Question

The amount of charge flowing per second per unit area normal to the flow is called

• A. electrical conductivity
• B. electrical resistivity
• C. mobility
• D. current density
• E. areal current

Hint:

Current density = how concentrated the current is across a surface.

Answer 1

The Correct Option is D

Concept: Current density is defined as the amount of electric current flowing per unit cross-sectional area perpendicular (normal) to the direction of flow. Mathematically, \[ J = \frac{I}{A} \] where:
• $J$ = current density
• $I$ = current (charge per second)
• $A$ = cross-sectional area

Step 1: Understanding “charge per second”
Electric current is defined as: \[ I = \frac{Q}{t} \] So, “charge flowing per second” directly represents current.

Step 2: Incorporating “per unit area”
The question adds “per unit area normal to flow,” which means we divide current by area: \[ J = \frac{I}{A} \]

Step 3: Combining both ideas
Substituting $I = \frac{Q}{t}$: \[ J = \frac{Q}{tA} \] This matches exactly: \[ \text{charge per second per unit area} \]

Step 4: Direction aspect
Current density is a vector quantity:
• Direction is along flow of positive charges
• Same direction as electric field in conductors

Step 5: Why other options are incorrect
• Conductivity → material property (not flow rate)
• Resistivity → opposition to current
• Mobility → drift velocity per unit field
• Areal current → not standard physical term Final Conclusion: \[ \boxed{\text{Current density}} \]