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Flying circus of physics

The case of the burning hospital gurneys

Tuesday, April 01, 2014


The case of the burning hospital gurneys
Jearl Walker
April 2014  Often, a burn victim is treated while lying on a gurney (a bed with wheels) in an enclosed chamber filled with oxygen-enriched air (a hyperbaric chamber). Once a treatment is over, a hospital worker pulls the gurney and patient from the chamber, to be rolled away. On several occasions, in different hospitals, a gurney caught fire at the end that was the last to leave the chamber. Obviously, a burning gurney holding a patient already suffering from burns is a dangerous situation, and obviously fires burn easily in air rich in oxygen, but the question remains: What caused the gurneys to catch fire?

Investigators realized that charge separation occurred between the patient’s skin, the hospital gown on the patient, and the sheet on the gurney, much like you might experience on a low-humidity day when your clothes slide over your skin. Where the patient’s skin made contact with the cloth, some of the loosely bound electrons of the skin molecules were pulled onto the cloth surface, making the gurney negatively charged. As the gurney gained electrons, it drove electrons out of the underlying metal framework of the chamber, leaving the framework positively charged.

The negatively charged gurney and the positively charged framework then formed a capacitor. In my textbook I introduce a capacitor in its simplest form: Two metal plates face each other with a small separation. The plates are then connected to a battery, which makes one plate positively charged and the other plate negatively charged. There is then a potential difference (measured in volts) between the two plates, and energy is stored in the electric field between them.

In the gurney case, the charging occurred because of the skin–cloth contact rather than a battery. Still, there was a potential difference between the top plate (the gurney) and the bottom plate (the framework), with a potential difference and stored energy between the two plates.

When you have two oppositely charged objects near each other, the loosely bound electrons on the negative object might jump to the positive object in a spark. Did such a spark jump between the gurney and framework and ignite the gurney? Measurements by the investigators revealed that a spark could have occurred only if the potential difference on the gurney–framework capacitor exceeded 2000 volts and that a fire could be started by a spark only if the capacitor’s energy exceeded 0.20 millijoules. However, the potential difference on the gurney–framework capacitor was only 600 volts, not enough to produce any spark and certainly not a spark that was energetic enough to cause a fire.

So, how did the fire start? The answer came when the investigators considered the motion of the gurney as it was pulled from the chamber, which was when the fire began as the gurney was nearly all the way out. During that motion, the charge on each capacitor was trapped and thus could not change. However, as less and less area on each plate faced each other, the charge on each plate was squeezed into a smaller and smaller area. The increased concentration of charge on each plate increased the potential difference between the plates. Could the potential difference have reached high enough value that a spark jumped between the gurney and framework?

I used this story of the burning gurney in an earlier edition of my textbook Fundamentals of Physics (or the international version, Principles of Physics). (It is still available in the associated online material in WileyPlus). In the example I explained that the amount of charge q on either plate of a capacitor is proportional to VA/d, where V is the potential difference between the plates, A is the area of each plate facing the other plate, and d is the plate separation. As the gurney was pulled out of the chamber, the charge q and the plate separation d were constant but the area A decreased. So, the potential V had to increase to offset that decease in A. When area A had decreased to 1/10 of its original value, the potential V had increased to 10 times its original value, that is, to 6000 V. That potential difference was large enough to drive a spark between the gurney and framework at the end of the gurney that was the last to leave the chamber. Moreover, with the potential difference that high, the energy released by the spark was 0.45 millijoules, more than enough to cause a fire.

The investigators published their analysis in 1980, but the same problem occurred later, as you can see in the linkage below to a newspaper report from 1990.

Dots · through ··· indicate level of difficulty
Journal reference style: author, journal, volume, pages (date)
Book reference style: author, title, publisher, date, pages
·· Hughes, J. F., and A. W. Bright, “Electrostatic charge control on hyperbaric chambers---a case history,” IEEE Transactions on Industry Applications, IA-16, No. 6, 762-766 (November/December 1980)
· Halliday, D., R. Resnick, and J. Walker, Fundamentals of Physics, 7th edition, John Wiley & Sons, 2005, pages 656 and 667-668, 8th edition, 2008, pages 668-669
· newspaper report, May 29, 1990,3437006


Pub tricks
Do you want the pub-trick stories here at the FCP site? Use the following links and then scroll down a page or search for "pub trick". Keep in mind my point --- anyone can do a pub trick but the real trick is to explain it without bluffing or just waving your hands in the air. Physics = the power to explain.
Chapter 1 archives
Water and the disappearing cigarette  click this
Balancing a coin on a folded paper edge  click this
Lifting a bottle with a thumb and one finger, click this
Hanging spoons from the nose, click this
Hanging bottle caps on your face, click this
Standing eggs on end  click this
Removing a lighter from under a bottle  click this
Removing a bill from between balanced bottles  click this
Removing the cork from a wine bottle  click this 
Balancing a hammer and a lorry  click this
Champagne cork as a morter round   click this 
Removing a coin from under a mug   click this 
Hanging a bottle on the wall  click this
Matchstick rocket  click this
Transferring a steel ball between beer mats   click this 
Tying a ring hitch click this

Chapter 2 archives
Reversing an egg in a tequila glass  click this
Blowing out a candle   click this
Escape from a cellophane pocket  click this
Exchanging water and whiskey   click this 
1000 drops from an empty bottle   click this 
Yard of ale and beer boot click this
Collecting grains of black pepper click this
Tia Maria wormy action click this
Vortex in a bottle and the vortex beer bottle   click this
Making straw paper stretch and crawl  click this
Making a ketchup packet float and dive  click this
Lifting rice with a rod click this
Inflating a long sandwich bag click this
Toothpick design trick click this
Using glug-glug to clear beer foam  click this
Egg tricks  click this
Raisin in champagne  click this
Inverted can of Red Bull   click this
Chapter 3 archives
Moving match sticks on a glass rim
Popping a plastic straw
Chapter 4 archives
Slam-freezing a beer or soda
Lighting a candle
Cooling beer on a hot day
Picking a shot glass with the palm
Rattling a coin
Chapter 5 archives
Rotating a matchstick balanced within a glass container

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