# major Physics AS help needed!

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#1
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.

The question..
-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
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#2
bythe way , i madwe a crappy drawing of the circuit ..it's an attachment of the first post..

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16 years ago
#3
Is this the OCR spec one?
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16 years ago
#4
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,
Ida.
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16 years ago
#5
(Original post by hihihihi)
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.

The question..
-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
you want to have a voltmeter with a resistance as close to infinity as possible. Current is not going to pass through paper easily so you want a very small piece of paper and a very high potential difference. Does what im help you or have i not really explained what your stuck on?
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#6
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

thanks again!
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#7
(Original post by iiikewldude)
you want to have a voltmeter with a resistance as close to infinity as possible. Current is not going to pass through paper easily so you want a very small piece of paper and a very high potential difference. Does what im help you or have i not really explained what your stuck on?
yes it says the volemeter has resistance of 1M ohms.

if i use a small paper then the current on my ammeter will be large ie a good/large enough quantity to measure?

lets say i move the ammeter so it's within the voltmeter. ie the voltmeter is parallel with both ammeter and material.
how do i calculate resistance of the material. do i just V= IxR it or must i consider the reesistance of voltmeter (which is 1Mohm)
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