Introduction

In this practical the pd along a piece of conductive paper is measured. Conductive paper is carbon impregnated paper that has a resistance of about 10kΩ per square. I find it quite convenient to use with electricity experiments since it draws little current from the source; this means circuit breakers don't pop and batteries don't go flat. Here a constant pd is applied to a strip of paper, and a voltmeter used to measure the pd along it. Knowing the resistivity of the paper it is possible to find the current flowing through the paper.

R=ρL/A

and V=IR

⇒ V/I =ρL/A

So a graph of V against L will be a straight line. The current can then be found from the gradient.

 

Assessment

There isn't so much data processing in this example since V and L are plotted without manipulation however if repeat measurememts are done the student will have to find averages and also do some processing once they find the gradient, this is enough to be considered for assessment of DCP (but only just).

 

DCP

• Students should construct a table for collection of raw data, the table should have units and uncertainties in the headers. The number of decimal places in the data should not exceed the decimal places in the uncertainty.
• The average value of pd should be calculated from the repeated measurements.
• The uncertainty in pd should be estimated from the spread of data.
• A graph of V against L should be plotted, axis should have the correct units and lables, custom error bars and best fit line.
• The gradient of the best fit line should be used to calulate the current in the paper and its uncertainty.

 

CE

The current through the paper is very small but can be measured if you have a sensitive ammeter, it is then possible to see if the calculated value is acceptable.

• The conclusion should state whether, acording to the results, the Pd was proportional to the Length of the paper.
• A value for the current should be quoted with it's uncertainty, this should be compared with a value obtained by direct measurent.
• An attempt should be made to explain large random errors in the data.
• If the line does not pass through (0,0) then it should be explained.
• There are several assumptions that have been made in the derivation of the equation, these should be considered when explainng the source of uncertainty.
• Suggested improvements should address areas of weakness.

Conducting paper marked report 1
Conducting paper marked report 2