Science Questions

On a recent trip to Sweden I noticed at the check in desk that my mobile battery was running low and was just about to turn off. As I went through airport security, my mobile was sent through the x-ray machine. When I next looked at my phone there was almost a full charge. I was wondering if you had any ideas as to why this might be so? Adamski via email

There are a couple of reasons but it's unlikely that it's anything to do with the x-ray machine itself, but more to do with the phone being turned off. One explanation might involve it being cold when you went into the airport. Batteries work by a chemical reaction going on inside. When they get cold, the chemical reaction slows down, which means that they can't produce as much current. This makes the battery appear to be a lot emptier than it is. This is why you sometimes get a flat battery on a cold day. If you have a battery and you've been using it a lot, it will drain quickly and the voltage will go down. If you turn it off, that voltage will slowly build up as long as you're not drawing a current. This means that when Adamski turned his mobile phone back on, it was full.

September 2006