Could a camera flash move a piece of plastic above it?

02 August 2009

Question

Could a camera flash move a piece of plastic above it?

Answer

Helen - He says he's tried this, actually, when he was a young boy quite a few years ago. And basically he put a flash gun on a flat table and set up a system where he could fire it off when he wanted to. He put a piece of plastic on top of it, then when the flash went off, it moved.Chris - And he's saying how could a flash move the plastic?Helen - How could that work? Any thoughts on that?Chris - I think it's perfectly feasible because a flash discharges quite a lot of energy. If you take a flash gun and look at the nuts and bolts of how it works, basically, what you're doing is using a circuit to charge a very big capacitor. The capacitor then discharges a fairly high voltage, about 300 volts through a gas tube, usually xenon is what is used, and this unleashes enormous amounts of energy very, very quickly, which is why you see this flash of light. But what that also does is to produce lots of heat so what I suspect is going on is that the flash goes off, this unleashes some heat. This heats the surface of the piece of plastic plus it heats the air separating the plastic and the flash bulb. These two things together contribute to both the change of shape of the plastic but, moreover, change of shape of the air. It expands. This may lift up and push the plastic up off the flash a little bit or especially if it's a light piece of plastic or a piece of paper. I did a few simple back-of-the-envelope calculations. If you look at the capacitor in a flash gun, it's about a one millifarad capacitor. It's quite a lot of capacitance and you can calculate how much energy comes out of a capacitor by the equation, E=1/2 CV2 [where E is the energy, C is the capacitance and V is the voltage.]If you put those numbers in with a one millifarad capacitor and you're using a voltage of about 300 volts, that's actually about 45 joules of energy. [ E=1/2 CV2 = 1/2 1x3002 = 1/2 90000 = 45000mJ or 45 J]That would lift a book about 45 metres up in the air, if you think about it. A book weighs about 1 Newton, I suppose, if you had a light or small paperback. So yeah, it's a reasonable amount of work you get out of it. When you do the electrodes on someone's chest to do a cardiac arrest resuscitation, that's about 300 joules, so it's probably about a fifth of the amount you would use to restart someone's heart. It's a reasonable amount of energy I think. So I don't think it's unfeasible.

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