Section A – Mandatory experiments

Section A is worth 120 marks, with each question being 40 marks and based on one of the 24 mandatory experiments on the syllabus. You will need to answer three questions out of four from this section, spending about 15 minutes on each. It is important to note that each of these questions is worth 10% of your total grade. With a bit of time and effort spent revising the experiments full marks are very achievable for this section.

Preparing for the experiment questions

In preparing for your Physics exam make sure you can do the following for each experiment:

• Draw a labelled diagram.

• Describe the procedure, including what measurements were taken and how they were taken.

• Be able to use the appropriate formula to calculate the required value or to verify a law.

• Graph the results correctly for the experiment being described, be able to calculate the slope if needed and understand its significance in the context of the question.

• Be able to state at least two sources of error and the relevant precautions to be followed for each experiment as well as how to reduce percentage error.

Approaching graph questions

Of the 24 mandatory experiments 16 of them involve drawing a graph; therefore it is important to be able to draw each graph correctly as this can earn you a significant proportion of the marks on offer.

When drawing your graph note the following tips:

• Draw a large, clear graph on graph paper.

• Be aware that it is often necessary to modify the data provided in some way before drawing the graph. This may mean changing the units, e.g. milliseconds to seconds, squaring a set of values or finding the reciprocal. Use the relevant formula to help you decide if this needs to be done.

• Choose appropriate scales based on the data.

• Label the axes with both the quantity and its units.

• Plot the points using a sharp pencil with a dot surrounded by a circle.

• Using a transparent ruler draw the line of best fit so that there is an even distribution of the dots either side of the line. Note that in many cases this line will be going through the origin; if this is the case it makes drawing the line of best fit easier.

• When calculating the slope do not use the given data, choose points on the line of best fit and mark these on your graph. It is good practice to use points that are spaced far apart but within the values given. Let us look at an example of an experiment question from a past paper.

2009 QUESTION 1 – MEASURING G BY FREEFALL

In an experiment to measure the acceleration due to gravity, the time t for an object to fall from rest through a distance s was measured. The procedure was repeated for a series of values of the distance s. The table shows the recorded data.

Your diagram should indicate a timer, ball, release mechanism (e.g. electromagnet) and trap door (or pressure plate). When marking s on the diagram ensure you mark the perpendicular

distance from the bottom of the ball bearing to the top of the trap door.

Describe how the time interval t was measured.

The timer is connected to the electromagnet and the trap door. With the timer set to zero release the ball bearing. This starts the timer. When the ball bearing hits the trap door the timer stops. Reset the timer and repeat at least twice more on that distance, use the smallest time recorded as the value for t for that distance.

Calculate a value for the acceleration due to gravity by drawing a suitable graph based on the recorded data. You will need to graph s against t2 in this case, so the data in the table needs to be modified; firstly s should be changed to meters and t to seconds and then the values for t squared to get the data for the graph.

We can calculate a value for the acceleration due to gravity using the slope of the graph. Remember do not use the data given but calculate the slope of the line of best fit, as our line goes through (0, 0) we only need one more point and can calculate slope by rise/run. Acceleration due to gravity is then 2 x slope, in our case:

g = 2 x 4.98 = 9.96 ms2 The reasoning behind this is based on our equations of motion:

This is provided that s is on the vertical axis (rise) and t2 is on the horizontal axis (run).

Give two ways of minimising the effect of air resistance in the experiment.

Use a smooth, heavy object and ensure that the experiment is carried out away from doors or windows to minimise draughts.