What unit measures the number of electrical charges or ion pairs in a kilogram of air?

• A. Coulomb per kilogram
• B. Gray
• C. Sievert
• D. Roentgen

Answer: (A)

Exposure quantifies how much ionization is produced in air by radiation. The SI unit that directly expresses the electric charge generated per kilogram of air is coulomb per kilogram. This unit literally measures the amount of charge (in coulombs) per unit mass (kilograms), which corresponds to the number of ion pairs formed per kilogram in a straightforward way.

The Gray measures energy absorbed per mass, which is a different physical quantity; the Sievert relates to biological effect (dose equivalent) and also differs in meaning; and the Roentgen is a historical unit of exposure tied to ionization in air but not the SI standard for charge per mass. Since the question focuses on the actual charge per kilogram, Coulomb per kilogram is the best fit.