Page 544 - Physics Coursebook 2015 (A level)

532
Cambridge International A Level Physics
WORKED EXAMPLE
You might also think of any difficulties in carrying out the experiment. For example, draughts must be avoided and you must wait until the balloon has stopped swinging before taking a reading of θ .
Safety
It may seem strange, but you should always comment on safety when asked to carry out any experiment. In some situations the risks may be unimportant, and it may be sufficient to mention simple ideas such as wearing goggles to protect the eyes when heating liquids or handling
1 Plan an experiment to measure the resistivity ρ of glass, which is about 1010 Ω m. You have available a number of sheets of glass of the same size but different
thicknesses. Resistivity ρ is defined as ρ = RA .
Step1 Identifythevariables.
stretched wires, using a safety screen, ensuring that the apparatus is stable and not easily knocked over, using
a sand tray under heavy weights and making sure that weights do not fall on your foot, and switching off currents when not in use so that wires do not overheat.
In our example with the balloon, keeping away from the rotating blades in the fan and wearing goggles to avoid air blowing into your eye should be sufficient. Do give some detail in your suggestions and do not just say ‘use goggles’.
The logical steps are then to record ammeter and voltmeter readings with one thickness of glass.
Then repeat the readings with different thicknesses, suggesting sensible thicknesses of glass, perhaps every mm from 1 mm to 10 mm. If you are going to perform the experiment these thicknesses may be available, but if you are merely planning the experiment then you must suggest sensible values.
Step3 Addanyadditionaldetails.Howcanyouobtain reasonable values? Think about the size and thickness of the glass to be used and whether you can detect
a reasonable change in the dependent variable, the resistance. You might, for example, suggest using a sheet of glass 1 m2 in area and 1 mm thick. Its resistance is then:
R= ρl =1010 ×0.001=107 Ω A
Can this be measured with ordinary laboratory apparatus? What voltages and what meters are suitable? A voltage of 10 V produces a current of 1 μA, which is measurable, but 100 V gives a current of 10 μA, which may be easier to measure but more dangerous. With glass of thickness between 1 and 10 mm the current will be 1–10 μA and so the ammeter should measure from 1 to 10 μA or up to 10 μA.
As you can see, this means that you need some idea
of the size of quantities that can be measured. In this example you need to know what currents and voltages can be measured with ordinary laboratory equipment.
You may also give additional detail by describing how to attach the metal foil as contacts onto the large faces of the glass sheet with weights on top, or suggest that the glass be cleaned and dried.
■■ The independent variable is the thickness l of the glass.
■■ The dependent variable is the resistance R of the glass. Finding R involves measuring the p.d. across the glass and the current in the glass.
■■ The control variable is the area of the glass. Since this is mentioned in the question, suggest also that the temperature must be constant.
Step2 Describethemethodofdatacollectioninlogical steps.
To alter the independent variable, use glass sheets of different thickness but the same area. The thickness of each piece of glass is measured with a micrometer at several places and averaged.
The area A is required. This can be found by measuring the length and breadth of each sheet with a rule and multiplying the values together.
Draw a circuit diagram of an ammeter in series with the glass sheet and a power supply, with a voltmeter across the glass. Connections are made to the large surfaces
of the glass. This can be done using aluminium foil, or metal plates as in a capacitor, which closely touch each large face of the glass sheet. Use a diagram to show how this is done.
l