Now that winter is most assuredly here, I thought of an interesting topic to discuss for readers here. It has to do with things I’ve discussed before regarding electromagnetism, but in includes something else I haven’t really discussed before.
Pretty much every morning these days I have to run the rear window defroster in my car. If it’s not too cold, the snow and ice will be able to melt and run off the window as liquid water. I noticed though that it doesn’t just run off smoothly. It seems to stick to the defroster element lines or be deflected by them. The wires themselves are not on the outside of the glass, so the water isn’t physically interacting with them. What’s going on here?
First, I have to make the disclaimer that my chemistry knowledge is several orders of magnitude weaker than my physics and electromagnetism knowledge, so hopefully I don’t mess up too much. Nevertheless, I’ll do my best to explain this phenomenon. To understand what’s happening with the water on my window, we have to go down to the water molecule itself. The chemical makeup of water is H2O. That’s two hydrogen atoms and a single oxygen atom. These atoms are just like any atoms. They have cores composed of positively charged protons that are orbited by negatively charged electrons. When atoms bond to each other to form molecules, they do so in a number of different configurations. Atoms group themselves into molecules either with electrostatic force (attraction of atoms with opposite overall charges) or via the sharing of electrons (covalent bonds) and these bonds can occur at different angles. In the case of water, the bonds involved are covalent. Water molecules are not linear, and the imbalance of relative electrical charges (oxygen atoms have a slight negative charge, hydrogen atoms have a slight positive charge) causes the resulting water molecule to have a positively charged end and a negatively charged end (called a dipole). Molecules like this are called polar molecules.
Which molecules come out as polar and which do not are based on pretty complicated chemistry. In each case it depends on the relative electronegativities of the atoms involved as well as the way they configure themselves in the molecule. A carbon dioxide molecule has polar covalent bonds holding it together, but because it configures itself in a linear fashion (a carbon atom flanked by two oxygen atoms in a straight line) there is no overall dipole. Though the ends of the molecule (the oxygen atoms) do have dipoles, they just happen to cancel out as far as the overall molecule is concerned. This is what allows CO2 (a non-polar molecule) to dissolve into water (a polar molecule).
Ok, if you managed to survive all that (I’m not even entirely sure I did), hopefully I can tie all this back to what I observed happening on the rear window in my car. Water is a polar molecule, meaning that it has a positively charged end and a negatively charged end. This allows it to be influenced by electromagnetic forces in the environment. When I engage the defroster element in my car, current flows through the narrow wires, encountering resistance which causes them to heat up. The flowing current also creates a magnetic field around each of the wires. The water molecules, being polar, are influenced by this magnetic field and therefore do not simply flow off the glass unimpeded. The water molecules tend to “stick” to the element wires even thought they don’t touch physically. Electromagnetism!
This property of water is more commonly demonstrated via a static electrical charge since the water will respond better to that than to a magnetic field. The demonstration usually consists of taking a plastic comb and combing your hair with it. This strips electrons from your hair and deposits them on the comb creating a static electrical field. If you create a thin stream of water from a faucet and hold the comb next to it, the water will be deflected instead of falling straight down. Again, this is because the water is a polar molecule and can be influenced by electromagnetic fields.
Hopefully this was an interesting topic. It’s always interesting to find complex science in the simplest things.