# basic voltage query

Discussion in 'Homework Help' started by Albert Symes, Oct 22, 2015.

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1. ### Albert Symes Thread Starter Member

Oct 22, 2015
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got a question i cant find the answer to,can anybody help.cheers

a 12v dc supply is connected to 2 100k resistors in series.a voltmeter has sensitivity of 10,000 per volt is swithed to its 10v range,

calculate 1)voltage reading

2) the percentage error of meter reading in part (1)

Sep 25, 2008
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3. ### Albert Symes Thread Starter Member

Oct 22, 2015
41
0
got a question i cant find the answer to,can anybody help.cheers

a 12v dc supply is connected to 2 100k resistors in series.a voltmeter has sensitivity of 10,000 per volt is swithed to its 10v range,

calculate 1)voltage reading

2) the percentage error of meter reading in part (1)

4. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
10,000 what per volt? Cats? Geraniums?

Since this is homework, you need to show YOUR best attempt to solve YOUR homework problem. It doesn't have to extensive or correct, but you need to give us something to start from. We aren't going to just work your homework for you.

5. ### Albert Symes Thread Starter Member

Oct 22, 2015
41
0
12 x 10,000 ohm =120,000 /10 = 12000 v

Last edited: Oct 22, 2015
6. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
1,264
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It's not a well-formed question.
e.g. The voltage across the 12 V source is 12 V/

They probably want the voltage across one resistor. You also have to assume that the the voltage source is ideal. You can only measure one resistor at a time.

So find the voltage across the resistor with no meter.

Use the ohms/V relation to figure out what the meter's resistance would be at that voltage.
ohm/V * Volts = ohms(meter)

Calculate it again with the calculated resistance in parallel with 100K.

7. ### WBahn Moderator

Mar 31, 2012
18,085
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12 x 10,000 ohm = 120,000 ohm, it most certainly does not equal 1200 V.

8. ### Albert Symes Thread Starter Member

Oct 22, 2015
41
0
your right,i humbly apologise for that mistake

Last edited: Oct 22, 2015
9. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
Not a problem -- take another shot at it.

Hint: Be sure to get the correct units for the meter sensitivity and then track the units through your work -- most mistakes can be caught that way.

10. ### Albert Symes Thread Starter Member

Oct 22, 2015
41
0
12v x 10,000 ohm =120,000 /10 = 12000 v

11. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
Again,

12 V x 10,000 ohm = 120,000 ohm

It does NOT equal 120,000/10.

Even if it did, 120,000/10 = 12,000 and NOT 12,000 V.

And what is this 12000 V supposed to be? The reading on the voltmeter? Does that make any sense whatsoever?

You haven't yet indicated how the voltmeter is connected to the circuit. Don't make us guess.

What is the definition of the sensitivity for a voltmeter? What are the proper units for it?

Don't guess -- look it up.

12. ### Albert Symes Thread Starter Member

Oct 22, 2015
41
0
voltmeter has sensitivity of 10,000ohm per volt

13. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
Okay. So the sensitivity of your voltmeter is 10,000 Ω/V.

Now, what does this mean? What IS voltmeter sensitivity? What does it tell you?

14. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,393
1,211
Here is your problem:

on edit: Added one additional question.

This happens to be a favorite question of mine.

• ###### Question-meter-sensitivity.png
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Last edited: Oct 22, 2015
15. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
1,264
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I pretty much gave you how to do it. I just didn't do it for you.

Oct 22, 2015
41
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100,000

17. ### Albert Symes Thread Starter Member

Oct 22, 2015
41
0
thanks for the drawing,thats pretty handy

18. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,393
1,211
So ... What are the answers.

You could have done the same drawing.

Oct 22, 2015
41
0
100,000 ohms

and 10%

I think

20. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
Don't just "think" up an answer. Analyze the circuit and determine the answer!

And SHOW YOUR WORK!

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