Did you know that you can charge up a piece of ice like a battery? How about making tiny electric storms when a drop of water turns into a puff of steam? We'll tell you about the little-known Thermodielectric Effect, and why it's present everywhere, including your body.
The Basics of Dielectrics
To get this effect going, you need a dielectric material. Fortunately, dielectric materials are not tough to get. Distilled water, paraffin wax, and palm wax are all dielectrics. Dielectrics are insulators - materials through which electric current cannot flow. Still, dielectrics are not totally indifferent to electricity. Put a dielectric in an electric field and it polarizes. All of its atoms move into a configuration that makes the slightly-more-negative sides of the atoms face one way, and the more positively-charged sides of the atoms face the other way. This is pretty stable. As long as the material stays at the same temperature, and in the same phase state, no charge builds up anywhere.
The Thermodielectric Effect
Once the material, let's use water as an example, starts getting heated up or cooled down, things get interesting. In these supposedly insulating materials, electric currents start flowing. This effect, discovered by Joaquim da Costa Ribeiro, is also called the Costa Ribeiro Effect. The effect happens during phase transitions, the transition of a material from solid to liquid, liquid to gas, or back down to solid again. Although it can happen during any transition, we'll look at the specific case of a material going from solid to liquid.
First, we have a few solid crystals in a mostly-liquid substance. Focus just at the boundary between the solid and the liquid, and you'll see an interesting phenomenon known as a "double layer." Along the edge of the layer of the solid, there is a build-up of charge. This can be charge of any kind, depending on the type of material. It could be electrons or negatively-charged ions. It could also be protons, or positively-charged ions. Either way, the border of the solid is lined with charged particles. This elicits a response in the liquid. Opposite charges line up along the edge of the liquid. If the solid has a positively-charged layer of particles, the layer along the liquid is negative. If it has a negatively-charged layer of particles, the layer along the liquid is positive. This situation can't last.
Because the entire concoction is (in this case), slowly freezing, the double layer can't stay in the same place. If the substance is freezing from the bottom up, the line of solid ice sweeps upwards. If the charge on both sides could move equally fast, nothing of note would happen. When it comes to the movement of charge, solid isn't as agile as liquid. The charge in the solid material doesn't dissipate when more material freezes above it. The liquid keeps charging up the ice and retreating, charging up the ice and retreating. As more of the material freezes, a large chunk of the newly-made solid is charged. The more solid material there is, the more charge builds up. The liquid, meanwhile, carries the opposite charge. Sometimes the difference between the charges gets to be too high, and charge flows from one place to another in an electric current.
The Thermodielectrics of Everyday Things
As dielectric materials are so common, the Thermodielectric Effect is something that happens all the time. When we see lightning in the sky, we're seeing a macro version of what can happen when individual water droplets get heated to vapor, or when water freezes to ice. Sudden stampedes of charge move from one side of the material to the other. This kind of thing doesn't just happen in inorganic materials. Sérgio Mascarenhas has noted that the Thermodielectric Effect can happen in biological substances, including proteins and DNA. Granted, not much stuff boils or freezes in the human body, but if even a few molecules in the body undergo a phase transition, they can generate tiny electrical currents, and tiny "electrical storms." More importantly, not only can heat be converted into electricity, it can be converted into electricity inside a living being.
Top Image: flagstaffotos.com.au