Which of the following statements regarding the interaction of electromagnetic fields with food materials are CORRECT?
Options:
(A) Penetration depth ($d_p$) is defined as the distance from the surface at which the power density drops to $1/e$ ($36.8\%$) of its surface value.
(B) At microwave frequencies ($915\text{ MHz}$ and $2450\text{ MHz}$), dipolar rotation of water molecules is the primary mechanism of thermal energy generation.
(C) In high-salinity liquid foods, ionic conduction contributes substantially to dielectric loss, especially at lower radiofrequencies ($13.56\text{ MHz}$ and $27.12\text{ MHz}$).
(D) Microwave penetration depth in food increases as the moisture content and dielectric loss factor increase.
Answer Key:
A, B, C
Step-by-Step Solution & Explanation
Analysis of statements:
- Option A is CORRECT: By definition, penetration depth $d_p = \frac{\lambda_0}{2\pi \sqrt{2 \varepsilon'}} \left[\sqrt{1 + (\varepsilon''/\varepsilon')^2} - 1\right]^{-1/2}$, at which power attenuates to $e^{-1} \approx 36.8\%$.
- Option B is CORRECT: Free water molecules rapidly reorient with the alternating electric field ($2.45 \times 10^9$ cycles/s), converting kinetic agitation into thermal energy through molecular friction.
- Option C is CORRECT: Dissolved ions ($Na^+, Cl^-$) migrate under the oscillating field, generating resistive Ohmic heating ($\sigma E^2$), which dominates at lower RF frequencies.
- Option D is INCORRECT: Higher moisture and higher dielectric loss factor ($\varepsilon''$) cause faster absorption of energy near the surface, resulting in a *smaller* penetration depth.