The Human Body’s Resistance
Most recent answer: 10/22/2007
- thiru (age 21)
india
Thiru -
There are a lot of factors involved and not every person has the same electrical resistance. For instance, men tend to have lower resistance than women. Just like for the resistors used in electronics, the resistance of a person’s arm depends on the arm’s length and diameter. Resistance goes up with length and down with diameter. Since men tend to have thicker arms and legs (more muscle), they usually have lower resistance. A rough value for the internal resistance of the human body is 300-1,000 Ohms. Naturally, the resistance also depends on the path that electricity takes through the body - if the electricity goes in the left hand and out the right foot, then the resistance will be much higher than if it goes in and out of adjacent fingers.
Within the body, the tissues with the greatest resistance are bone and fat - nerves and muscle have the least resistance. That said, the majority of the body’s resistance is in the skin - the dead, dry cells of the epidermis (the skin’s outer layer) are very poor conductors. Depending on the person, the resistance of dry skin is usually between 1,000-100,000 Ohms. The skin’s resistance is much lower if it is wet or burnt/blistered. This means that when a person is electrocuted in real life, the body’s resistance drops as the skin is burned. To determine a person’s total resistance, just add together the resistance of each part of the body - remember that the electricity must pass through the skin twice (on the way in and on the way out), so the total resistance is:
Rtotal = Rskin(in) + Rinternal + Rskin(out)
Another interesting point to consider is that in addition to acting like a resistor, the epidermis acts like a capacitor if placed in contact with a piece of metal (the underlying tissue is like one plate of a capacitor and the metal surface is like the other plate - the dry epidermis is the less-conductive material or "dielectric" in between) . In cases of electrocution by a DC voltage source, this capacitive property has little importance. But if the electrocution is by an AC source, the epidermis’s natural resistance is "shorted out", allowing the current to bypass that part of the body’s resistance and making the body’s total resistance much lower.
-Tamara
Reference: R. Fish & L. Geddes, Medical and Bioengineering Aspects of Electrical Injuries, c2003 Lawyers & Judges Publishing Company, Inc.
(published on 10/22/2007)
Follow-Up #1: Human capacitance (in Farads)
- aru (age 25)
chennai,tamil nadu,india
For R = 1 meter C = 111 picoFarads. Actual measured values of human body capacitance (to distant ground) vary from 100 to 200 picoFarads. By the way, epsilono is the experimentally measured permitivity of free space.
LeeH
(published on 10/22/2007)
Follow-Up #2: human conductance
- Marco (age 32)
San Diego, USA
Mike W.
By the way, you can see the original post, it's # 6793.
LeeH
(published on 08/04/2009)
Follow-Up #3: Series or parallel resistance in electrical shocks?
- raghavendra
bangalore,karnataka ,india
LeeH
(published on 08/25/2010)
Follow-Up #4: Stopping watches and laptops?
- Rosemary (age 37)
Dickinson, TX USA
LeeH
(published on 08/22/2011)
Follow-Up #5: Dangers of low and high voltage shocks
- Ariel (age 17)
gensan.phil.
I have marked this answer as a follow-up to question number 6793 that discusses some other aspects of electrical shocks.
LeeH
(published on 06/20/2012)
Follow-Up #6: low reading for hand-held resistor
- matt (age 29)
Canada
Mike W.
(published on 04/04/2013)
Follow-Up #7: safety circuit on boat
- Richard Meckstroth (age 69)
Vonore
I don't think there's any way that the water connection between the shafts could be in series with the motor. The resistance is much too high. Instead, I bet there's a separate circuit including the water between the shafts through which a small current flows that controls some switch on the main motor circuit. Thus the shaft-to-shaft connection would be in series with the on-off switch, as you found, but not in series with the motor.
Presumably that's done for some sort of protective reason. Perhaps if the motor runs freely in air it overheats.
Mike W.
(published on 08/18/2013)
Follow-Up #8: boat safety switch
- Richard Meckstroth (age 69)
Vonore
Yes, I was also wondering if it relies more on resistance or capacitance. If it works with de-ionized water I bet it's capacitance, since the resistivity is so high. It doesn't really matter much for the safety switch circuit, which is unlikely to notice the phase of the current.
Mike W.
(published on 08/19/2013)
Follow-Up #9: Controlling an I-Pad touch screen
- scott (age 45)
haverhill, ma
The capacitative sensors on the Ipad screen will pick up the presence of any conductor or very high dielectric-constant material very close to the screen, since those increase the capacitance of the closest capacitors. You can get styli with conducting tips for the Ipad. You don't need the capacitance to ground provided by the body. The styli could be manipulated by anything.
(Most other touch screens use little resistive switches activated by mechanical pressure. Their styli usually aren't conductive, so they won't work with the Ipad. )
Mike W.
(published on 08/23/2013)
Follow-Up #10: contacting ipad
- Eli (age 23)
California
Actually, what we said couldn't have been what the previous questioner already knew since we contradicted one of his assumptions. There's no need to emulate the electric field of a finger connected to a body. The only thing that's needed is to have some conductor or very high dielectric constant material to perturb the field coming from the screen. If for some reason your robot wanted to closely emulate a finger, it could use a little bag of saltwater.
Mike W.
(published on 05/16/2014)
Follow-Up #11: home-made touch-screen stylus
- Eli (age 23)
California
Avoiding the high prices of store-bought styli sounds like a nice idea. Here's a guess about what might work well. Take a a little metal rod a few mm thick with a nice rounded end. You could even round an end with a file. Then coat the rod with some thin plastic stuff. Coatings like that are available iin hardware stores for not much cost. The purpose of the coating is to prevent damaging the screen when your home-made stylus accidentally touches it.
Since we don't really have any experience in this business, please do not sue us for damaging your screen if something screws up.
Mike W.
(published on 05/16/2014)
Follow-Up #12: cheap capacitance measurement
- Jordan (age 25)
Singapore
How to measure the capacitance of some little capacitor depends a lot on what you have available. The easiest way is (no surprise here) to use a capacitance meter. It looks like you can get one for about $70 (US) that will measure small enough capacitance. If you happen to already have an oscilloscope with a high-impedance input, you can charge up the capacitor with a battery and then let the charge drain off through a big resistor while monitoring the voltage on the scope. The drain time (to about 40% of the initial voltage) is RC, the resistance times the capacitance. There are all sorts of variations on this theme, such as seeing how much ac current flows through the capacitor for a particular ac voltage, in case you have an ordinary ac meter to measure current and voltage and some suitable source of ac voltage. The current magnitude will be 2πVCf, where f is the frequency. Don't use voltage from standard outlets. It's too big and could kill you or damage your little capacitor. You want a small voltage, and probably at much higher frequency to make the current through the capacitor big enough to measure.
Mike W.
(published on 06/10/2014)
Follow-Up #13: Capacitive coupling & you
- Peter (age 46)
Beaver Dam, WI, USA
It's okay to disagree with the standard response. But ask yourself this: "What would convince me that I'm wrong?"
Be specific. What standards would a study have to adhere to? Who would you trust to do it right? Would it need to be repeated by others? If the result didn't support what you already believe, would you suspect the truth was being covered up somehow? If you can't think of anything that would convince you, that should be a warning that you're letting yourself be guided more by how you feel about the issue than by evidence. Scientists have to be careful about this stuff, too.
With that said, I'm not aware of any studies on the long-term safety of capacitive touch screens. It's always possible that there's some harmful effect we haven't predicted, but the "standard response" that the amount of charge involved is very small makes a lot of sense. Rocks and water don't have anything to do with it.
One more thing to think about—if touch screens were harmful, every other conductive surface you touch (metal, water, other people) probably would be too. Capacitive coupling happens between any two conductors seperated by an insulator such as air, glass, or dirt. Touch screens just provide a conductive surface, an insulator, and a way to measure the local change in capacitance caused by your finger.
Rebecca Holmes
(published on 09/22/2014)
Follow-Up #14: Testing touch screens
- Peter (age 46)
Beaver Dam, WI, USA
Peter,
Thanks for all your thoughtful points. If you could show that there's really a sensation associated with using touch screens, you might be able to get someone interested in studying it more. One way to do that would be to ask an assistant with a random number generator to randomly present you with a real or "fake" touch screen (maybe a piece of glass made to look and feel identical, or maybe just the same touch screen turned off) many times, and see if you can reliably tell the difference between them (reliably = better than 50% with statistical significance).
It would be even more convincing to get a bunch of volunteers, have half of them touch active touch screens, and have half of them touch fakes. Then you could ask them to rate something about the way their fingers feel, and look for a difference between the two groups. None of this would prove anything about a harmful effect, but it would be somewhere to start.
As a physicist, I have to stick to my educated guess that there's no physical reason why touch screens would be dangerous.
Rebecca Holmes
(published on 09/29/2014)