Mass flux is defined as which physical quantity?

• A. Mass flow rate per unit area
• B. Mass flow rate
• C. Mass per unit volume
• D. Volume flow rate

Answer: (A)

Mass flux represents how much mass crosses a surface per unit area each second. It is the mass flow rate divided by the cross-sectional area, giving units of kg per square meter per second. In one-dimensional steady flow, this flux density is simply the product of density and normal velocity, j_m = ρ v_n. This means the total mass flow rate through the surface is obtained by multiplying the flux density by the area, Mdot = j_m A.

Why the other ideas aren’t the same: mass flow rate is the total mass per unit time across the area, not per unit area. Mass per unit volume is density, which tells you how much mass is present per volume, not how fast it crosses a surface. Volume flow rate is the volume per unit time, not the mass crossing per unit time.

So, the quantity that matches mass flow through area per unit time is the mass flux density, with units kg/(m^2 s).