,

where t is the density of the liquid,

g the acceleration due to gravity and h the depth below the surface of the liquid

The manometer shown in Figure 1

The difference in pressure is measured by the height difference h between the two columns (Figure 1

atmospheric pressure to gas supply

Figure 1

Sample question Some students are playing a ball game in the sea and the ball is pushed 60 cm below the surface of the water (see Figure 1

the pressure on a point on the ball 60 cm below the surface of the sea with the pressure just below the surface

General Physics

Figure 1

the pressure on the ball 60 cm below the surface of the sea with the pressure 60 cm below the surface of a fresh water lake

[2 marks] c) Calculate the pressure on a point on the ball 60 cm below the surface of the sea (density of fresh water = 1000 kg/m3

[1 mark] b) The pressure on the ball below the surface of the sea is greater because sea water has a greater density

[1 mark] c) Pressure = tgh = 1000 × 10 × 0

Examiner’s comments

a) The statement is correct but no reason is given

b) The statement is correct and the reason is also correct,

The student should have mentioned that the comparison was at the same depth below each surface

c) The pressure has been calculated in the correct way but at a depth of 60 cm below the surface of fresh water instead of sea water

Correct answer

a) The pressure increases because the ball is at a greater depth in the same liquid

[2 marks] b) The pressure on the ball below the surface of the sea is greater because sea water has a greater density and both balls are at the same depth

[2 marks] 3 c) Density of sea water = 1

The answers are given on p

Figure 1

Write down at which of the points A,

B or C a) the pressure is greatest,

TOPIC 2 Thermal Physics Key objectives

● To know how the kinetic theory explains the nature of solids,

liquids and gases ● To know how the flow of thermal energy affects the properties of solids,

liquids and gases and their changes of state ● To know how thermal energy is transferred by conduction,

convection and radiation ● To be able to explain the behaviour of solids,

liquids and gases in a wide range of practical situations from everyday life

Key definitions

Key ideas

Definition

Molecule

A tiny particle consisting of one,

Particle

Any small piece of a substance

could be a molecule or billions of molecules

Temperature

How hot a body is

Thermal energy

Energy that flows into or out of a body by conduction,

Kinetic theory of molecules ● ● ●

All matter is made up of molecules in motion

The higher the temperature,

the faster the motion of the molecules

Almost always,

matter expands with increase of temperature

Solids ● ● ● ●

Figure 2

Molecules are close together

Molecules vibrate about fixed points

The rigid structure of solids results from these fixed positions

As the temperature increases,

the molecules vibrate further and faster

This pushes the fixed points further apart and the solid expands

The positions of molecules in a solid are fixed because the attractive and repulsive forces between neighbouring molecules are balanced

There is only a very slight expansion of a solid with increase of temperature,

the length of an iron rod increases by about 0

Liquids ● ● ●

Molecules are slightly further apart than in solids

Molecules are still close enough to keep a definite volume

Molecules move randomly in all directions,

Thermal Physics

Figure 2

As the temperature increases,

the molecules move faster and further apart so the liquid expands

One exception to this is: liquid water heated from 0 °C to 4 °C changes its structure so it contracts instead of expands

The forces between molecules are too weak to keep them in a definite pattern but are sufficient to hold them to the bulk of the liquid

There is a small expansion of a liquid with increase of temperature,

the volume of many liquids increases by about 4% when heated from 20 °C to 100 °C

Everyday uses and consequences of thermal expansion Examiner’s tip

䉴 You must be able to state and explain some of the everyday uses and consequences of thermal expansion

As a minimum,

learn one constructive use and one disadvantage each for solids and liquids

Uses of expansion of a solid: shrink-fitting,

curling of a bimetallic strip in a fire alarm

Figure 2

TOPIC 2

In the fire alarm circuit of Figure 2

the heat from the fire causes the lower metal in the bimetallic strip to expand more than the upper metal

This causes the strip to curl up,

which completes the circuit and the alarm bell rings

Disadvantage of expansion of a solid: gaps need to be left between lengths of railway line to allow for expansion in hot weather

Use of expansion of a liquid: a mercury or alcohol thermometer

see the later section on Measurement of temperature

Disadvantage of expansion of a liquid: the water in a car’s cooling system expands when the engine gets hot

A separate water tank is needed for the hot water to expand into

Sample question The lid is stuck on a jam jar

How could you use hot water to release it

? Explain in terms of the molecules how this works

[4 marks] Student’s answer Put the jam jar in hot water and the lid will come off [1 mark] because the molecules expand

[0 marks] Examiner’s comments The student did not specify where exactly the hot water should be used and gave a vague,

incorrect explanation of the role of the molecules

Correct answer Put the lid in hot water so it expands and can be released

their mean positions move further apart so the lid expands

The answer is given on p

are bonded together at room temperature

Bronze expands appreciably when heated but invar expands very little

Describe the shape of the strips when heated in an oven

Gases ● ●

Perspex tube

Figure 2

Molecules are much further apart than in solids or liquids

Molecules move much faster than in solids or liquids

There is no definite volume

Molecules move throughout the available space

Molecules constantly collide with each other and the container walls

Gases have low densities

The higher the temperature,

the faster the speed of the molecules

In fact,

temperature is a measure of the average speed of the molecules

The higher the temperature,

the larger the volume of a gas at constant pressure

Except when they are actually colliding,

the forces between the molecules are negligible

There is a considerable expansion of a gas with increase of temperature at constant pressure,

the volume of a gas increases by about 27% when it is heated from 20 °C to 100 °C

Thermal Physics

Brownian motion ●

In the apparatus in Figure 2

smoke particles reflect the light,

which is seen in the microscope as bright pin pricks

They move around randomly

They also move in and out of focus as they move vertically

This movement is caused by the irregular bombardment of the smoke particles by fast-moving,

This is clear evidence for the kinetic theory

This was first observed by Robert Brown who observed in a microscope pollen particles suspended in water moving due to bombardment by fast-moving,

Smoke/Pollen particles are visible and relatively massive

Air/Water molecules are invisible and fast-moving

Figure 2

Common error ✘ Incorrect use of the words ‘particle’ and ‘molecule’ shows a lack of understanding of their different properties and roles

■ Sample question A student looks in a microscope at a cell containing illuminated smoke particles

Explain a) what is seen,

Examiner’s comments

Correct answer

a) smoke particles b) moving around c) The smoke molecules are bombarded by air

not smoke particles that are seen

b) ‘Moving around’ is too vague

c) The student has made incorrect/incomplete use of the terms ‘molecule’ and ‘particle’

a) bright specks of light [1 mark] b) moving around haphazardly in all directions [1 mark] c) The bright specks are light reflected off the smoke particles,

which are bombarded by air molecules

Measurement of temperature ●

A physical property is needed that varies in a regular way over a wide range of temperatures

A practical thermometer is a simple piece of apparatus that measures how this property changes

Examples of properties that can be used: expansion of a solid (coiled bimetallic strip) or liquid (mercury-in-glass),

electrical voltage between two junctions of different metals (thermocouple),

Common errors ✘ Confusing heat and temperature

✓ Temperature measures how hot a body is

Heat is energy that flows from a hot body to a cold body

TOPIC 2

The thermometer must have sufficient sensitivity

This means the property must change enough to be measurable,

enough coils in a coiled bimetallic strip thermometer to give sufficient movement,

a narrow tube in a liquid-in-glass thermometer

The thermometer must have sufficient range

This means it can be used over a wide range of temperatures,

a water-in-glass thermometer would be of limited use because it could only be used between 0 °C and 100 °C

The thermometer’s reading must show linearity

This means it must change by the same amount for every degree of temperature change,

a property that changes little in one half of the temperature range and much more in the other half would not be suitable

The fixed points of a thermometer are essential to give it its scale

The thermometer must read exactly 0 °C at the freezing point of pure water,

and exactly 100 °C at the boiling point of pure water,

at normal atmospheric pressure

In between these fixed points the scale is divided into equal divisions

Liquid-in-glass thermometers 0 °C

100 °C

Figure 2

The liquid only expands by a few per cent,

so the expansion of the large amount of liquid into the narrow tube gives a good amount of movement,

Mercury and alcohol are suitable liquids because: 1) they expand enough to make a sensitive thermometer 2) they can be used over wide ranges of temperatures – alcohol from –115 °C to 78 °C (and higher if under pressure) and mercury from –39 °C to 357 °C

The answers are given on p

a) What Celsius temperature should the thermometer read

? The thermometer is now placed in steam above boiling water and the bead expands to 82 mm long

b) What Celsius temperature should the thermometer now read

? c) Work out the length of the bead at 50 °C

d) Work out the temperature reading when the bead length is 61 mm

Thermal Physics

Thermocouple V

Figure 2

The voltage produced between the two junctions of wires of different metals is proportional to the temperature difference between the junctions

Only the hot junction is attached to the object whose temperature is being measured

This junction is very small and light,

so reacts quickly to rapidly changing temperatures

It is also very robust and resistant to damage from vibrations of machinery

Thermocouples are widely used in industry because they can be used over a wide temperature range from below –250 °C to 1500 °C

In addition,

the voltage output is very convenient for data logging

an industrial test rig may have hundreds of thermocouple thermometers all connected to a computer for analysis

Sample question A new petrol engine is being tested and the engineers need to measure the temperature of the exhaust pipe close to the engine

State with reasons the type of thermometer that would be used

[4 marks] Student’s answer A thermocouple because it is small and goes up to a high temperature

[2 marks] Examiner’s comments The student’s answer is on the right lines but rather vague and incomplete

(It would earn the first and last marks below

) Correct answer A thermocouple would be used [1 mark] because the hot junction is small [1 mark] and will not be damaged by the vibrations of the engine

[1 mark] A thermocouple can measure high temperatures

Changes of state ●

Molecules at the surface of a liquid,

have enough energy to escape the attractive force of the rest of the liquid and evaporate to become molecules of a gas

Evaporation takes place at all temperatures

Energy is needed to break the bonds between molecules so evaporation causes the remaining liquid to cool down

TOPIC 2

Sample question A student is playing football on a cool,

He feels comfortably warm because he is moving around vigorously

His kit then gets wet in a rain shower

Explain why he now feels cold

[2 marks] Student’s answer The wet T-shirt makes him feel cold

[0 marks]

Examiner’s comments The student’s answer is far too vague and does not mention the cooling caused by evaporation

Correct answer The water in his wet kit is evaporated by the wind

[1 mark] The thermal energy needed for this evaporation is taken from the water in his T-shirt and shorts as well as from his body so he feels cold

The answers are given on p

After a few minutes some of the ice has melted

State whether the following increase,

decrease or stay the same: a) temperature of the ice,

d) total mass of the ice and water

Rate of evaporation increases with: 1) higher temperature,

as more molecules at the surface are moving faster 2) increased surface area,

as more molecules are at the surface 3) a wind or draught,

as the gas molecules are blown away so cannot re-enter the liquid

Boiling occurs at a definite temperature called the ‘boiling point’

Bubbles of vapour form within the liquid and rise freely to the surface

Energy must be supplied continuously to maintain boiling

Examiner’s tip

䉴 You must be able to distinguish between boiling and evaporation

Carefully learn the features of each

Condensation occurs when gas or vapour molecules return to the liquid state

Energy is given out as the bonds between molecules in the liquid re-form

Melting,

takes place at a definite temperature called the ‘melting point’

Energy must be provided to break the bonds between molecules in order for them to leave the well-ordered structure of the solid

Solidification,