Advanced #50

The electrical conductivity of water increases considerably with frequency

At high frequencies, water shows considerably increased conductivity.

Scientific Explanation

At direct current or low frequencies, pure water has very low electrical conductivity — it is a poor conductor. With increasing frequency, however, the conductivity rises by several orders of magnitude. In the gigahertz and terahertz ranges, water becomes a far better conductor than at low frequencies.

This increase originates in dielectric relaxation. At low frequencies, the water molecules can fully follow the alternating field and store the energy essentially without loss. At higher frequencies, the molecules can no longer keep up with the rapidly changing field — a phase shift develops between the field and the polarization. This phase shift leads to energy dissipation, which is measurable as increased conductivity.

The main relaxation frequency of water lies at approximately 20 gigahertz at room temperature. In this range, the energy loss is at its maximum. The effective conductivity is the sum of the ionic DC conductivity and the frequency-dependent dielectric loss, with the latter dominating at high frequencies.

Electrical Conductivity of Water vs Frequency Line chart showing that the electrical conductivity of water increases substantially with increasing frequency, rising by several orders of magnitude from low frequencies to GHz and THz ranges. Conductivity (S/m, log) 10² 10° 10⁻² 10⁻⁴ DC MHz GHz THz IR Frequency (log scale) Orders of magnitude increase Water Conductivity vs Frequency
Frequency dependence of the electrical conductivity of water.

Everyday Relevance

This phenomenon is the reason microwave ovens work. Microwaves operate at 2.45 gigahertz — a frequency at which water absorbs energy efficiently because the molecules can no longer fully follow the field. The absorbed electromagnetic energy is converted into heat, warming the food. Moist foods heat up quickly while dry ones barely warm at all. In medical technology (diathermy), radar absorption by rain, and satellite communications, the frequency-dependent conductivity of water also plays a critical role.