Question about Full scale deflection

[Tony Meloni]

Homework Statement

hello, just came across this type of question for first time.
A voltmeter with a range of 0-30volts is to be used to measure a 120 volt circuit. calculate the value of the resistor to be placed in series with the meter. the sensitivity of the meter is 1000 ohms per volt ?

Homework Equations

im taking a guess its 120-30 = 90 vd x 1000 = 90k ohms ? any input would help thanks

The Attempt at a Solution

 

[mfb]

Don't guess. What does the sensitivity tell you about the resistance at 30 V at the voltmeter?

 

[Tony Meloni]

its telling me at 30volts its 30k ohms since its 1k per volt ..needs 90volts to drop across the resistor .. 120volt x 1000 k = 120k - 30k = 90k x .001ma = 90vd ?

 

[mfb]

120volt x 1000 k

What do you multiply here and what are the units? k without a unit doesn't have a proper meaning.

120volt x 1000 k = 120k - 30k

That equation is wrong.

120k - 30k = 90k x .001ma

That equation is wrong as well.

90k x .001ma = 90vd

That equation has at least an issue with units.

 

[Tony Meloni]

so what are the proper steps to find the answer ?

 

[mfb]

If the voltmeter has 30 kOhm, what is the maximal current flowing through it?

At this current, what do you need as additional resistance to get 90 V voltage drop there?

 

[Tony Meloni]

my choices to pick from the homework are

60k ohms 90k ohms

90ohms 150k ohms

if you take 90k x .001 you get 90vd

 

[mfb]

if you take 90k x .001 you get 90vd

With the right units this can be useful.

Work with units. It helps spotting most errors you'll make if you don't work with them.

 

[Tony Meloni]

what do you mean by units ?

 

[mfb]

Ohm, Volt, Ampere and so on.

 

[Tony Meloni]

how would you write out the problem using ohms law ?

 

[mfb]

That's your homework task. You need a few simple steps in a row, going from the voltmeter to the additional resistor step by step.