Ohm-Law Circuit Connections and V-I Test
To test Ohm's law, vary the current through a resistor while measuring series current and parallel terminal voltage; an ohmic conductor at constant physical conditions gives a linear V-I relation whose slope is resistance.
Why this shows up in the exam
Verifying Ohm's law · Choosing meter connections · Finding resistance from a V-I graph
Learn the idea
An ammeter samples series current while a voltmeter samples the potential difference across the test resistor. Current must pass through the ammeter, so it belongs in series and should disturb the circuit little; voltage is compared across two nodes, so the high-resistance voltmeter belongs in parallel.
🧠 Memory hook: Ammeter in the path; voltmeter across the part.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- V = IR — ohmic relation at constant temperature and other physical conditions
- R = slope of V versus I — resistance from a graph with V on the vertical axis
How to approach it
- 1Mark the test resistor terminals
- 2Place ideal low-R ammeter in series and high-R voltmeter across
- 3Read graph axes before taking a slope
Common slip-ups that cost marks
- •Putting an ammeter directly across a source
- •Putting a voltmeter in series
- •Calling every nonlinear device ohmic
🌟 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
Vernier Least Count and Direct Reading
For a direct vernier, the least count is one main-scale division minus one vernier-scale division, and the observed reading is the main-scale reading plus the coinciding vernier division multiplied by that least count.
Vernier Zero Error and Correction
Vernier zero error is the signed reading shown when the jaws are in true contact; the zero correction is its negative, so the corrected measurement equals the observed measurement minus the signed zero error.
Screw-Gauge Pitch and Least Count
The pitch is the axial distance moved by the spindle per complete rotation, and the screw-gauge least count is the pitch divided by the number of equal divisions on the circular scale.
Screw-Gauge Reading and Zero Correction
The observed screw-gauge reading is the pitch-scale reading plus the circular-scale reading times the least count, and the corrected value is obtained by subtracting the signed zero error measured with the studs in contact.
Resolution and Propagation of Measurement Uncertainty
For independent small limiting uncertainties, the maximum fractional uncertainty of a product of powers Q = product(x_i raised to n_i) is the sum of |n_i| times Delta x_i/x_i; a directly read scale cannot justify resolution finer than its least count.
Stokes-Law Viscosity by Terminal Speed
For a small sphere moving slowly through an effectively unbounded Newtonian liquid, Stokes drag is 6 pi eta R v; balancing drag with weight minus buoyancy gives terminal speed proportional to R squared and inversely proportional to viscosity.