What are the key learning points regarding practical skills?
Measure and record data from practical experiments.
Analyse results of practical investigations.
PlotA way of presenting data for two variables that are numbers, that helps to show if there is a link between the data. to identify the relationship between the dependent and independent variable.
Measure the In a graph, the gradient is the steepness of the line. The greater the gradient, the greater the rate of change. of straight-line graphs to determine the value of a constant.
Understand the term When one variable is zero so is the other. As one variable increases the other does at the same rate. When 𝒚 is plotted against 𝒙 this produces a straight-line graph through the origin..
Understand the term Two variables are said to be inversely proportional when one increases by a factor of two (doubles) the other decreases by a factor of two (halves)..
How are practical skills assessed?
The practical skills component of GCSE Physics (CCEA) is worth 25% of the overall grade.
It is made up of two parts.
Part A
A two-hour practical examination (worth 7.5%) carried out in the school science lab.
In this session you will re-do two of the nine prescribed practicals from Unit 1 and Unit 2.
Unit 1
PP1 – Motion down a ramp
PP2 - Hooke’s law
PP3 - The principle of moments
PP4 – The relationship between mass and volume
PP5 – Measuring personal power
Unit 2
PP6 – Refraction of light
PP7 - Ohm’s law
PP8 – Factors affecting the resistance of a metallic conductor
PP9 – Factors affecting the strength of an electromagnet
Part B
A 75 minute (higher tier) or 60 minute (foundation tier) written examination (worth 17.5%) relating to ANY practical work from Unit 1 or Unit 2.
Some examples of non-prescribed practicals to revise include:
Dispersion of light
Measuring the power of an electric motor
Finding the focal length of a convex lens
Measuring the critical angle
Newton’s second law
Freefall motion
V-I graph for a filament bulb
Echo calculations
What are variables when carrying out practical work?
Often an experiment involves things that can change, known as variables.
Variables need to be identified, so they can then either be changed or controlled.
There are three kinds of variable:
Dependent
Independent
Control
Scientists often want to find out if changing one variable makes a difference to other variables.
In many (though not all) investigations variables are kept constant - the control variables.
Apart from one which is varied - the independent variable.
The effect of the independent variable is then determined by monitoring the dependent variable.
An example would be investigating whether increasing the length of a pendulum might alter its time to make one complete swing (the period).
As it is the length of the pendulum which is changing, that would be the independent variable.
The changing length alters the time for one complete swing, therefore the period is the dependent variable.
When carrying out the experiment, care has to be taken that other variables that might affect the period, such as mass of the pendulum bob, are kept constant.
These are control variables.
Values and readings
The values are the measurements used for the independent variable.
If, for example, one of the variables in an experiment was length, it would be important to decide the maximum and minimum values, and also the intervals between values.
If electric current at different voltages was being investigated, a decision would have to be made on what range of currents to use.
This decision would take into account elements such as available equipment, the scales on the measuring instruments and safety.
When measurements are being taken, it is usually appropriate to repeat them.
Sometimes, there are lots of possible readings that could be taken.
For example, if the length of a pendulum was being varied, it wouldn’t be necessary to measure every centimetre; however, it wouldn’t be a good idea to measure every 10 centimetres.
How to choose the most appropriate equipment
Shini and Simon demonstrate the key points to consider when using equipment in science experiments
The next step is to think about the most appropriate equipment to use.
For example, the length of a pendulum could be measured using a 50 cm rule, a metre rule or a 20 m tape measure.
In different circumstances, one of these might be more accurate than others, which would affect the choice.
If you need to measure 20 cm of string then a 50 cm rule would give a more accurate length than using a metre rule or a 20 m tape measure.
Also, using balances that measure mass to the nearest 0.01 g will give a more accurate measurement than using ones that measure mass to the nearest gram.
If you need to measure out 5 cm3 of liquid then a 10 cm3 measuring cylinder would give a more accurate volume than using a 100 cm3 measuring cylinder.
What observation skills are needed during a practical?
Shini and Simon discuss the things to consider when observing a science investigation
The experiment should be conducted in a clear and systematic way to ensure the data is complete and of a high quality.
In an experiment into the relationship between force, mass and acceleration a toy car of different masses runs down a ramp.
The acceleration needs to be measured several times at each mass.
The repeats would all need to be done in the same way and with care to ensure precise data.
If you observe that a repeat is not similar to the others then it is good idea to repeat it.
It is also important to pay careful attention while the experiment is being carried out.
It might be that the car starts to deviate from a straight line path; which if significant may mean that the method should be modified.
Taking accurate measurements
When using observations to collect data during an experiment it is important to be as accurate as possible.
For example, when measuring the The distance between the centre of the lens and the focal point. of a converging lens using a distant object.
The distant object is focused onto a screen.
Different people might make the call as to whether the object has been sharply focused at slightly different positions, so it might be decided to use the same person to make all the observations.
How to apply good analytic skills to practical work
Analysing Shini and Simon talk through the steps that should be taken when analysing a science investigation
Studying the data
Data collected during an investigation is normally displayed in a results table.
At this point you can study your repeats to see how close they are.
Repeats that are similar are said to be precise.
Sometimes you may have an A result or measurement is anomalous if it doesn’t show the same pattern as the other results in a set of data collected under the same conditions. An anomalous result is also called an outlier.
If this is the result of a measurement error it can be ignored, although it is good practice to repeat that measurement again.
Example:
An investigation was carried out to see how the current flowing through an electromagnet affected its strength.
The results table below shows the data from the experiment.
| Current / A | Number of paper-clips collected |
|---|---|
| 0.5 | 2 |
| 1.0 | 4 |
| 1.5 | 6 |
| 2.0 | 8 |
| 2.5 | 10 |
It is obvious that as the current increases the number of paper-clips collected and hence the strength of the electromagnet increases.
Notice that as the current doubles, i.e. from 0.5 A to 1.0 A or from 1.0 A to 2.0 A, the number of paper-clips collected also doubles, i.e. from 2 to 4 or from 4 to 8.
This is a good example of When one variable is zero so is the other. As one variable increases the other does at the same rate. When 𝒚 is plotted against 𝒙 this produces a straight-line graph through the origin..
We say that the number of paper-clips collected is directly proportional to the current.
This means that as the current doubles, so does the number of paper-clips.
How to display the data
It can be difficult to see the relationship between the variables from a results table so often the means are plotted on a graph or chart to analyse the results further.
It is important to choose the most appropriate type of graph or chart.
If both the independent and dependent variables are continuous data then a line graph (also called a scatter graph) is the best choice.
Usually a line of best fit will be drawn to show the trend in the data.
This will allow you to see the relationship between the variables, for example if they are directly proportional.
A directly proportional relationship can be shown by a scatter graph which:
is a straight line,
through the origin (0,0).
Question
Using the previous example of the electromagnet, click on the graph which shows the relationship between the number of paperclips and the current.
Also, you can see if any of the values are anomalous as they will be placed far away from the line of best fit.
The example below shows the results when several students carried out the measurement of the focal length of the same lens and plotted their results on the graph shown below.
Each student is given a number from 1 to 6.
How to measure the gradient of a straight-line graph
In an experiment to investigate how the resistance of a wire depends on the length of the wire the following graph was obtained:
The resistance R and length of wire L are related by the equation:
R = K L
Where K is a constant.
As resistance has been plotted on the y -axis and length on the x-axis this equation can be mapped to the general equation for a straight-line through the origin: y = mx
The mapping above shows that the value of the constant, K, is equal to the In a graph, the gradient is the steepness of the line. The greater the gradient, the greater the rate of change. of the line.
To measure the gradient:
Pick two points on the line of best fit as far apart as possible.
(if the line goes through the origin, you can use 0,0 as one of your points).
Measure the change in the y-values.
Measure the change in the x-values.
Calculate the gradient by dividing the change in y by the change in x.
Notice that the gradient, (and hence the constant), has units of the y quantity / the units of the x quantity, in this case Ω / cm.
Inverse proportionality
The other main relationship between two variables is Two variables are said to be inversely proportional when one increases by a factor of two (doubles) the other decreases by a factor of two (halves)..
Inverse proportionality means that as one variable doubles, the other halves.
An example of this would be the relationship between the resistance of a conductor and its cross-sectional area.
If the cross-section area doubles, the resistance halves.
If the variables are plotted then the graph is a curve.
To obtain a straight line, a graph of resistance against 1/ area must be plotted.
The relationship between the acceleration of a car and its mass was studied when a constant resultant force was acting on the car.
Which of the graphs below would be obtained for this investigation?
Evaluating
Shini and Simon present steps that should be taken when evaluating a science investigation
The final stage is to consider what has been learned from the investigation and the quality of the data.
If it is decided that the experiment could have been improved in some way; suggestions should be considered of how and why.
Drawing conclusions
In this part you will say what your results show and identify the relationship between the independent and dependent variables.
Evaluating data
You need to consider if the data is of high quality.
As well as looking at accuracy of the results, you can also consider reliability, repeatability and reproducibility.
An accurate result is one judged to be close to the true value.
Accuracy can be improved by using appropriate, high quality measuring apparatus and by using the apparatus skilfully.
Reliability is improved by repeating AND averaging the measured data.
When calculating the average, be careful not to include data which is obviously incorrect.
Example
A student takes several measurements of the The distance between the centre of the lens and the focal point.of a lens as shown in the table below.
| Focal length / mm | 297 | 40 | 302 | 301 |
The best value of the focal length would be obtained by ignoring the value of 40 and finding the average of the other three values.
| Focal length / mm | 297 | 302 | 301 |
i.e. average focal length = (297 + 302 +301) / 3 = 900 / 3 = 300 mm.
Results are said to be repeatable if similar results are obtained when you repeat your investigation.
To check reproducibility, you need to get someone else to follow your method and see if their results are similar to yours.
If the data is considered to not be of high quality then the method used might not be suitable.
