= Solution
Take the longitudinal scalar <piezoelectric coefficient> appropriate to the load and <electrode> direction. For a uniformly stressed slab of face <area> $A$ and thickness $t$, the source charge is $\Delta Q=d\Delta F=dA\Delta\sigma$. Its <capacitance> is $C=\epsilon_0\kappa A/t$, so the ideal initially uncharged, electrically isolated sensor gives
$$
\boxed{\left|\frac{\Delta V}{\Delta\sigma}\right|=\frac{dt}{\epsilon_0\kappa}.}
$$
The sign depends on the <poling> direction and <electrode> convention. Equivalently, the incremental constitutive law is $\Delta D=d\Delta\sigma+\epsilon_0\kappa\Delta E$; an ideal <open circuit> keeps free <electrode> charge, hence $D$, fixed and gives $\Delta E=-d\Delta\sigma/(\epsilon_0\kappa)$. <area> cancels because both generated charge and <capacitance> scale with <area>. Thickness does not cancel: the material comparison is through $d/\kappa$ only when geometry is fixed. Here $\kappa$ must be the <relative permittivity> under the relevant mechanical and frequency conditions.
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