Things You Should Know For Physics Paper 5: Design, Analysis and Planning

VARIABLES

Scientists use an experiment to search for cause and effect relationships in nature. In other words, they design an experiment so that changes to one thing causes something else to vary in a way that the scientist can describe as a 'trend'. The most useful way to describe a trend is a mathematical one.

These changing quantities are called variables, and an experiment usually has three main kinds: independent, dependent, and controlled.

The independent variable is the one that is changed by the scientist. In an experiment there is only one independent variable. This is usually plotted on the X-axis of the graph that the scientist uses to display his/her results in.

As the scientist changes the independent variable, he or she observes what happens.

The dependent variable changes in response to the change the scientist makes to the independent variable. The new value of the dependent variable is caused by and depends on the value of the independent variable. For example, if you turn on a water tap (the independent variable), the quantity of water flowing (dependent variable) changes in response - the water flow increases. The more open the tap - the faster the flow of water. The number of dependent variables in an experiment varies, and there is often more than one.

Experiments also have controlled variables. Controlled variables are things that would have an effect on the dependent variable. S/he must be sure that the only thing affecting that variable is his/her adjustment to the independent variable.

So, controlled variables are quantities that a scientist needs to keep constant, and s/he must observe them as carefully as the dependent variables.

For example, if we want to measure how much water flow increases when we switch on a tap, it is important to make sure that the water pressure from the water supply (the controlled variable) is held constant. That's because both the water pressure and the opening of the tap valve have an impact on how much water flows. If we change both of them at the same time, we can't be sure how much of the change in water flow is because of the faucet opening and how much because of the water pressure.

Most experiments have more than one controlled variable. Some people refer to controlled variables as "constant variables."

INTRODUCTION

Whenever you design an experiment you have to first 'set the scene'.

You are not ever finding anything out without any preconceptions. You always have ideas about what you are going to find out - you have expectations!

In a science experiment these expectations will be based on:

- what you have experienced in life,

-experiments you have carried out before and

-scientific knowledge (things you have been taught about science at school, or have found out from books).

In your report you need to explain to the reader what you expect to find out and why!

You do not have to look into a crystal ball and write down numeric predictions... just predict a general trend. A good way to do this is to sketch a graph!

You do have to explain the main scientific ideas that your prediction is based on. Try to use scientific keywords in this section and explain in simple terms what you understand them to mean.

A Fair Test

A fair test situation is vital for an investigation's results to be meaningful. You therefore have to use the scientific knowledge you have explained to identify the variables in your investigation - things you have to control, otherwise it will not be a fair test. Say what will need to be controlled and why - using theory to explain it.

One of the variables will be the variable you are going to change. Say which on you are going to change and by how much (the range over which you will change it). Say how you found out that was a suitable range. It may well be your preliminaries that helped you decide on a suitable range! Then say have you are going to control all of the others you have identified.

Your fair test must be linked to your scientific knowledge.

PRELIMINARY READINGS

You will have a rough idea of what you want to do, but will need to 'tweak' your idea by trying things out practically. You therefore sketch out a rough experiemental procedure and test out the best way to do it in a preliminary session.You may want to:

- choose materials to work with: check that you will get a big enough range of readings with the ones you have chosen to investigate.

- find out if you are controlling the other variables well enough to have a 'fair test'.... maybe you will spot some you hadn't thought of!

- practise using the equipment, and see if you need to make adjustments to avoid or minimise errors.... or make it safer!

- spot dangers in your procedure that you ought to avoid.

Always check with a teacher before you carry out preliminary experiments - they have more experience at spotting potential dangers than you do!!

What you find out from your preliminary readings will influence your final design of your experiment.

Remember to say in your report if you found out a better way to do it from preliminary work.... and how you checked your ideas were sound before proceeding.

PROCEDURE

The procedure has several parts to it:

A fully labelled diagram of the experimental equipment.

· This should be so detailed that a person could carry out the experiment just from the diagram!

It must be fully labelled with specifications of the equipment (e.g. '250 ml beaker' rather than just 'beaker'). Measuring instuments must have their range as well as increments marked on them (e.g. rather than 'thermometer' you would put a mercury filled thermometer with a range of -10^oC to 110^oC in 0.5 ^oC increments)

A full side of A4 should be given to this!

A full list of equipment - including minor parts

This should be done on a separate sheet of paper as a list to be given to a technician.

Full specifications of the equipment (e.g. '250 ml beaker' rather than just 'beaker') must be requested. Measuring instuments must have their range as well as increments marked on them (e.g. rather than 'thermometer' you would put a mercury filled thermometer with a range of -10^oC to 110^oC in 0.5 ^oC increments).

Odds and ends such as: 4 connecting wires, 2 croc clips, sellotape etc. must be itemised.

A risk assessment

List all of the possible hazards you have identified and how you intend to avoid them.

A set of instructions

These should be in past impersonal tense.... 'The apparatus was set up as shown in the diagram. The beaker was filled with 100 ml water..... etc.'. This is far better than a list of instructions with bullet points - but you may want to start off with such a list and then translate it into the correct form of English for scientific writing.

The order must be logical!

Don't forget to say that the experiment was repeated, how many times it was repeated etc.

Remember to say that results were recorded - in a table of whatever, averaged, and that a graph was plotted.

RESULTS/ANALYSIS

When successive measurements of the same quantity are repeated there is a distribution of values obtained. In experimental physics it is vital to be able to measure and quantify this uncertainty. The words "error" and "uncertainty" are often used interchangeably by physicists - this is not ideal - but get used to it!

Some important questions can only be answered if, in addition to performing an experiment, an error analysis has been conducted. These include:

• Do the results agree with theory?
• Are they reproducible?
• Has a new phenomenon or effect been observed?

Types of Error

We need to identify the following types of errors:

• Systematic errors - these influence the accuracy of a result
• Random errors - these influence precision
• Mistakes - bad data points.

Accuracy and Precision

These are two terms that have very different meanings in experimental physics. We need to be able to distinguish between an accurate measurement and a precise measurement. An accurate measurement is one in which the results of the experiment are in agreement with the ‘accepted’ value. Note this only applies to experiments where this is the goal – measuring the speed of light, for example. A precise measurement is one that we can make to a large number of decimal places.

ERRORS

These cause reading to be different from the true value. For example; Error is a measure of how close you can be sure about your measurement.

Percentage error = (smallest measurement you can measure/your measurement)*100

e.g. a ruler in mm divisions measure
es a length of 10 mm. The smallest
that the ruler can measure is to within 0.5 mm. So the error in my
measurement of 10mm is;

(0.5 mm/10 mm ) x 100 = 5%

This means I have measured 10 mm +/- 5%
The measurement may actually have been as big as 10.5 mm or as small as 9.5 mm.

Types of Errors

- Random

Random errors may be detected and compensated for by taking a large number of readings.

For example: Random errors may be caused by human error, a faulty technique in taking the measurements, or by faulty equipment. These cause readings to be spread about some value other than the true value; in other words, all the readings are shifted one way or the other way from the true value.

- Systematic

These cause readings to be spread about some value other than the true value; in other words, all the readings are shifted one way or the other way from the true value.

- Zero

For example: A zero error occurs when a needle on an ammeter fails to return to zero when no current flows, or when a top-pan balance shows a reading when there is nothing placed on the top-pan balance.

(These have been compiled from various sources including Wikipedia and Cyberphysics )