Well, I have now done themobile phone experiment in a lecture. The question was, is a bucket of water enough to shield the electromagnetic communication between a cellphone and the nearest mast? So, I wrapped my phone in glad wrap (or cling film, for those who don’t live in NZ), put it in a sealable plastic bag (like those you are allowed to take through airport security), and then put it in a snaplock plastic container. We did a quick control experiment, to check that it could receive through those layers of plastic, and then I held it in the middle of a bucket of water.
Did it ring. No it didn’t. Interesting result. Took it out of the bucket and did another control expt. Didn’t ring this time. Rather puzzling. At this point the student who was phoning it realized he’d been phoning the wrong number. So, let’s try again.. Back in the bucket. And it rang. Not that I could hear it, but I could certainly feel the ‘vibrate’ part of the ringing through all that plastic.
Conclusion: A few centimetres of Hamilton tapwater isn’t enough.
But what happens when we boost the conducticity of the water. The more the conductivity, the more quickly an electromagnetic wave should be stopped. So, I put some salt in there, and we tried again. This time the phone didn’t ring.
OK, so the method probably needs a bit of work to get it controlled properly, but hopefully it got the point across to the students about skin depth and how conductive materials kill off electromagnetic waves.
Finally, opened up the snaplock plastic container, poured the water out (so much for that being waterproof), opened up the resealable bag and unwrapped the phone, which, I am pleased to report, is still working.
The Easter break is almost upon us, so don’t expect much blogging next week, but I shall be back
Daniel Oosterwijk says:
I am doing this experiment for a school science fair project, and am wondering how much salt per litre of water you think would stop the phone ringing? And by the way, I have been in the Waikato Uni pool XD
Marcus Wilson says:
Well, I leave that for you to estimate or find out (you are the one doing the project, not me). But some guidelines.
The ‘skin depth’ for electromagnetic waves, which is broadly the distance into a conductive medium (the salty water) that a wave can travel before being absorbed, is given by:
skin depth = 1/sqrt( pi * conductivity * frequency * mu_0)
i.e. one divided by the square root of (pi (3.1415…) times conductivity times frequency times mu_0).
mu_0 is the permeability of free space, having a value of 4*pi times ten to the power of minus 7 Henrys per metre. (4 * pi * 1E-7 Henrys per metre).
You’ll need to find out the frequencies that mobile phones use (not that difficult) and the conductivity of a solution with a given amount of salt (that can be looked up too).
When I did the experiment I emptied about 100 g of salt into a bucket of water and that stopped the phone ringing, so that should give you a guideline.
The best thing is for you to experiment and find out. That’s what science is about.