Science Questions

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

Sun, 2nd Aug 2009

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Reuben asked:

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


Flash on SumoHelen - He says heís 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 heís saying how could a flash move the plastic?

Helen -  How could that work?  Any thoughts on that?

Chris -  I think itís 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 youíre 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 itís 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, itís about a one millifarad capacitor.  Itís 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 youíre using a voltage of about 300 volts, thatís 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, itís a reasonable amount of work you get out of it.  When you do the electrodes on someoneís chest to do a cardiac arrest resuscitation, thatís about 300 joules, so itís probably about a fifth of the amount you would use to restart someoneís heart.  Itís a reasonable amount of energy I think.  So I donít think itís unfeasible. 


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Ruben Razquin asked the Naked Scientists: Hello, First of all, thank you for your wonderful programme. There is an interesting and puzzling experiment I did when I was a young boy (about thirty years ago) that I want to share with you and ask you for an explanation, for I did not see clearly what happened. I used an electronic flash and a small, thin, flat, light and opaque piece of black plastic. I connected two pieces of electric wire to the contacts of the electronic flash, so that I could trigger the flash without touching it, just connecting the two electric wires. Then I put the flash on a table on its back, so the light would go upwards, and I put the small piece of plastic on top of the flash, covering the area that emits the light. I then short-circuited the flash and it flashed MOVING THE PIECE OF PLASTIC upwards just a little bit. I was astonished because I could not believe light could move objects and I could even hear a click, as if I could hear fotons banging on the plastic. Could you please explain this? Thank you. Rubťn What do you think? Ruben Razquin , Wed, 8th Jul 2009

There is a mechanical shock when the electrical discharge occurs in the Xenon flash tube.
This is responsible for the "Pop" or "click" noise when the flash is fired.
Not caused by photons, caused by changes in the Xenon gas in the tube : a mini explosion.

RD, Wed, 8th Jul 2009

If you fire a slash gun at a thin piece of dark plastic you will heat the side of the plastic that faces the flash. That warm plastic will heat the air near it which will expand. That might be enough to move the plastic. Bored chemist, Wed, 8th Jul 2009

I think RD is correct.  The fact that you can hear the flash  means that there's been some physical movement of part of the flash.

Many years ago I occasionally used to use a hand-held flash (for bounce lighting), which was powered by lead-acid batteries, and you could just about detect very slight heating if you put your hand over the flash head and fired it.  It was quite an old unit even then though, and was probably quite a bit less efficient than modern units; you only want light out of flashguns and any heat produced is just wasting your power (heh - unless you're doing IR photography perhaps). LeeE, Wed, 8th Jul 2009

When I flash I make people move ! neilep, Wed, 8th Jul 2009

LOL.....I bet you do too! LOL...LOL.. Karen W., Wed, 8th Jul 2009

Another effect that could move the plastic was the electrostatic effect of unleashing the several hundred volts that powers the flash Soul Surfer, Wed, 8th Jul 2009

The energy stored in the capacitor of a flashgun is
E =CV2/2
1mF and 300V are typical, giving
total energy.
Not a lot in terms of heat but enough to take the chill off a resistor (I happen to have done that very thing last Sunday whilst mending my flashgun)
Could it also heat up enough air to cause expansion? I'm not sure what sum to use. lyner, Wed, 8th Jul 2009

1mF seems very large would not 1μF be more typical ? syhprum, Thu, 9th Jul 2009

Slash gun? What's that? High tec Jack the Ripper? rhade, Thu, 9th Jul 2009

The capacitors used in the flash in "disposable" film cameras are around 200μF 300V.
A professional camera flash would easily be 5x more powerful.

RD, Thu, 9th Jul 2009

The one in my Hi-end consumer flash is 1mF. Why just use 1muF? It wouldn't store enough energy. (0.047J) lyner, Thu, 9th Jul 2009

Slash gun? What's that? High tec Jack the Ripper?

Oops! Bored chemist, Thu, 9th Jul 2009

I admit my error having grownup at a time when 8 μF capacitors were considered large I forget how compact present day capacitors are. syhprum, Sat, 8th Aug 2009

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