Wednesday, June 20, 2012

Two Common Scientific Misconceptions

1.  Surprisingly many people think that the rubber in tires will insulate your car, and this will protect you from lightning.

Lightnings sequence 2 animation

Come on!  That electric spark has just jumped across a mile or more of air, and you think that 4 or 5 inches of tire are going to stop it?  Besides which the tire is usually wet, and water, of course, conducts electricity just fine.

Cars actually still do a good job of protecting people from lightning, but for a different reason.  Most cars still have a metal frame, and that metal frame is a great conductor of electricity.  As a result, almost all the current is conducted away from the people in the car, although some current can still go through them.

All this assumes that the car has a metal frame.  In cars with less metal, so they will be lighter and more fuel efficient, lightning is a more significant risk. 

2. Many people also think that hot water kills germs.  Actually, many germs love warm temperatures.  They might be killed by being held for a while at temperatures at or above boiling, which is how surgical instruments are sterilized in an autoclave and part of the reason we cook food. Think about it, though:  If you put a raw egg into a sink full of "hot water" like you use to do dishes, then remove it after 10 minutes, what do you have?  A wet, warm, raw egg.  Water you can put your hands in will not kill germs. 

Is there a reason for using hot water, then?  Yes!  Hot water makes it easier for the detergent to help dissolve the dirt (or food particles, or whatever).  This is like what happens when you try to dissolve a spoonful of sugar in a drink:  it dissolves much better in hot coffee than in iced tea.  So hot water helps you get things clean, and if they're clean, they should not have anything for the germs to grow on.  If you want them really sterile, though, you have to add something like bleach that will kill the germs.


  1. My car was hit by lightning when I was driving it. You wouldn't believe how many people believe it is impossible to be hit by lightning because of the rubber tires. The emergency room doctor actually told me that the tires probably saved my life!

  2. Dogs and cats prefer running water to drink from, and it makes sense that water "you can put your hands in will not kill germs". Still water = bad. What does kill germs then?

  3. Running water in nature has probably come, not long ago, from springs or rain, both of which should make it pretty clean. Also, fast-running water (rapids, waterfalls, etc.) will have more oxygen in it than stagnant water. This might help break down organic molecules, help support animals like fish that would eat pond scum, or simply favor a different range of microorganisms; one way or the other, it is more likely to be relatively clean.

    As for what *does* kill germs, I already mentioned one option: bleach. You could also bring the water to a good rolling boil for several minutes.