I measured resistors in series using an ohmmeter and using a power supply/ammeter then applied ohm's law.

The values differed.

Which is more accurate and why?

This post was edited by ROM on Sep 23 2014 11:43am

How different are your values?

Not very. Example, theoretically the total resistance value of the 330ohm and 560ohm resistors in series should be 890 ohm.
Measuring with the just ohmmeter I get 877 ohm.
Measuring with the ammeter ran in series, and voltmeter in parallel. Applying 15volts with a variable output power supply then applying ohm's law with the values, I get 881ohm.

This post was edited by ROM on Sep 23 2014 05:50pm

So first of all, we know that resistors have tolerances. If given a 330 ohms resistor, it may not actually have a value of 330 ohms, see picture above for tolerances.



As for why your ohmmeter and experimental calculations differ slightly, the simple answer is that your laboratory equipment such as your ammeter or your ohmmeter have accuracy ratings too. You can check the specifications of the equipment and it should tell you a range or percentage of accuracy.

This post was edited by DynastyFury on Sep 23 2014 06:12pm

Thank you, this is exactly what I was looking for. Donating 4fg.

Thanks for Don8.

the power supply + ammeter should be more accurate even though you have uncertainties from 2 devices
They most likely have more precision/accruacy. the main thing is that the power supply is variable and you're able to dial in that 15v
That voltage probably will be fairly accurate while the multimeter's measurement will depend on its battery.

I understand. But this is year one stuff for me. The prof is just looking for the simple answer. I understand in reality measuring just the resistors in series in parallel with an ohmmeter isn't the most practical thing to do in the workforce.

Saber is trying to answer your question since I did not really answer it fully other than point out that lab instruments have accuracy ratings in the specifications.

Assuming no human error, your professor may actually be asking you to understand how an ohmmeter works and how it measures the resistance versus how an ammeter works and how it measures the current. Then, come up with a conclusion based on the relationships. Saber gave the hint about voltage/battery.

This post was edited by DynastyFury on Sep 24 2014 09:28am

I don't think that's what he is asking since we haven't covered anything like that. I am not sure. It's not marked too hard so as long as I get an answer that could be considered right I should be okay.

Thanks though guys. I appreciate the help and it is good for me to know. It gives me a head up over the rest of my class.

/edit

While we are on the subject.
In the experiment we had to measure each resistor resistance before proceeding. Why did we do this?

The only reason I can think of was to see if the individual resistors were working before running them in series.

This post was edited by ROM on Sep 24 2014 11:52am