# Physics Planning Sheet OCR (AS)

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16 years ago
#21
hmm i think i understand the wheatstone bridge -

so initially Vout = 0

but resistance increases due to force stretching the wirse so u get a value for Vout.

what do u do with this Vout to get find the resistance (or the increase in resistance)?
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16 years ago
#22
i was going to do that or mabye even just connect the wores of the strain gauge to a multi meter but my physics teacher told me that it wouldnt be accurate enough and that wheatstone bridges as designed to measure small changes in resistance. also the knowledge that when a wire is stretched that the resistance increases in that wire is on the ocr syllabus and a strain gauge is just an application of that
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16 years ago
#23
what you do is adjust the resistors until Vout=0. it should be clear which one you need to adjust from the equation. this will then give you a new value for the resistor you have adjusted, you then rearrang the equation so the strain gauge is the subject to get then answer

i forgot to mention earlier that the other three resistors in the strain gauge are known
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16 years ago
#24
is it not just possible, rather than using the wheatstone bridge configuration to find the resistance of the strain gauge, to use a digital multimeter set on resistance?? i know it is less accurate, but i have not came across this "wheatstone bridge" before and do not know how to use it or incorporate it into my circuit.

I have searched google, still none the wiser.
Any help would be appreciated!

cheers
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16 years ago
#25
if you search for galvanometer in google you should be able to filter through the information you dont really need, when i did my planning i found these websites useful.

http://www.play-hookey.com/dc_theory...ne_bridge.html
http://www.frca.co.uk/article.aspx?articleid=336
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16 years ago
#26
thankyou very much, i will have a look now

cheers
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16 years ago
#27
Sorry.....has anyone got an idea what the intitial resistance value of a uni-axle strain gauge could be...or do i just make a value up?

cheers
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16 years ago
#28
(Original post by shoe_box10)
what you do is adjust the resistors until Vout=0. it should be clear which one you need to adjust from the equation. this will then give you a new value for the resistor you have adjusted, you then rearrang the equation so the strain gauge is the subject to get then answer

i forgot to mention earlier that the other three resistors in the strain gauge are known
what equation? the Vin Vout one?? 9this one hewre http://civil.colorado.edu/courseware.../tquarter.html ) also we dont want to know the strain gauge, but the resistance change.

is it possibly to measure Vout by putting a voltmeter in parallel where Vout is? ? and rearrange the equation to get deltaR
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16 years ago
#29
I was knida content with just attaching voltameter and ammeter to the strain gauge to record the resistance using R=V/I. But i think the problem is with:
1. What material to use
2. What range the the meters used to record the resistnace would be in.
3. approx range of forces to use.
The range for the meters just seems so hard, as you would have to calculate how much you expect the resistance of the strain gauge to change with extra force applied, but how can you be sure without any knowledge of the strain gauge to be used and the wires properties.
This is just so much harder than any other planning exercises weve had to do.
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16 years ago
#30
if you look at fig 2-6 at

http://www.omega.com/literature/tran...3/strain2.html

The bridge is considered balanced when R1/R2 = Rg/R3 and, therefore, VOUT equals zero.
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16 years ago
#31
OK so lets say, if u ahve 2 resistors, 1 variable resistor and 1 strain gauge in WB circuit like on those diagrams..once u apply a force, resistance in straingauge increases, so u get a Vout value

then u use the variable resistor to balacne it again- ie to make Vout=0

to balance it again do u just use Traial adn Error?? ie fiddle around with the resistors?
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16 years ago
#32
hi everyone.
you don't need to use a wheatstone bridge because that is really complicated, and too difficult to arrange etc. While the circuit with the ammeter and voltmeter is inaccurate. Instead you use a potential divider circuit to calculate the resistance - i'm not too sure on how to set one of these up but there's bound to be something in an AS physics txtbook, if someone finds anything it would be very much appreciated, seeing as i havent actually started this bloody exercise yet. hope this helps anyhow.
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#33
any more help pleeeeeeeeeeeeeeeeeeeeez
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#34
help pls, any more ideas
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16 years ago
#35
yeah i asked my teacher if it involves the wheatstone bridge circuit, and she goes "let me put it this way, its not on your syllabus" which means no.

i really need help on what material to use! anyone? please??

also are people doing it vertically or horizontally?

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16 years ago
#36
Hmmm... This post from another thread should help:

(Original post by t-t-t-tommo)
here's what i got so far:

the test specimen is "stiff rubber"
the strain gauge will be attached to it by "epoxy adhesive"
the stiff rubber will be clamped to a table
an increasing load will be hung from the stiff rubber at the edge of the table
the resistance will be measured by means of a basic circuit with a voltmeter in parallel and ammeter in series with the strain gauge
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16 years ago
#37
thank u soooo much but i need 2 know where he got that information from because we need 2 rite our resources, and i wouldnt have though of that myself.
are u sure this is right tho? cos my brother (doing a degree in physics) said it might be easier to clamp it vertically and hang weights of it without using a pulley.

thank u
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16 years ago
#38
I have no idea where he got that info; but I thought up a similar experiment myself (Just think of what you need to do, and how you need to do it)

As for the pulley idea, you could do it, but i think it'll become very complicated, and difficult to write up with the amount of words you have.

The experiment details I gave you is basically what's expected of you - there is no need for the wheatstone bridge arrangement (although, from what I've seen, if you do electronics it's easier to do)
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16 years ago
#39
(Original post by LS.)
I have no idea where he got that info; but I thought up a similar experiment myself (Just think of what you need to do, and how you need to do it)

As for the pulley idea, you could do it, but i think it'll become very complicated, and difficult to write up with the amount of words you have.

The experiment details I gave you is basically what's expected of you - there is no need for the wheatstone bridge arrangement (although, from what I've seen, if you do electronics it's easier to do)
but wouldn't it be simpler without the pulley, cos then u just clamp the rubber and hand the weights off it, without the need for a pulley or means of attaching the rubber to the weights using a string or whatever to go over the pulley.

could you tell me where you got the thread from, of the title of the thread please.

about me not being able to think of it myself, i meant the idea of using stiff rubber and using an epoxy adhesive...i don't even know what epoxy adhesive is!
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16 years ago
#40
Im settin my experiment up vertically, where i attatch the material (wood or somin) to the wall and then hang weights off it and measure the change in resistance using multimeters.... pretty simple design really
Ive shown it to my teacher and he says the method is fine and i just have to make improvments to the bit where u say what results u expect and why u expect them.
anyway, practical exams this wednesday so good luck to everyone
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