Calculating Internal Resistance in a Series Circuit

[bengaltiger14]

Homework Statement

Calculate internal resistance of meter. There is a 10V power supply with a variable resistor connected in series. The meter will be connected across the resistor. I adjust the resistor until the reading in the meter read about 5.37V. The value of resistance is 15 ohms. How do I calculate the internal resistance? This is the equation I used

5.37V=10V((Rm*15)/(Rm+15))

I solve for Rm and get 559kOhm. Does this look correct?

 

[dlgoff]

Looks correct to me. Just curious, how did you measure the resistance being 15 ohm? With the same meter?

 

[alphysicist]

Homework Statement

Calculate internal resistance of meter. There is a 10V power supply with a variable resistor connected in series. The meter will be connected across the resistor. I adjust the resistor until the reading in the meter read about 5.37V. The value of resistance is 15 ohms. How do I calculate the internal resistance? This is the equation I used

5.37V=10V((Rm*15)/(Rm+15))

I solve for Rm and get 559kOhm. Does this look correct?

I don't see how you are getting 559kOhm from that equation; is there a calculation error?

Also, perhaps I'm visualizing the circuit incorrectly, but could you show how you got that equation?

 

[bengaltiger14]

That 15 ohms was what it took to make the meter reading drop to half of 10V. It is a variable resistor. I tried many ohmage settings and 15Ohms dropped the reading to half of 10V to 5.??.

alphysicist: As for a the miscalculation, you may be right. I calculated that quickly in lab and was unsure if I was even approaching the problem in the correct manner. I will check that but can't do it at the moment.

 

[alphysicist]

That 15 ohms was what it took to make the meter reading drop to half of 10V. It is a variable resistor. I tried many ohmage settings and 15Ohms dropped the reading to half of 10V to 5.??.

alphysicist: As for a the miscalculation, you may be right. I calculated that quickly in lab and was unsure if I was even approaching the problem in the correct manner. I will check that but can't do it at the moment.

What bothered me was that the equation you had in your post was not dimensionally consistent (volts on the left, volts*ohms on the right); I was wondering if there was more to the equation that you had already dealt with that took care of that. Unfortunately I think I am not visualizing your circuit correctly, so there could something that I'm not thinking about.

 

[dlgoff]

I tried many ohmage settings and 15Ohms dropped the reading to half of 10V to 5.??.

So you were using a decade box? There may be tolerances of the resistors to consider when determining the internal resistance of your meter.

 

[Redbelly98]

Are you supposed to calculated the internal resistance of the power supply or of the meter?