Practical Techniques for physics ,chemistry, Biology (Paper 3 practical)

[ismailmz]

Friends could you please tell me the short cuts that could be applied in these 3 subjects paper 3 practicals . To manage time this is very vital, so I believe someone experienced would have much relevant idea on this, and to be frank this post would also help out other students in this forum so please do-not skip this post ;ones entered at least answer something.

 

[CaptainDanger]

I've posted them before, but anyway, here you go:

Oscillations

* Perform 3 sets of measurements for each different oscillation, (you will get more marks if you do 2 sets of repeats).
* Measure at least 30 periods in total.
* Preferably make each measurement 20 periods long.
* Precaution: minimize oscillations in any other plane other than the one being observed
* Always write times to two d.p., never to the nearest second.
* Precaution: do small amplitude oscillations
* Precaution: say that you did several periods at once to minimize reaction time errors.
Moments

* Precaution: balance the ruler being used first.
* Use distances from the pivot of greater than 25 cm (250 mm).
* Weigh the unknown and known masses in your hands before using the ruler, and place the lighter mass as far from the pivot as possible.
* Measure distances from the pivot to the centre of mass of the object.
* All measurements should be to 1 mm accuracy.
* To make sure the rule is balanced, pull each end down. If it comes up again then the rule must have equal moments acting on it on either end.


Density Measurements

* When using a micrometer, one full rotation of the barrel is 0.5 mm (50 on the barrel scale).
* The micrometer reads to 0.01 mm accuracy.
* Vernier calipers can read to 0.05 mm accuracy. However, it is advisable to quote the figure to 0.1 mm accuracy, as this will give a larger uncertainty, which will make comparisons in later parts of the question easier.
* If measuring a very small dimension, measure several "thicknesses" of it. Remember to divide by the number of thicknesses after the measurement!
* Precaution: check the zero error on the micrometer and/or Vernier calipers. Wipe the jaws of the micrometer to remove grease.
* Give answers to 2 or 3 s.f. : it is meaningless to write, for example, "density is 7785.654 kg m-3"!
* Always take at least 3 readings for each measurement, and take an average.
* When measuring string diameters or foil thicknesses (or similar), use a minimum of 10 thicknesses.


Uncertainties

* % Uncertainty =
* When multiplying or dividing quantities, add their % uncertainties together.
* When adding or subtracting quantities, add their absolute errors together, then divide by the result of the addition/subtraction of the measurement, e.g. for , where , and , the absolute errors added = 0.102 mm. Therefore the % uncertainty is:
* If you have to calculate the error in, e.g. , the absolute error in d must be multiplied by pi and then added to the absolute error in x. The percentage uncertainty is this total error divided by the calculated and the result multiplied by 100.
* If a measurement is to be raised to a power, then multiply the % uncertainty in the measurement by the power to get the % uncertainty in the overall term.
* If two values, for say, a density are available, calculate the % difference between them. If a value is given by the examiner, then use this as the "correct" value, and calculate the % difference the following way: where x is your measured value, and c is the examiner's value. If you have obtained two values, then the expression changes: where x1 and x2 are your measured values, and is the median of the two, (not necessarily the mean!).
* Compare the % difference with your % uncertainty. Any relationship suggested, such as that the two densities should be equal, can be considered correct if your % difference is less than your % uncertainty. This will gain you marks!
* If your % uncertainties look small, check that you have multiplied by 100!

Electrical Experiments

* With capacitor discharges, either take readings every 5 seconds for the first part of the discharge, or I think that every 10 seconds is sufficient.
* If a range is specified over which you should take measurements, do not exceed it: you will be penalised.
* With an analogue ammeter, use the top scale. This reads (generally), from 20, to 0, to 10. These are in fact divisions of 10 m A, and the meter actually reads from -20 to 100 m A. If you are out by a factor of 10, (e.g. you get a calculated cell voltage of 0.15 V), check that you have read the meter correctly. Always remember that the polarity on the meter must be correct.
* Any small discrepancy in your results can be explained by "electrical resistance at the contacts in the circuit".
* With most electrical experiment where a curve will be obtained (e.g. the V/I characteristic of a diode), 9 points on a smooth curve are sufficient.

Graphs

* With any graph, a minimum of 6 to 8 points are needed, and you must have at least 4 points on a curve.
* When measuring the gradient of a graph, carry your tangent on to the sides of the graph paper, however big your graph. The triangle you use should be greater than 10 cm in length and height, although in some mark schemes 100 cm2 is fine.
* Your graph does not have to go through the origin. If the data does not indicate that it does so, do not force it to. Comment on the fact that there must have been a systematic error.
* When choosing values to read of a graph, it is better to take them from the middle part of the curve, as this is where you will have more points per change in y co-ordinate.
* When describing your "plan" in Experiment C, state that the graph you plot will be a straight line through the origin (if this is the case!), of gradient = to an expression which will help you confirm the relationship suggested by the examiner.
* Turning points on graphs require at least 4 points.
* If points near the origin deviate substantially from your line of best fit, point out that for small measurements there is a greater uncertainty.

Experiments Involving Temperature

* Readings should be accurate to fractions of a degree.
* Stir any liquid being heated.
* Insulate the apparatus if possible.
* The thermometer should not be touching the sides of the container it is in, and should be in the middle of the liquid you are measuring the temperature of.
* Precaution: read the thermometer at eye level to avoid parallax error.
* The bulb of the thermometer should be completely submerged.
* Comment on the result obtained being the right order of magnitude.

 

[ismailmz]

Can i even get for chemistry and Biology of this sort please

 

[CaptainDanger]

I didn't get any such for those subjects..

 

[Unicorn]

Oh and before i forget for:
electrical practicals:
*always check the electrodes of the battery casing and the ends of the wires because any rust/oxidation on them will affect the results heavily [and are clean]
*if you are using an analogue reader always tap the reader just to make sure the needle is not stuck

Oscillations (or any other experment)
*when asked to justify the number of s.f. you gave in your answer always say it is because you have used the same number of s.f. before in your previous readings and you are always meant to give s.f. to the same number as before or 1 better

Uncertainty
*always give your absolute error to 2 [depending on units] (plus or minus) [3max] to be on the safe side as this is what all mark schemes have but however this depends on the type of reading as for some it may be greater but never give an absolute error in fractions you will lose marks
Graphs
*sometimes the question (mostly question 1 in AS) will ask for you to show that a particular formula equals to the graph you have drawn break up the equation finding the letters that correspond to [gradient], [y intercept] and [x value] it will making the rest of the queation much easier to solve


All the best

 

[Unicorn]

Don't we have to draw straight line graphs?

 

[CaptainDanger]

Don't we have to draw straight line graphs?

We have to...

Already mentioned :
* When describing your "plan" in Experiment C, state that the graph you plot will be a straight line through the origin (if this is the case!), of gradient = to an expression which will help you confirm the relationship suggested by the examiner.

 

[Unicorn]

Will we have enough time to do another repeat of the whole experiment? (to get 2nd set of data)

 

[CaptainDanger]

Will we have enough time to do another repeat of the whole experiment? (to get 2nd set of data)

I don't think so...

 

[Unicorn]

We have to...

Already mentioned :
* When describing your "plan" in Experiment C, state that the graph you plot will be a straight line through the origin (if this is the case!), of gradient = to an expression which will help you confirm the relationship suggested by the examiner.

does this apply to AS?

 

[CaptainDanger]

does this apply to AS?

I haven't come across it yet...

 

[Just call me MJ]

In AS you have plenty of time if you have done some practical before!

-Focus and read carefully!

-As for Biology! don't warry! its gonna be simple if you studied the way you should!
-Chemistry however, is quite annoying! one drop of water could make a noticeable difference! so I guess the most important thing you need to watch out for is that all of your tools are dry! even the spatula, glass rod, thermometer.......etc.

-Try to do as many practicals as you can! and as many times as you can! this will improve your speed and accuracy!

-oh! and the materials you get are limited! so you have a specific amount for a specific number of recordings (which you must choose!). if they are finished and you want to do more recordings or experiments, they won't give you!
-If you have anyproblem with any instrument, ask the invigilator to give you a new one! and don't just sit there try to fix the one at hand too! you need all the time you could get!

-careful and accurate readings are essential! e.g if you get a 27 on your first reading and then a 20 for the second reading of the same expirement then they (the examiners)would deduct some marks for that!
to avoid this outcome, take a set of value and cancel out the unrealistic values! e.g you have a set of 5 numbers {20,22,21,27,24}. you take the average of three which are closest to eachother and cross out the rest!

-and last but not least! GOOD LUCK!

 

[MindStealth]

Here are some good tips for Chemistry Practicals.
http://www.xtremepapers.com/community/threads/tips-for-chemistry-practicals-p3.11810/