What other graph can be plotted in Worked example 1, and how is its gradient used to find ρ?

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WORKED EXAMPLE 1 Plan an experiment to measure the resistivity p of glass, which is about 1010 2 m. You have available a number of sheets of glass of the same size but with different thicknesses. Resistivity p is defined as p = RA 4. Step 1 Identify the variables. • The independent variable is the thickness I 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 A of the glass. Since this is mentioned in the question, suggest also that the temperature must be constant. Step 2 Describe the method of data collection in logical 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. 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. Step 3 Add any additional details. How can you obtain reasonable values? Think about the size and Activate Window thickness of the glass to be used and whether you can detect a reasonable change in the Settings to activ dependent variable, the resistance. You might, for example, suggest using a sheet of glass 1