Meter Loading and Measurement Error
Model an ammeter by resistance RA in series at its insertion point and a voltmeter by RV in parallel with the measured element. Recompute the circuit before comparing measured and true values.
Why this shows up in the exam
Voltmeter loading correction · Ammeter shunt circuits · Resistance measurement error
Learn the idea
Real meters alter a circuit: an ammeter has finite resistance and a voltmeter has finite resistance. An ideal ammeter is a wire and an ideal voltmeter is an open branch. Real instruments deviate from those limits, changing the very current or voltage being measured.
🧠 Memory hook: Ideal A is zero ohm; ideal V is infinite ohm.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- R_measured = V_meter/I_meter — apparent resistance depends on connection topology
- R_parallel = R R_V/(R+R_V) — voltmeter loading across a resistor
- Delta V_A = I R_A — ammeter burden voltage
How to approach it
- 1Draw each meter as its resistance
- 2Reduce the modified circuit
- 3Compare indicated quantity with the unperturbed target
Common slip-ups that cost marks
- •Removing a finite meter from the circuit model
- •Confusing meter range with internal resistance
- •Using V/I without identifying what each meter senses
🌟 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.
A cell of emf 6 V and internal resistance 1 ohm is connected to a 2 ohm resistor. Find the circuit current.
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