# H11. justifies the appropriateness of a particular investigation plan (Physics)

 BikiCrumbs: H11. justifies… plan (Physics)

## 11.1 identify data sources to:

#### b) determine the type of data that needs to be collected and explain the qualitative or quantitative analysis that will be required for this data to be useful

The type of data collected can vary. The data may be numerical quantities such as the mass of an object, or it may be observations such as the colour of something or its smell.

Data can be analysed qualitatively or quantitatively. Qualitative analysis refers to analysing it in regards to its qualities. For example you may observe the colour of the emission spectra of an element. You are not performing any numerical calculations, therefore you are qualitatively analysing the data. Quantitative analysis refers to analysing it in regards to numerical calculations. You may do an experiment where you change length and measure time. From this data you collect you may apply some mathematical formula to come to some conclusion. As you have performed calculations on this data, you have quantitatively analysing the data.

Data is just data. On its own it is useless. You must analyse this data in order for the information gathered from this data to be useful.

#### c) identify the orders of magnitude that will be appropriate and the uncertainty that may be present in the measurement of data

The orders of magnitude of measurements of data can be explained with the concept of significant figures. I’ll assume you all know what decimal places are, you may already know what significant figures are but I will explain them anyway. The number of significant figures you have in a particular quantity is just how many digits you use to make up the quantity, omitting leading zeros. For example the number 52 has two significant figures, that being the ‘5’ and the ‘2’. The quantity 3.00 has three significant figures, that being the ‘3’, the ‘0’ and the ‘0’. These three digits define the accuracy of a quantity. For example the quantity 3.00 specifies that the tenths and hundredths of the quantity are zero. This is different to the quantity 3 which means that we only know that the units are 3, we don’t know the tenths or the hundredths. And so 3.00 has three significant figures and 3 has one, this means that 3.00 is much more accurate.

When we get to very small numbers such as 0.0001, it is best to explain these in terms of scientific notation. 0.0001 is represented as 1 × 10-4 in scientific notation. They are equivalent. However now that the quantity is in scientific notation we can see only one digit is used to define the quantity, hence is has one significant figures.

Now you can see why significant figures are more important than the number of decimal places.

Hence when we are measuring data we should ensure that we collect enough significant figures, not decimal places.

When we measure data, we never know exactly the quantity that we measure. We may measure a length to be 35mm. However the distance may vary from 35mm by ±0.5 and still have this measurement. For example in the diagram below the piece of wood is measured to be 15mm. This is because you can only measure to the lines given, however it is actually a bit greater that 15mm. There is a region of values that are all measured to 15mm, that being from 14.5mm to 15.5mm. Hence the uncertainly of the measurement is ±0.5mm

#### d) identify and use correct units for data that will be collected

SI Base Units:

These are the fundamental measurements, upon which all other measurements are based. So of the SI base units are shown in the table below. Please note that the first three length, time and mass are the three most important.

 Quantity Unit Symbol Length metre m Time second s Mass kilogram kg Temperature degrees Kelvin °K

For each of these quantities, a standard has been decided upon. For example the standard of a metre is defined to be the distance travelled by light in a vacuum during a certain fraction of a second.

SI Derived Units:

Other units can be defined in terms of the base units. These are known as derived units. For example $\mbox{speed} = \frac {\mbox{distance}}{\mbox{time}}$. As distance and time are base units, the units of speed can be derived from these base units. Hence the unit of speed will be $\frac {\mbox{metre}}{\mbox{second}}$, ie. metre per second, ie. ms-1. The units of many quantities can be derived from the formula of that quantity.

Using the SI Prefixes:

 Factor Prefix Symbol 109 giga G 106 mega M 103 kilo k 10-2 centi c 10-3 milli m 10-6 micro µ 10-9 nano n

The above table shows the most commonly used SI prefixes in this physics course. These are used to change units. For example, in many exam questions that require you to calculate the force of gravity, they tell you the distance in kilometres (km). You cannot just put this into the gravity formula, as if you do, you will not get newtons for the force. Similarly in questions involving Kepler’s Law of Periods, r must be in metres, as the units for G on the data sheet is N m2 kg-2. To be safe, it is best practice to always change your units back to the base SI units before doing any calculations.

So referring to the table shown above, if you are told a length is 5 cm, then this is $5 \times 10^{-2} \mbox{m}$.

Unfortunately in exams they do expect you to know the symbols and factors shown in the above table.

Converting Units of Higher Powers:

Sometimes you may have units such as cm2 which need to be converted into m2. You cannot just multiply the cm2 by 10-2, this only works for converting cm to m. So to convert cm2 to m2, it is best to draw a picture.

As seen in the diagram above, if the area of the square is $x \ \mbox{cm}^2$, then the length of each side is $\sqrt{x} \ \mbox{cm}$. Now the side length can be converted to m by multiplying by 10-2, and so the side lengths are $\sqrt{x} \times 10^{-2} \ \mbox{m}$. Now this can be squared to get the area in m2, $\left ( \sqrt{x} \times 10^{-2} \right ) ^2 = x \times 10^{-4} \ \mbox{m}^2$

#### e) recommend the use of an appropriate technology or strategy for data collection or information gathering that will assist efficient future analysis

In modern scientific laboratories data is collected electronically. This is usually done using specialised hardware devices which collect the data. This data is then sent to a computer where it is analysed using powerful computer software. There are many advantages of this including:

• There is no chance of human error. For example if the electronic balance says 5.21g, but then the human accidently records this as 5.12g then a transcription error has occurred. If however the electronic balance sends the 5.21g straight to the computer then there is no chance of a transcription error.
• Humans do not need to be present to monitor the experiment. This means that someone can set up an experiment and get the computer to automatically monitor the experiment and collect data. This is reduces the labour needed and means that for experiments that are done over a long time do not need to be sat and watched by a human 24 hours a day.
• Humans are not exposed to potential safety hazards. For example if you were conducting an experiment which involved radioactive substances, it would be too dangerous to have a human walk into the room and read some measurement. Instead this can be done by a computer and the human can stay outside in a protected room.

Specific technology includes Texas Instruments calculators, which can electronically collect and process data.

## 11.2 plan first-hand investigations to:

#### a) demonstrate the use of the terms ‘dependent’ and ‘independent’ to describe variables involved in the investigation

Independent variables, as the name suggests, are variables that do not depend on some other value. For example if you were performing an experiment to model the temperature changes of a cooling body. The independent variable would be time. For every value of time there is a temperature. And so temperature would be the dependent variable. The temperature depends on the time. This relates to functions in mathematics. Where the independent variable is x and the dependent variable is f(x). As such the independent variable is usually plotted on the horizontal axis and the dependent variable on the vertical axis. The dependent variable is changed and the independent variable is measured.

#### b) identify variables that needed to be kept constant, develop strategies to ensure that these variables are kept constant, and demonstrate the use of a control

For every experiment you should only change one thing at a time and measure one thing. The rest of the variables should be kept constant. If need be you can do an experiment for each of the variables one at a time, ensuring that you never change two variables.

A control is where you leave it unchanged and unaffected by the experiment. This is used so that you have a reference to compare to.

#### c) design investigations that allow valid and reliable data and information to be collected

Reliability refers to whether or not you get the same result when you repeat the experiment. For example if you perform the same experiment 10 times with the same conditions and each time you get relatively large differences between all 10 results, then your experiment is not very reliable. If however you get the same result all 10 times then your experiment is very reliable. To ensure a reliable experiment you should ensure that the same conditions are met when repeating the experiment.

Validity refers to if the results are correct. To ensure validity ensure that you perform the correct calculations with no conceptual errors, and also ensure that the way you perform your experiment will test and give the correct results for the aim of the experiment.

However, just because your experiment is very reliable does not mean that it is valid. You may just keep getting the same wrong result all this time. In this case your experiment would be very reliable, but not valid. Similarly your experiment could be extremely valid, however not very reliable. The best experiments are valid and reliable.

#### d) describe and trial procedures to undertake investigations and explain why a procedure, a sequence of procedures or the repetition of procedures is appropriate

A procedure is just a sequence of steps that are taken to perform an experiment. A procedure for an experiment is necessary so that others can perform the same experiment as you (ie. repeat it) the same way and under the same conditions. This is necessary as for an experiment to be valid, it must be able to be repeated all the time by different people and still yield the same result.

## 11.3 choose equipment or resources by:

#### a) identifying and/or setting up the most appropriate equipment or combination of equipment needed to undertake the investigation

Equipment is the tools you use to perform the experiment. Some of the most common equipment used in this physics course includes:

• Transformer (used to get safe voltages from the power supply for use with experiments)
• Pendulum
• Stopwatch
• Ruler
• Magnets
• Electronic Balance
• Induction Coil

#### b) carrying out a risk assessment of intended experimental procedures and identifying and addressing potential hazards

A risk assessment involves investigating and identifying any potential risks or hazards and then implementing methods to eliminate or reduce the risk or hazard. Some common risks/hazards and why they are risks/hazards in this physics course include:

• High Voltage Electricity is dangerous as it can cause electric shock. To reduce this risk, you should not touch the apparatus while the electricity is turned on.
• Super Strong Magnets can cut skin if they are brought too close together. To reduce the risk, you should be careful when handling these magnets, ensuring you don’t allow them to get to close to each other.
• Glass equipment can break and shatter causing flying shards of glass. To reduce this risk wear safety glasses and be careful and cautious when handling the fragile equipment.
• Radioactive substances or electromagnetic radiation can cause cancer to living things. To reduce the risk, ensure methods are taken to label radioactive substances with warning signs, and store them is layer of lead.

It is important to remember the three main aspects of risk assessments:

Identify the risk.
Explain why it is a risk.
Explain measures taken to reduce or eliminate the risk.

#### d) recognising the difference between destructive and non-destructive testing of material and analysing potentially different results from these two procedures

In destructive testing of materials, the testing process destroys the material. For example if I was testing the strength of a piece of glass, I would need to keep applying force until the glass broke. As the glass is damaged in the process of the testing, the method was destructive. In non-destructive testing the testing process does not alter the material. For example testing the resistance of a wire does not destroy the wire (unless the voltage is so great that the resistance causes the wire to melt, as this would be destructive testing).

In some cases the failure of the material in destructive testing can affect the results of the experiment. However also in some cases the non-destructive testing methods do no test the material with enough rigor and the results are not entirely accurate or correct. In some cases the material must be broken in order to test it.