Current density is typically expressed in which units?

• A. A/m^2
• B. V/Ohm
• C. mA/cm^2
• D. W/m^2

Answer: (C)

Current density measures how much current flows through a given area. That’s why the practical unit used in many clinical and electrode-based contexts is milliampere per square centimeter (mA/cm^2): it directly expresses current relative to the electrode’s contact area, which matters for how the tissue is stimulated. While the SI unit could be expressed as A/m^2, in real-world applications electrode sizes are typically known in square centimeters, and currents are often in milliamps, so mA/cm^2 is the most convenient, widely used form. For perspective, 1 A/m^2 equals 0.1 mA/cm^2 (since 1 A = 1000 mA and 1 m^2 = 10,000 cm^2). Other options don’t represent current density: V/Ohm reduces to current, not current per area, and W/m^2 describes radiant flux density, not electrical current per area.