Unit 3 Guide

8/9/2019 Unit 3 Guide

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Unit 3 Guide – Part I

Unit 3 (exam date: Thursday, May 9, 2013) is a paper which tests knowledge

on experimental technique and analysis mainly on Unit 1 and unit 2 topics.

Typically the paper consists of multiple!choice questions and 3 or " longquestions. #elow $ will outline the di%erent types of long questions that are

usually set&

i) The ‘cm!arisn" #uestins: 'uch questions require one to

compare and contrast two di%erent methods to conduct an experiment

or sometimes to compare two di%erent experimental apparatus.

or example&

“Two students are discussing an experiment to plot a cooling curve for

a liquid. One says that it is always better to use a suitable datalogging

device. The other says that using a liquid-in-glass thermometer and

stopwatch is better.Discuss the advantages and disadvantages of each method for an

experiment which involves taking the temperature of water in a beaker

over a period of ! minutes and plotting a temperature against time

graph" (rom ay 2*1*)

$n such cases most of us would +e inclined to prefer to use a

datalogging de,ice for se,eral reasons. #ut there are alsodisad,antages that must +e considered (-e will discuss such questions

in art $$). $u must ma%e a &ist ' such ad(antaes and

disad(antaes and &earn them* +uch a &ist must e !art ' yur

-n ntes 'r Unit 3* .(eryne must ma%e their -n ntes/

ii) The ‘ex!eriments" #uestins: 'ometimes you are required to gi,e a

detailed description of an experiment on Unit 1 or Unit 2 theory. Usually

you will +e guided on which points they speci/cally want you to

address. 'ometimes such questions carry a lot of marks thus you

should +e prepared0 ou should learn some typical experiments thattend to come up.

et"s see -hich ex!eriments ha(e cme u! s 'ar:


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.x!eriment $ear mar%s

Resistivity of iron wire un!* 4To determine the %loss in KE ofbouncing ball an!1*

Resistivity of constantan wire an!1*

Hooke's Law an!1* 4

Resistivity of a metal un!1*

$un Mdu&us ' a -ire an!11 12To investigate the variation in R of NTthermistor un!11 11

!iscosity of oil an!12 13E"# and internal resistance of a cell un!12 13

$cceleration of free fall an!13 "E"# and internal resistance of a solarcell an!13 1"

un!13 555

6ll these experiments are suggested in the sylla+us. 'ome experiments which

are also suggested +ut ha,e not come up are&

.x!eriment % !ae7stimate power output or e8ciency of anelectric motor

'tress!strain experiment for ru++er

orce!compression experimentseasure the speed of sound in air usingstanding wa,es

easure the speed of sound in a solid

easure the refracti,e index for a liquid

$!9 graphs experiments

iii) The ‘midd&e" #uestins: These are questions that usually descri+e

an experiment and require you to carry out calculations: a,erages:

work out uncertainties: ; uncertainties: to comment on di%erent things

etc. (sme exam!&es& an11!q


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This was Dust an introduction. $n art $$ we will see speci/c examples of

questions.

Unit 3 Guide – Part II

$n part $$ of our guide we will concentrate on cm!arisn E type

questions*

erhaps the most trou+lesome measurement one is required to consider:

at 6!le,el: is that of time. uman reactin time often introduces a

considera+le uncertainty in our measurements when these are +eing

made with a stopwatch. Thus we often employ electronic de,ices such

&iht ates connected to data &ers to impro,e the accuracy of our

results. -hen the light gate +eam (usually $F) is interrupted a signal is

sent to the data logger to start timing.

+ay yu ha(e t measure the time 'r a a&& t 'a&& thruh a

distance* #elow you can /nd an examinerGs summary of the pros and

cons of using light gates connected to a data logger ,s the stopwatch

method.


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There are other cases where data loggers can +e employed +ut sensrs other

than light gates are used. or example if one wants to take temperature

measurements of a hot liquid as it cools down they will need a tem!erature

sensr connected to the data logger. >f course the other way to do it would +e

to use a thermmeter. Here are the pros and cons.

6lso e&ectric current measurements can +e tricky to make especially when we

a ha,e an experiment where the current decreases or decreases with time. $n

such a case one would need a current sensr connected to data logger as this

can accurately take many measurements automatically o,er a small time

inter,al. ost importantly the data logger sa,es the data and can e,en plot a

graph as the experiment is taking place. 6lso the sensor can respond fast to a

Iuctuation in current which happens in a ,ery short time inter,al i.e the current

that Iows in a lamp once it is switched one. -here a current sensor is


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una,aila+le one has the choice of digital multimeters ,s analogue meters. #elow

you can see the pros and cons.

Unit 3 Guide – Part IIIerhaps the most interesting type of question is ‘ex!eriments" question.

$tGs worth learning the details of some experiments that are considered to

+e important. Jearly all the experiments that can potentially +e in a paper

are descri+ed in your +lack text+ook Ede&cel hysics for $(.

#elow we will outline some of these experiments.

.x!eriment iaram rie' descri!tin4esisti(ity ' a -ire

$un"s mdu&us


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5iscsity ' a &i#uid

.x!eriment iaram rie' descri!tin6cce&eratin ' 'ree

'a&& 7)

.M8 and r ' ce&&


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+!eed ' sund -ith

standin -a(es

.x!eriment iaram rie' descri!tin


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the form yAmx BC so that we o+tain a straight line from which we can get

something from the gradient and@or the y!intercept.

hy d -e -ant ra!hs

$n une 2*12 paper there was a 3!mark question where students had to discussthe ad,antages of using a graph. #elow you can see the 7xaminerGs marks

scheme for this question.

6ll these points are really good. $Gd like to discuss some of them.

'ay you are carrying out an experiment to discuss how extension (Kx) ,aries with

force applied () on a piece of wire. $f you carry out the experiment and you

o+tain a straight line thruh the riin this means that the two ,aria+les: Kx

and : are direct&y !r!rtina&. 'o:

F ∝ Δx

-e can now write an equation of the form yAmx as:

F =k Δ x

'o the gradient of a graph of # against $x is equal to k (the sti%ness of the wire

or if it was a spring the spring constant ).

>n the other hand if we plot $x against the gradient will +e equal to1

k .

6nother important point is that if we ha,e many measurements itGs hard to pickout the anma&us readins which could ha,e arisen due to a mistake while

carrying out the experiment. -ith a graph these can easily +e identi/ed.

+ystematic errrs are errors in the experiment that a%ect all our results

equally. These usually arise due to careless usage of measuring instruments or

?ero errors in the instruments themsel,es or wrong cali+ration of the instrument.

Let me gi,e you examples.

'ay that you want to measure the height of a +ench using a ruler which doesnGt

ha,e the ?ero mark right at the edge of the ruler. 'ay that thereGs a *. cm

inter,al +efore the ?ero mark. $f you take your measurements forgetting a+outthis then all your measurements will +e +igger than what they should +e +y *.


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cm. This is what we call a ?ero error and itGs a systematic error. 'uch errors can

e,en arise when we use electronic equipment as well0 or example ammeters:

electronic +alances: electronic force sensors etc often ha,e ?ero errors. >ne has

to cali+rate an instrument or correct ?ero errors +efore using the instrument. $f

this is nt done then systematic errors might gi,e rise to unex!ected y;

interce!ts. or example you may carry out a current! ,oltage experiment for anohmic conductor and o+tain a straight line that does not pass through the origin

as expected0

4andm errrs are errors which occur due to unpredicta+le changes which

might occur in the en,ironment of the experimental setup or in the instruments

themsel,es.

'ay you are carrying out an experiment at room temperature and thereGs a

sudden change in T due to change in weather. ou canGt do much a+out this and

your results might +e a%ected. Minimisin randm errrs in(&(es ta%in a

&are numer ' measurements and ca&cu&atin an a(erae r dra-in a

&ine ' est ne typical question that arises in this type of exam question is to criticise the

data gi,en to you. or example.

The points accepted in such questions are ,ery typical so learn them as most

pro+a+ly you will face such a question. The marks scheme for thi one&


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$ found another one. $n their tension ,alues they used g as 1* J@kg0

Thanks for your attention0 ore Unit 3 stu% ,ery soon0

Unit 3 Guide –Part 5

=a&cu&atin uncertainties, !ercentae uncertainties and

!ercentae di>erences

ThereGs always an uncertainty in e,ery measurement we make. $n hysics

experiments we always stri,e to minimise the e%ect of such uncertainties

in order to impro,e accuracy.

'ay you ha,e to measure the length of this piece of paper you are

reading. $f you measure it with a ruler you will /nd it to +e 2.4 cm. Using

a ruler: in this case: seems to +e a reasona+le choice as the smallest

di,ision on the ruler is 1 mm which is small compared to the 24 mm we

measure.

$nstruments such as the micrmeter

>r the (ernier ca&i!er

ha,e +etter resolutions (or smaller di,isions). The micrometerGs resolution

is *.*1 mm whereas the resolution of the ,ernier is *.* mm (and *.*1

mm for the electronic ,ernier)

=an these instruments e used t tain a mre accurate

measurement 'r the !a!er"s &enth


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$nstruments such as the electronic +alance we use at school ha,e

resolutions up to a 1@1**th of a gram although more expensi,e models

ha,e +etter resolutions. or the o+Dects that we measure the +alances we

ha,e are ,ery good as the ? uncertainties in the measurements are

small.

hat '&&-s is a cm!act uide n h- t ca&cu&ate ?

uncertainties*

6n exception is time measurements where we follow the following guide&


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'o far there ha,e +een few marks per paper dedicated to calculating uncertainties and ;

uncertainties.( une 2**: q=(+) (2marks): an 2*1*: q4(e) (1mark): une 2*1*: q (c)

(2marks): q= (+) (1): une 2*11 q< (+ii) (2marks): une 2*12 q< (+) ("marks).

Thank you so much for your time0

• 8r ca&cu&atin ? uncertainties see the mre detai&ed uide

n this su@ect*

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Unit 3 Guide

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