Uncertainty in Resistance from Voltmeter and Ammeter
For R = V/I and small independent limiting reading uncertainties, the maximum fractional uncertainty is Delta R/R approximately Delta V/V plus Delta I/I, with each direct uncertainty taken consistently from the instrument specification.
Why this shows up in the exam
Voltmeter-ammeter resistance experiment · Selecting suitable meter ranges · Reporting resistance with justified precision
Learn the idea
Resistance found as V/I inherits the fractional uncertainties of both meter readings. A voltage reading and a current reading each occupy an interval set by their scales. Dividing them does not erase either limitation, so both relative contributions enter the resistance estimate.
🧠 Memory hook: A quotient keeps both fractional uncertainties.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- R = V/I — resistance inferred from simultaneous voltage and current readings
- Delta R/R approximately Delta V/V + Delta I/I — maximum fractional uncertainty for a quotient
How to approach it
- 1Find R from central readings
- 2Convert meter least counts to fractional uncertainties
- 3Add fractions and convert back to absolute or percentage uncertainty
Common slip-ups that cost marks
- •Adding volt and ampere absolute errors directly
- •Using full-scale range when least count is specified
- •Reporting an uncertainty with excessive significant figures
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
In a meter bridge, resistance 2 ohm is in the left gap and balance occurs at 40 cm from the left end. Find the right-gap resistance.
More from Experimental Skills
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