CHAPTER 12 -- WATER, RESONANCE, AND DIELECTRIC RESPONSE
Water is fundamental to biological physics and exhibits unique electromagnetic properties.
12.1 MOLECULAR DIPOLES
A water molecule (H2O) is polar. The oxygen atom is more electronegative than the hydrogen atoms, pulling electron density towards it.
- This creates a permanent electric dipole moment: a positive charge center near the hydrogens and a negative center near the oxygen.
- In liquid water, hydrogen bonds form a transient network between molecules.
12.2 ROTATIONAL MODES
Molecules can rotate. In the gas phase, these rotations are quantized. In the liquid phase, rotation is hindered by collisions and hydrogen bonding.
- The dipole moment allows the molecule to interact strongly with oscillating electric fields (like microwaves).
- The field exerts a torque, causing the molecule to rotate.
12.3 DIELECTRIC LOSS
When a dielectric material (like water) is placed in an alternating electric field, the dipoles attempt to align with the field.
- At low frequencies, dipoles align easily.
- At very high frequencies (optical), they cannot keep up.
- At microwave frequencies (e.g., 2.45 GHz), the rotation lags behind the field due to friction/viscosity.
- This lag (phase difference) creates "Dielectric Loss." Energy from the field is dissipated as heat (molecular kinetic energy). This is the principle of microwave ovens.
12.4 FREQUENCY-DEPENDENT ABSORPTION
The complex permittivity epsilon describes the material's response:
epsilon = epsilon' - i * epsilon''
- epsilon' (real part) represents energy storage (polarization).
- epsilon'' (imaginary part) represents energy loss (absorption).
The absorption peaks at the relaxation frequency of the water molecules (approx 20 GHz at room temperature), but is significant over a broad range including the GHz band.
This strong absorption by water is why radar and communications signals at certain frequencies are heavily attenuated by rain.
