PhysSandbox

Ohm's Law: Finding an Unknown Resistance (V–I)

Sweep the supply voltage across a fixed mystery resistor; record (I, V) pairs with small instrument noise and recover R as the slope of the linear fit V versus I.

Beginner· 22 min·Related simulator: Ohm's Law

Goal

Verify Ohm's law V = I R for an ohmic resistor and determine its resistance R from a linear fit of measured voltage versus current.

Equipment

  • Adjustable DC supply
  • Voltmeter path (simulated)
  • Ammeter (simulated)
  • Unknown carbon resistor

Theory

For an ideal resistor, current is proportional to applied voltage: V = R·I. Plotting V on the vertical axis against I gives a straight line through the origin with slope equal to R (in ohms when V is in volts and I in amperes).

Procedure

  1. The circuit contains one unknown resistor R at room temperature (treated as ohmic).
  2. Set the supply voltage V with the slider; read the ideal operating point I = V/R on the schematic.
  3. Press “Record measurement” to log (I, V) with small voltmeter and ammeter noise.
  4. Repeat for at least 6 different voltages spread between ~1 V and ~20 V (avoid clustering all points at one end).
  5. Inspect the linear fit V vs I: the slope equals R; the intercept should be close to 0.
  6. Compare your R with the reference value and write the conclusion.

Experiment

Unknown resistor R (fixed in this model): operating point on the V–I line; table rows add instrument noise.

Ideal operating current: I = V/R ≈ 0.0400 A

Take ≥6 points over a wide V range. The fit uses V on the vertical axis versus I on the horizontal axis (slope = R).

Measurements

№
Current I
A
Voltage V
V
No measurements yet — take your first reading.

Data processing

Add at least 2 measurements to compute the fit.

Uncertainty

Instrument uncertainty (simple propagation)

±0.050 V

±0.0015 A

Pointwise R ≈ V/I: δR/R ≈ √((ΔV/V)² + (ΔI/I)²). Values use your sliders on each recorded row.

Take measurements to estimate propagated uncertainty.

Your graded result is the regression slope of V vs I; pointwise R is for intuition and error budgeting.

Lab report

Opens the system print dialog — choose “Save as PDF” or your printer. Header and footer are hidden when printing.

One click opens the print dialog — choose “Save as PDF”.

Ohm's Law: Finding an Unknown Resistance (V–I)

Generated: 23 Apr 2026, 05:25

Goal

Verify Ohm's law V = I R for an ohmic resistor and determine its resistance R from a linear fit of measured voltage versus current.

Measurement table

#Current I (A)Voltage V (V)
No measurements yet — take your first reading.

Fit and derived value

Add at least 2 measurements to compute the fit.

Conclusion

The fitted resistance agrees with the reference value within tolerance. Main error sources: finite number of voltage steps, instrument noise, and assuming a perfectly linear ohmic element.

PhysSandbox virtual lab — values come from your session; add your own discussion of error sources.

Conclusion

The fitted resistance agrees with the reference value within tolerance. Main error sources: finite number of voltage steps, instrument noise, and assuming a perfectly linear ohmic element.