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A Level Physics P3 [Notes]

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Paper format:
There are two questions in each paper. Each question should take one hour as both questions are of 20 marks.

Question number 1:

Outline: it requires candidate to collect data, plot a graph and draw simple conclusions.

Question number 2:

Outline: No graph will be required. It will require candidates to follow an inaccurate method, take several readings and then evaluate the results. It requires candidates to identify the problems faced in experiment and suggest improvements that can be made.

Now, before starting on the tips to ace the Physics Practical Paper 3, you should keep this thing in mind that most of the marks are for working, presentation and conclusions. So even if your practical work is not very accurate you should move to the tables, graph and working without wasting time in making it more accurate. There are only 2 marks of quality on whole paper. Why lose rest of the marks due to 2 marks only? Don't get me wrong though. I don't mean that don't do the practical accurately but I mean to say that don't waste extra time in making it more accurate and leaving no time to complete rest of the paper.

I will discuss each point given in the Mark Scheme below.

Question 1:

In this question, first set up the apparatus in exactly the same manner as shown on the question paper. The first part of this question usually involves measuring something like diameter of a wire, length of some part of experiment apparatus, potential difference, current etc. While measuring you should ensure that you present the data to the appropriate number of significant figures so that if reflects the least count of the device being used for example:

  1. Micrometer Screw Gauge*: 0.01 mm
  2. Meter rule: 0.5 mm
  3. Vernier Caliper**: 0.1 mm
  4. Protractor: 0.5 degrees
  5. Graduated cylinder: 1/2 of the least count
  6. Time: 1 decimal place
And, yes, represent your all data in the SI units..meter (m) for length, radius e.t.c

* Below is the animation, showing: How to use Micrometer Screw Guage?


http://www.gceguide.com/wp-content/uploads/2016/07/Micrometer_no_zero_error.gif

** Below is the animation, showing: How to use Vernier Calipers?


http://www.gceguide.com/wp-content/uploads/2016/07/using-a-caliper.gif

In some cases, you have to measure something and judging by the space provided you have to show the evidence that you have taken repeated readings and averaged them out. Say you have to measure the diameter of a sample of wire, so using the micrometer screw gauze take 3 readings in three different parts along the length of the wire and show:

d = (d1+d2+d3)/3

and show the value calculated. Also remember to add appropriate units along with the individual readings you measure.

Then it says to repeat the procedure and get six different sets of data in a table. Students often have this thing out of their mind that the presentation is important and CIE in its examiner’s report terms such students as ‘weak candidates’. So first thing first, know how many variable you have to measure and/or calculate so you could draw appropriate columns.

Now before moving onto how to construct a 'nice' table, let's first consider what actually is demanded by the examiner in the table. A 'nice' table should have these features:

1) Range and distribution

Largest possible range is required. It means that for example if you have to measure length (L) from 0-100 cm, so we must take highest range while keeping the difference constant and getting 6 set of readings. The difference you can take here is 15, so the readings of L you'll take will be: 15 cm, 30 cm, 45 cm, 60 cm, 75 cm, 90 cm. These values of L you have cover almost the whole range of values possible from minimum to maximum. Same applies if you have set of resistors and you are asked to make different combinations of resistors to yield different values of resistances. These values of resistance must cover the whole range possible, like from minimum to maximum value possible (here keeping the difference between individual values of resistance for different combinations common is not necessary!)

2 ) Quality of data

In a nutshell this one mark is for how close your readings are to the readings of supervisor and does your readings have the points which make them look actual readings instead of made-up readings such as: (1) scatter of points about the graph, due to random error the points will never lie on a straight line (2) the trend is correct like dependent variable increasing with increasing independent variable and so on.

You will get accuracy marks if you actually write the values which are there on the equipment instead of making your own and if you did the experiment as accurately as supervisor.

3 ) Table

(i) Layout:

You will draw one single table with headings. Each heading will have the name or symbol of quantity with it's standard units in brackets of after slash such as “L /m” or “Temperature (K)”. using T can cause confusion so better write temperature or time instead of T or t unless the question explicitly says something like “ t=time period”. Writing “L m” or “temperature K” is not accepted.

(ii) Raw data

The data must be up to to the same precision. All the raw readings of a particular quantity should be recorded to the same number of decimal places which should in turn be consistent with the precision of the measuring instrument.
 
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(iii) Calculated quantities

For example, you record the values for current (I) using the ammeter. Then the question asks you to include the values of 1/I in your table. That 1/I is calculated from I.
Lets say, I was given to 3sf. Then the calculated form (1/I) must have same number of sf, i.e. 3sf or one more sf, i.e. 4sf. These number of significant figures for calculated quantity should be kept same throught out the colum for that quantity.
However, if you are to calculate resistance from p.d and current, and the p.d was up to 2 significant figures while current was up to 3 significant figures. Then the number of sf in the calculated quantity must be equal to the least number of sf used in the calculation or one better. Therefore, the resistance calculated can only be given to either 2 sf (least sf used in calculation) or 3 sf (1 better sf).

Now after this discussion of what are the features of a nice table, lets move on to : HOW to draw the table?

Use the full space provided. First draw a rectangle covering whole of the space and then draw a upper row relatively wide. Then draw a narrow column headed, S.No. (serial number), then draw equal sized columns for the variables, then draw equal sized 6 rows below the heading row the column heading carries one mark ‘quantity/unit’. Finally, record your raw data in to the table which is obtained from the experimental procedure. Afterwards, use this data to calculate other quantities. A 'nicely' made table looks like this (Here, I have taken the table from a question which involved measure of two quantities, x/m & I/A, and then involved a calculated quantity, 1/I) :

S.No..........X/m.....I/mA.....1/I / 1/mA
1..................0.100.......1.1 ............0.9
2..................0.250.......1.0............1.0
3..................0.400.......0.9............1.1
4..................0.550.......0.8............1.3
5..................0.700.......0.7............1.4
6..................0.850.......0.6............1.7

However, if you have to measure time period of an oscillating pendulum, make sure that the amplitude is not greater than 5 cm, then display 3 sets of data in 3 small columns for 10 oscillations 10t1 ,10t2, 10t3 and show in a separate column the calculated value of ‘t’ stating the formula in the column heading. Again significant figure of the raw data should represent the precision of the instrument used, and s.f of any calculated value from those data should be in same or one more s.f - correct calculation carries one mark!

DO NOT panic if your data has some flaws; inform the supervisor and if he gives replacement of some instrument carry on or if he doesn’t, hit him with the same instrument!! Just kidding. Any type of malfunctioning of instrument will be reported to CIE and you are not penalized for it as your practical skills are being assessed here.

After this comes the graph. Now what is required for the graph? Read it below!

Graph

Layout:

The axes must be labeled with their appropriate units (same as the headings of table). The scale must not be odd such as each 1 cm block = 3 N. Appropriate scales are 1,2 and 5 units = 1 block. Scale must be chosen to give at least 50% of the graph in both of x- & y- directions. On the graph grid provided, there are about 8 big boxes horizontally & 12 big boxes vertically (when the paper is viewed in portrait form). Therefore, the graph you draw must cover ATLEAST 4 boxes horizontally and 6 boxes vertically - appropriate scales must be chosen to ensure this. The line drawn must be extended beyond the points to occupy full graph. False origin should be used if the values start far away from the origin. The numerical labels must be regularly spaced. Scale markings should be no more than three large squares apart so to be on the safe side label all the
marks.

Plotting:

All points must be plotted accurately so they are not more than 1mm away from where they must be plotted(slight offsetting is pardoned). The point must be plotted sharply. If the points are not visible due to sharp lead then cross them or encircle them. Personally, I recommend using small crosses instead of dots (points), because blobs (points with diameter > 0.5 small square) are not accepted. Otherwise, if you find it easy to work with dots, use them, but make sure they are not blobs.

Trend:

The graph is a straight line. But it is not possible that all points lie on the line. A best-fit line has to be drawn. Most people don't get the idea of best-fit line. By best-fit we mean 'average of all points' line. There must be even distribution of points above and below the line. The scattering of points around the line is due to random errors.

Best fit line must have the balance of at least 5 points which means you can ignore any one point which does not fit into a trend . There must be an even distribution of points either side of the line along the full length, as we can call the best fit line ‘Insaaf Wali Line’ in Urdu, which means line doing fair treatment to all the points. So the vector displacement of the points from the line should cancel out to zero . Lines must not be kinked. Lines thicker than half a small square are not accepted so I recommend a sharp lead pencil and a transparent ruler for this job. All points in the table (minimum 5) must be plotted for this mark to be scored. All points must be within 2 cm (to scale) in x direction of a straight line.

When finding gradient from the line draw the triangle with the hypotenuse at least 70% of the graph. Label the points with their coordinates.

Analysis conclusion and evaluation

Finding gradient and y-intercept:

First you will need to revise the equation of linear lines if you don't remember them. A linear line can be written in equation as: y=mx+c

y is dependent variable, x independent variable, c is point where line touches y-axis(a constant), and m is gradient of graph.

To find gradient. From your points which you found by drawing triangle on the line, you can find gradient by this equation:

(Y2 - Y1) / (X2 - X1)

Both read-offs must be accurate to half a small square and sensibly quoted on the graph and in the calculations as well.

Finding the Intercept:

Either: Check correct read-off from a point on the line, and substitution into y = mx + c. Read-off must be accurate to half a small square.

Or: Check read-off of intercept directly from graph. then a calculation follows which requires you to substitute the values obtained in previous calculation of gradient and intercept. A method mark and a accuracy mark for the new calculated value.
 
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Messages
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wht does pt within 1/2 a small sq mean? is it abt the plot shd b within half small sq or size as you mentioned?
It is about the size of the point you mark. Your marked point shouldn't be larger than half of the smallest square.
 
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