Worked example: A quartz disc (piezoelectric coefficient d = 2.3 pC/N) of area 1 cm² receives a sharp tap of force 50 N. Estimate the charge generated and the voltage if its capacitance is 20 pF. Step 1: Direct effect: Q = d·F = 2.3×10⁻¹² × 50 = 1.15×10⁻¹⁰ C = 115 pC. Step 2: Voltage V = Q/C = 1.15×10⁻¹⁰ / 20×10⁻¹² = 5.75 V. Step 3: A mere 50 N tap makes ~6 V — stack many discs (as in lighters) and kilovolts appear, enough to spark. Step 4: Reverse: applying 100 V across a PZT actuator (d ≈ 300 pC/N) produces strain ~d·E — nanometre-precise positioning for microscope stages. Conclusion: piezoelectrics bridge mechanics and electronics with no moving parts — force in, volts out; volts in, motion out.