Manipulation, measurement and observation (9 marks)

Successful collection of data (7 marks) Measured values carry 1 mark. Care should be taken that the unit is consistent and the readings

are correct to the appropriate number of decimal places. Measurements (e.g. diameter of wire, p.d., current, length etc.) are considered to be raw data. All measured values or raw data should be noted correct to the respective instruments accuracy. For example, if length is being measured using a metre rule, it should be noted correct to the nearest mm (which is the smallest scale division on a metre rule). If the unit of measurement is cm, the smallest scale division would be 0.1cm (equivalent to 1mm), therefore all readings would be noted correct to 1 decimal place. Similarly, if the unit of measurement is m, the smallest scale division would be 0.001m (equivalent to 1mm) and all readings would be noted correct to 3 decimal places. The only exception to this rule is time. Time is not recorded in accordance to the smallest scale division of the measuring instrument (such as a digital stopwatch) simply because the Human Reaction Time Error (HRTE) is much larger than the instruments smallest scale division. Hence HRTE controls the significance. The HRTE is generally taken as +/- 0.5 seconds (1 decimal place) for every start/stop operation.

Repeated readings also carries 1 mark. Measured quantities such as p.d., current, time etc. are expected to be noted twice and then averaged to minimize experimental error. Candidates should appreciate the need for repeated readings where necessary.

Candidates are awarded 5 marks for six sets of readings, 4 marks for five sets of readings, 3 marks for four sets of readings etc.

Any major help regarding the setup of the experiment will result in a loss of 2marks and any minor help will result in a loss of 1 mark.

Any unreasonable values or incorrect trend will result in the deduction of 1 mark. Range and distribution of values (1 mark)

Range and distribution of values carries a mark. It is always preferred that the least and greatest values of the range mentioned in the

question are included in the six sets of readings i.e. if the range specified is 10cm l 100cm then the six sets of readings should start at 10cm and end at 100cm.

Once the extreme values have been selected, the remaining four sets of readings should be reasonably spaced so as to have a balanced distribution. The intervals between each reading do not necessarily have to be equal throughout the data.

Quality of data (1 mark) Quality of data also carries a mark.

This is judged by scatter of points about the LOBF (Line of Best Fit). The lower the scatter i.e. the closer the points are to the line with at least three points lying on the line the better the chances of scoring this point.

All points are assessed for this mark. At least 5 plots are necessary to score this mark.

Instructions for AS Practicals Type I Questions

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Presentation of data and observation (7 marks)

Table of results: Layout (1 mark) Column headings should be properly arranged i.e. S.No. first, measured quantities second and

calculated values last. Each column heading must include a quantity and unit where appropriate.

There must be some distinguishing mark between the quantity and its unit such as a solidus or brackets for example, V/V or I (A) etc. Care should be taken that whatever method is selected should be used consistently throughout the table. Solidus is the preferred way to go.

The second column based on either a specified range in the question or measured values should follow an ascending or descending order.

Table of results: Raw data (1 mark) All raw or measured data is governed by decimal places depending on the smallest scale division

of the instrument (with the exception of time) as mentioned earlier. All raw data should be consistently noted to the same number of decimal places (depending on

the smallest scale division of the instrument and the unit of measurement chosen in case the question does not specify any particular unit).

Table of results: Calculated quantities (2 marks) Calculated quantities are those that are derived using measured or raw data, for example if the

first two columns were V1/V and V2/V (which are measured quantities) then Vavg/V is a calculated

quantity.

All calculated quantities are governed by significant figures and not decimal places but do depend on the accuracy of the measured values. If we consider the example above, V1 and V2 are

noted correct to 2 d.p. if a digital multimeter is used.

V1/V V2/V Vavg/V

1.96 1.97 1.97 or 1.965

Note that when deciding on how many s.f. to consider for Vavg we have to determine how many

s.f. were our measured values (even though when we noted these raw values we did so

considering d.p. NOT s.f.). In this case both raw values are correct to 3 s.f. so our calculated value

could be the same (3s.f.) or on more (4s.f.).

Now consider another example. If values of a p.d. and of a current are measured correct to 2 s.f. and 4 s.f. respectively, then the corresponding calculated value of resistance should be given to 2

or 3 s.f. but not 1 or 4 s.f.

The rule is : Except where they are produced by addition or subtraction calculated quantities

should be given to the same number of significant figures (or one more) than the lowest number

of significant figures from raw data.

1 mark for writing calculated quantities to the correct number of significant figures. 1 mark for checking at random one calculated value to see if the calculation was correct.

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Graph: Layout (1 mark) The axes should be correctly drawn and labeled with the quantities they represent. Mentioning

the respective units is preferred. Sensible scales should be chosen. Scales such as 3:10 are considered awkward and do not gain

this mark. Though it is not necessary to choose a scale that occupies the whole graph grid it is compulsory

to choose the scale so that the plotted points occupy at least half the graph grid in both x and y directions.

Inverting axes is allowed. Graph: Plotting of points (1 mark)

This mark can only be scored if all points are plotted. Points should not be drawn as thick blobs. If the area occupied by any point is more than half a

small square on the graph grid this mark will be deducted. It is suggested that each point be circled to highlight it for the examiner. Any plotted point will be checked at random to see if it has been correctly plotted according to

the chosen scale. Candidates should work within an accuracy of half a small square.

Graph: Trend line (1 mark) At least 5 trend plots are needed for this mark to be awarded. The L.O.B.F. should be a reasonable one with low scatter. In case the scatter is large this mark

will not be awarded. There must be a fair scatter of points on either side of the line with each point being

approximately the same distance from the line.

Analysis, conclusions and evaluation (4 marks)

Interpretation of Graph (2 marks) Gradient (1 mark) The hypotenuse of the triangle must be at least half the length of the line drawn. The two points used should be clearly marked and their coordinates written clearly on the graph. Read-offs from the graph to calculate the gradient must be within accuracy of half a small

square. Any values used from the table to determine the gradient will not be accepted unless they lie on

the L.O.B.F. The ratio of y/x should be calculated correctly in accordance to the values shown. Y-intercept (1 mark) If the value is read off the graph it should be correct to half a small square. In case the graph has a false origin than it should be labeled on top as FO. In this case the y-

intercept can be calculated using the equation of a line y = mx + c.

Drawing conclusions (2 marks) A clear working is required to identify in the equation which quantity represents y, x, the

gradient and the y-intercept. The candidate is expected to realize how the values he obtained for the gradient and the y-

intercept will relate to the unknown variables in the equation. No mark will be awarded if the gradient or y-intercept calculated earlier is not used.

Clear working including all calculations should be shown. 1 mark is usually awarded for finding the unknown related to the gradient if within the

acceptable range. 1 mark is usually awarded for finding the unknown related to the y-intercept if within the

acceptable range.

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Manipulation, measurement and observation (7 marks)

Successful collection of data (6 marks) Measured or raw values should be taken so that the unit is consistent and the readings are

correct to the appropriate number of decimal places. Measurements (e.g. diameter of wire, p.d., current, length etc.) are considered to be raw data. All measured values or raw data should be noted correct to the respective instruments accuracy. For example, if length is being measured using a metre rule, it should be noted correct to the nearest mm (which is the smallest scale division on a metre rule). If the unit of measurement is cm, the smallest scale division would be 0.1cm (equivalent to 1mm), therefore all readings would be noted correct to 1 decimal place. Similarly, if the unit of measurement is m, the smallest scale division would be 0.001m (equivalent to 1mm) and all readings would be noted correct to 3 decimal places. The only exception to this rule is time. Time is not recorded in accordance to the smallest scale division of the measuring instrument (such as a digital stopwatch) simply because the Human Reaction Time Error (HRTE) is much larger than the instruments smallest scale division. Hence HRTE controls the significance. The HRTE is generally taken as +/- 0.5 seconds (1 decimal place) for every start/stop operation.

The rule of thumb above holds well for Type I questions but Type II questions are trickier as they have inherent limitations or flaws which the candidate is expected to realize and appreciate. Hence there can be exceptions to this rule depending on the design of the experiment. For example, in Experiment # 2 (in your Lab Manual), a micrometer screw guage was used to measure the diameter of the cardboard tube.

As d is measured in metres using a micrometer screw guage (which has a least count of 0.01 mm equivalent to 0.00001m), this value should be noted correct to 5 decimal places. But as this is a type II question, it requires you to realize the inherent limitation of the apparatus while you decide a reasonable accuracy to maintain.

In this case, the variation in the diameter of the irregular cardboard tube is nearly 2 mm (0.002 m) which is much greater compared to the accuracy which a micrometer screw guage offers (0.00001 m). Hence the last 2 d.p. are excessive and are not required.

For this experiment, due to these reasons all measurements in metres should be noted correct to 3 d.p. even though the micrometer is capable of measuring correct to 5 d.p.

Candidates should appreciate the need for repeated readings where necessary. For example, in the same experiment the determination of the diameter was tricky due to the irregular shape of the tube and it was expected that the candidates appreciate the need to repeat their readings in different directions across the circumference of the base of the tube.

Candidates are supposed to obtain two sets of readings in all Type II questions. Any major help regarding the setup of the experiment will result in a loss of 2marks and

any minor help will result in a loss of 1 mark.

Quality of data (1 mark) Quality of data also carries a mark.

This will vary experiment to experiment. In general any incorrect trend in values will result in the deduction of this mark.

Instructions for AS Practicals Type II Questions

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Presentation of data and observation (3 marks)

Display of calculation and reasoning (3 marks) First calculated value noted down correctly will score 1 mark. Second calculated value noted down correctly will score 1 mark. Both calculations will be checked to score the 2 marks mentioned above. Correct calculation to check for the stated relationship carries 1 mark.

It is expected that the candidate show all working clearly writing the algebraic relationship and then testing it using his two sets of readings to calculate the % deviation in the constant or the stated variable.

Analysis, conclusions and evaluation (10 marks)

Drawing conclusions (1 mark) Sensible comments relating to proportionality calculations and to the suggested relation score 1

mark. Incorrect ideas score zero. The conclusion should be based and stated clearly on the candidates calculation.

Estimating uncertainties (1 mark) The candidate will be challenged to decide for a reasonable absolute uncertainty. This would vary

experiment to experiment. For example, in Experiment # 2, the diameter was fairly easy to read, an error of 1 mm to 2mm in d was expected. In contrast, looking at Experiment # 9, the candidate should have appreciated that the highest position was difficult to obtain, an error of 1 mm (in accordance with normal convention) in h was too small given the motion of the rule. There is a misconception to equate uncertainty with the smallest division on the scale, without looking at the limitations of the experiment. A tolerance of 2-10 mm is acceptable in this particular case.

With repeated readings the uncertainty could be half the suggested range. Identifying limitations & Suggesting improvements (8 marks)

The first error/limitation and its improvement is standard for every type II question. Limitation: Two sets of readings are not enough to draw a conclusion. Improvement: Take more readings and plot graph.

Each of the four limitations scores one mark and each corresponding improvement scores one mark totaling 8 marks.

Improvements can only be made to the errors/limitations mentioned. If the improvement stated is not related to the error/limitation stated, marks will not be awarded.

If the limitation is written as limitation # 1 then its improvement should also be written as improvement # 1. Change in the order will result in loss of marks.

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