we were given a mock Plan to do (for practical exam):
A student is comparing materials for use as electrical insulation. The student decides to measure the resistivity of a number of different materials. The circuit shown is sued to determine the resistance of the sample.
The sample is connected to the circuit using a pair of small discs as shown.
The student finds that the milliammeter gives a small reading. However, when the voltmeter is removed from the circuit the milliammeter reads zero. The student concludes that the current registered on the milliammeter is due to the voltage and not the insulating material. The student decides to make several modifications to the experiment in order to obtain a measurable current in the insulating material.
-design a lab experiement to measure resistivity of an insulating material such as paper..
-steps which would be taken to ensure that the current measured isnot due to the current passing through the voltmeter
-modifications u’d make to the circuit in order to obtain a measurable current
ok i dont really understand.. the mA is in series and the voltmeter in parallel so surely it'd work?? i dont understand how it's measuring the resistance fo voltmeter.
i'd liek some help plz// if u could give a me some hints/a starting place. is there emf involved?
obviously there'd be resistivity= RA/l ,anything else??
thanks for anthing
Is this the OCR spec one?
But either way, I did something very similar last year. It measures the voltage running through the voltmeter. The voltmeter has a theoretically infinite resistance, but when there's a "choice" between the high resistance path of the paper and that of the voltmeter, the voltmeter wins. If you put the voltmeter around both the ammeter and paper you won't have that problem, but it brings up some other problems.
Hope this helps,
yeah this is from OCR march 2003
thanks for all replies.. i think i do have a starting point now
so if the voltmeter was in parallel around BOTH the ammeter and material then the ammeter would only measure the current of the material, yes??
also what do i do with the EMF of 3V
if u've any more suggestions /tips please reply!