Strain measurement fundamentals — operating principle: A conventional electrical strain gauge (as used in Wheatstone bridges for stress/strain measurement) operates primarily on which physical effect? The claim states: “A strain gauge uses the principle of capacitance for its operation.” Decide whether this statement is valid.

Introduction / Context:
Strain gauges are among the most widely used transducers in measurement and control for converting mechanical strain into an electrical signal. This question checks conceptual understanding of the underlying physical principle. While the word “gauge” is sometimes used for capacitive displacement sensors, the standard, bonded electrical strain gauge used in bridges relies on resistive change, not capacitance.

Given Data / Assumptions:

• Conventional bonded strain gauge (metal-foil or semiconductor type).
• Gauge is typically arranged in a Wheatstone bridge for sensitivity and temperature compensation.
• Small strains (microstrain level) are measured as small resistance changes.

Concept / Approach:
The defining quantity for standard strain gauges is the gauge factor GF, given by GF = (ΔR / R) / ε, where ΔR/R is the fractional change in resistance and ε is the mechanical strain. Metal-foil gauges exploit piezoresistive and geometric effects in a conductive grid; semiconductor gauges use stronger piezoresistive effects. Capacitive sensors do exist for displacement or pressure, where capacitance changes due to plate spacing or area, but that is a different transducer class than the common “electrical resistance strain gauge.”

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer datasheets specify gauge resistance (e.g., 120 Ω, 350 Ω), gauge factor (≈2 for metals, much higher for semiconductors), and temperature compensation—not capacitor values. Calibration procedures reference resistance change versus applied strain, confirming the resistive foundation.

Why Other Options Are Wrong:

• Correct: Not correct; the principle is resistive.
• Valid only for foil gauges / semiconductor gauges: Both metallic and semiconductor strain gauges are resistive devices (though semiconductor types have higher GF).
• Undetermined without bridge excitation: Bridge excitation affects sensitivity, not the underlying physics.

Common Pitfalls:
Confusing “capacitive displacement sensor” with “resistance strain gauge”; assuming any two-terminaI sensor is capacitive. The naming may overlap in casual speech, but the working principles differ.

Final Answer:
Incorrect.