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Q & A: The Human Body’s Resistance

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Q:
please tell value of human body resistance
- thiru (age 21)
india
A:
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.  (An implication of this is that the lethal current for men is higher than that for women.)  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 07/05/07)

Follow-Up #1: human conductance

Q:
Not a question, just a sincere THANK YOU for providing a clear and precise answer. I have come across several webpages trying to address this question with vague and sometimes ridiculuous answers. One other page in fact pointed out the AC/DC difference but did not explain the reason (epidermis as dieletric)- now it makes sense.
- Marco (age 32)
San Diego, USA
A:
Thanks!
Mike W.

By the way, you can see the original post, it's # 6793.
LeeH

(published on 08/05/09)

Follow-Up #2: Series or parallel resistance in electrical shocks?

Q:
do we get electricity shocks in periods when we touch high voltage AC source? also when dc current passes throgh us do we vibrate or experience shocks in period? in one of ur privious answers uve given that Rtotal = Rskin(in) + Rinternal + Rskin(out),i dont agree as u cannot consider as a series connection,it is a parallel one.
- raghavendra
bangalore,karnataka ,india
A:
I think the previous answer is correct.  In an electrical shock the current first has to pass through the skin, then it has to pass through the body,  finally it has to pass through the skin again. That sounds like series to me. 

LeeH

(published on 08/25/10)

Follow-Up #3: Stopping watches and laptops?

Q:
I know this will sound stupid but here it goes... Can the human body stop watches and laptops?
- Rosemary (age 37)
Dickinson, TX USA
A:
I find it hard to believe a human body stopping a watch, unless the human hand dropped the watch from a high distance.  Stopping a laptop though is a different matter.  I once destroyed my laptop's wireless card on a cold winter's day by scuffling across a rug and touching it before discharging my self.  A big spark did it.    Other than static electricity I can't see other human body phenomena that would do it, except clumsiness.

LeeH

(published on 08/22/11)

Follow-Up #4: Human capacitance (in Farads)

Q:
How does the human body act as capacitor?
- aru (age 25)
chennai,tamil nadu,india
A:
Electrical capacitance is defined by the amount of static charge on a body divided by the voltage of the body with respect to some reference ground.   For an isolated  spherical body of radius R with an accumulated charge Q the voltage with respect to a ground at infinity (or very far away compared to R), is V = Q/(4*pi*epsilono)/R   which results  in C = 4*pi*epsilono*R.
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 12/13/11)

Follow-Up #5: Dangers of low and high voltage shocks

Q:
which is the most dangerous low voltage or high voltage?
- Ariel (age 17)
gensan.phil.
A:
The danger of electrical shock depends mainly on the total current flowing through the body.  The current, according to Ohm's Law, is proportional to the voltage divided by the total resistance of the circuit.  This includes the skin resistance plus the body's internal resistance.  The skin resistance can vary by a factor of 100 or more depending whether the skin is dry or moist with salty sweat.   For the same value of skin resistance the higher the voltage, the more danger of severe shock.  For the same value of voltage, the wetter the skin contact, the more danger.   Take your pick.

I have marked this answer as a follow-up to question number 6793 that discusses some other aspects of electrical shocks.

LeeH



(published on 06/21/12)

Follow-Up #6: low reading for hand-held resistor

Q:
Hey, thanks for your time I was in the lab, and was testing a 100k resistor. With the resistor in my hands, the multimeter read 47k ohlms. Didn't matter how hard or soft i held it. 3 others of my piers all read 87k olhms or greater. Why would there be such a difference?
- matt (age 29)
Canada
A:
This sounds quite peculiar. What's particularly odd is that 47kΩ is a standard resistor value. Ignoring that, could your hands have been sweaty?

Mike W.

(published on 04/28/13)

Follow-Up #7: safety circuit on boat

Q:
I bought a remote control electric boat at a mall isle booth, which has a "feature" that it only works if it's in the water, or if you have the top off, if you touch each of the 2 brass propeller shafts with your hand. That circuit, which includes the hand or water is in series with the on/off switch for the boat. How can this work? It is a "Superlative" Misquito pre-assembled boat made in China.
- Richard Meckstroth (age 69)
Vonore
A:

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 08/18/13)

Follow-Up #8: boat safety switch

Q:
That's the most intelligent/useful response I have gotten so far. No one on the model boat forums seems to know. Yes, the on-off switch goes to a sealed electronic box, so I agree that that part of the circuit doesn't take the full current of the motors. I'm still surprised that either your hand across the brass shafts, or them being in the water will activate the boat electrical system. I wonder if it is resistance or capacitance that affects the circuit. My voltmeter (9 volt battery driven)won't measure any resistance across my hand, it shows infinite.
- Richard Meckstroth (age 69)
Vonore
A:

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 08/19/13)

Follow-Up #9: Controlling an I-Pad touch screen

Q:
Hi thank you for your great knowledge. Question: I would like to control my IPad touch screen without touching it. A stylus pen is a conduit for the bodies electric field. I want to create the bodies electric field mechanically with no human connection. Is that possible and what would it take? IE: batteries, chip?
- scott (age 45)
haverhill, ma
A:

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 08/26/13)

Follow-Up #10: contacting ipad

Q:
You didn't answer follow up #9 the way he wanted. He already knew what you said in the answer. He was asking if there is a way to emulate the electric field of a human finger, without connection to the human body. Not like existing styli. Say you're a robot, and you need to use an iPhone. How would you interface with the screen?
- Eli (age 23)
California
A:

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 05/16/14)

Follow-Up #11: home-made touch-screen stylus

Q:
Science isn't about NEED, it's about WANT. Yes, it fulfills needs, but it's no fun to only think about needs. We don't need lightsabers or hoverboards. But you know we all damn well want them. Interesting about the saltwater bag. I didn't know that. But let me be more direct and honest: I I want to use a stylus on my ipad that has the fine tip(ballpoint pen status) and projects the same electric field that a human finger does(since the sensor grid refuses to acknowledge anything less than 4mm in width), so as to change the voltage of the capacitors surrounding the tip just like touching the glass with a finger does. And before you say "buy one" I refuse. I will not pay 50 dollars for defective products(customer reviews are terrifying), and the Bluetooth styli are too expensive/fancy for me want them. But most important of all: I want to make it myself. It's fun. It's educational. So: how do I do it?
- Eli (age 23)
California
A:

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 05/18/14)

Follow-Up #12: cheap capacitance measurement

Q:
Hi, thank you for your help. May I can I measure the capacitance of a material? For example, the thin plastic stuff that was stated in the previous answer. How would you measure the capacitance, using a cheap way if possible?
- Jordan (age 25)
Singapore
A:

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 06/16/14)

Follow-up on this answer.