The case of the burning hospital gurneys
Jearl Walker www.flyingcircusofphysics.com
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
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