# Mobile phone physics

By Marcus Wilson 19/03/2010 1

Just occasionally, I have a crazy thought regarding a physics demonstration.   This is one that I’m thinking about inflicting on my third year electromagnetism class.

We’ve been discussing the way electromagnetic waves travel (or rather, do not travel) through electrical conductors. Basically, conductors allow electric currents to flow in response to an applied electric field (in simple terms this just means applying a voltage). Electromagnetic waves such as visible light, radio and X-rays contain electric fields, so when one hits a conductor electric currents flow. Flowing currents heat up a material. Where does this heat energy come from? From the wave. In other words, conductors suck out energy from an electomagnetic wave, and, broadly speaking,  the wave can only penetrate so far into the conductor. This distance is what’s known as the ‘skin depth’.

Skin depth depends importantly on two things – the conductivity of the material and the frequency of the wave. The higher the conductivity, or the higher the frequency, the smaller the skin depth.  Thus, if you consider the waves to/from a mobile phone (frequency of around 1000 MHz) travelling through aluminium (a very good conductor) the skin depth turns out to be small indeed – microns in size.  That means wrapping a phone in aluminium foil will prevent it from picking up a signal. I’ve already shown this in class.

But – here’s the crazy thought – what about water?   Distilled water is a pretty non-conductive, but what comes out of the tap is loaded with dissolved salts and has a moderate conductivity, albeit several orders of magnitude below aluminium foil.   What’s its skin depth for  mobile phone frequencies?  I’ve done some quick back-of-the-envelope, and I reckon something of the order  few centimetres.  So….I predict that if we put the phone in just a few millimetres of water (YES, it needs waterproofing first!) it will still receive a signal, but suspend it in the middle of a swimming pool and there’s going to be no reception at all.

I reckon that getting my class to estimate how much water would be required to shut out the signal, and then design an experiment (that might or might not need to include ‘borrowing’ the university swimming pool for a short while) would be a great way to get them to think about the various issues themselves.  There’s plenty of literature to back up that assertion – e.g. Etkina et al., American Journal of Physics 74(11), p979  (2006). The best thing is that I can’t be tempted to tell them the answer –  because I don’t know it – I haven’t done the experiment myself. Though I have found this YouTube…

### One Response to “Mobile phone physics”

• repton says:

Have you seen this visualisation of RFID tags: http://berglondon.com/blog/2009/10/12/the-ghost-in-the-field/ ?

Maybe you could do something similar with phones .. perhaps get a modern phone and download a youtube video (so that it’s transmitting for a longish period), then take some “muffled” detector and wave it around..