Question

The negative sign in Fick's first law indicates that diffusion flux occurs:

• A. Along increasing concentration
• B. Independent of concentration gradient
• C. Opposite to the concentration gradient
• D. Only at equilibrium

Hint:

Diffusion is always a “downhill” process in concentration space.

Answer 1

The Correct Option is C

Concept: Fick's first law of diffusion is: \[ J = -D \frac{dC}{dx} \] Where: - $J$ = diffusion flux - $D$ = diffusion coefficient (always positive) - $\frac{dC}{dx}$ = concentration gradient The negative sign is physically significant.

Step 1: Understand concentration gradient.
If concentration decreases with distance: \[ \frac{dC}{dx} < 0 \]

Step 2: Apply Fick’s law.
\[ J = -D \frac{dC}{dx} \] Since $D>0$, flux $J$ becomes positive in direction of decreasing concentration.

Step 3: Physical interpretation.
Particles always move: \[ \text{from high concentration} \rightarrow \text{low concentration} \] This is opposite to the gradient direction. Final Answer: \[ \boxed{\text{Opposite to concentration gradient}} \]