High voltage - low current

pjh · 19th July 2006, 01:36 PM

Randi used this phrase recently himself in one of his columns, and it's something that's never been clear to me.

Given Ohms law (I=V/R), what exacty is meant by the phrase "High Voltage - low current" What are devices like tazers and cattle fences actually doing? Is it that the discharge is almost instantaneous or is something else happening? The same with static eletricity and van der graaff machines - high voltages but and instant discharge.

Here's an example of a typical answer to this question (not posted by me)
http://forum.allaboutcircuits.com/showthread.php?t=1052

But is this correct? I've used 12v car batteries all my life and never heard they were dangerous - I've regularly been in contact with both terminals and never felt a shock.

LTC8K6 · 19th July 2006, 01:41 PM

A car battery can't knock you on your ass unless someone hits you with one.

People probably get hit with the spark plug voltage, and just assume the battery was what shocked them.

Jimbo07 · 19th July 2006, 02:24 PM

V = IR

Car battery:

12 V = I (approx. 1000000 Ohms across the skin at low voltage)

I = 12 V/1000000 Ohm

I = 12 microamps

Taser:

50000 V = I (1000000 Ohms)

I = 50 milliamps

In short, a Taser probably won't kill you. Now, if you connected the terminals of a car battery with two ends of a metal rod that you were holding onto...

roger · 19th July 2006, 02:39 PM

Not entirely sure what you are asking but I=v/r applies. Your skin resistance is high, so you grab onto the terminals of a car battery and you have 12 divided by a big number, so you get little current. Wet both your hands, and look out! Resistance drops, and current increases.

Typically, it's current that kills. We spent some time studying this in a power class, and it's not quite that straightforward. We respond differently to AC and DC. In AC different frequencies have different effects at the same voltage and amperage. You would think that since current kills, that high voltage is more dangerous. Not necessarily. 500~750V is especially dangerous, because it makes your muscles seize up. If you grab a power line at that voltage, your muscles will clamp your hand to it, and you will stay there until dead, unable to open your fingers even though you try with all your might (and you die because the same thing is happening to your heart). On the other hand, your muscles spasm at 10,000V, so touch a line at that level and you'll be "blown" away from the line by the force of your convulsions, moving you to safety. Electric chairs, designed to kill, typically will cycle through different frequencies and voltages to maximize effects. I.e. some time is spent at 500-750V to make your heart seize, then to a higher frequency and voltage to confuse your heart's pacemaker, etc.

So, how could a cattle fence be high voltage/low current? first, the cow itself has high resistance. Second, you can limit the current by putting a high value resistor between the power supply and the lines of the fence. This'll create an effective maximum on the amount of current the fence can produce.

davefoc · 19th July 2006, 02:56 PM

I didn't notice that anybody said anything wrong in any of the links or here, but I might say things a little differently.

The issue of what is meant by voltage is routinely misrepresented in the popular media. Knowing the voltage does not provide adequate infromation to know about the potential of an electric source to do harm.

One way to think about this is with an analogy to water. The voltage can be seen as the water pressure and the current can be seen as a measurement of water flow. A pump capable of delivering high water pressure but only through a very small outlet can't do much damage or deliver much energy . Conversely a low pressure pump with a big outlet might be capable of delivering much more energy.

This is the situation with electricity. Lots of voltage but with sufficiently small current won't harm anything.

Real voltage sources (like batteries) can be modeled as a combination of an ideal voltage source (a voltage source that regardless of the load maintains the same output voltage) and a series resistor. One side of the series resistor is connected to the output of the ideal source and the other side of the resistor is the output of the model for the real voltage source. The size of the resistor in ohms is referred to as the source impedance of the battery.

The size of the series impedance limits the maximum amount of current available out of the battery. For example a AAA alkaline battery has a much higher source impedance than a D cell alkaline battery but they both have the smae output voltage when they are new. The lower source impedance means that the D cell can supply a much larger current than the AAA cell.

The current transferred from a voltage source to a load is not only a function of the voltage and the source impedance of the source, it is also a function of the resistance of the load. The resistance of the human body is hign enough that even if the source impeadance were zero a low voltage source like a car battery under most circumstances can not push enough current through the body to do damage. So if you are trying to kill somebody with a 12 volt battery, your best bet is to him on the head with it as was mentioned above.

There are a few other factors in all this that affect how lethal a particular source is.

1. High voltages can drive significant currents outside the body and thereby allow survival from contact that would otherwise be fatal if all the current flowed through the body.

2. The area of the human body that the current flows through. Current that bypasses the heart but destroys a limb can be survived. Current that goes through the heart and destroys it will generally result in death.

3. Interestingly, AC current can be fatal at far less current that DC current. According to several articles on electrocution that I have read AC current can disrupt the heart rhythm and cause death at much lower levels than DC currents. The sweet spot for this effect is between 40 and 400 hertz.

One other thing I might mention here, in what I hope isn't too long a response, is that some voltage sources are best modeled as a capacitor with a series resistance. A capacitor is a device that stores a fixed charge . A capacitor is discharged when a current flows from it to the load. ESD (electrostatic discharge) is generally modelled as a capacitor. The capacitor has a small value so not a lot of energy can be stored unless the voltage is very high.

So the reason a 10KV ESD discharge into a human won't be fatal and a 10KV discharge from a high tension wire will be is that even though a very high current flows during the ESD event it only flows for microseconds whereas the discharge from the 10KV high tension source can continue untill the victim has been converted to a hot gas.

Terry · 19th July 2006, 03:01 PM

Output impedance may be a key factor here.

pjh · 19th July 2006, 03:04 PM

Originally Posted by roger

Not entirely sure what you are asking but I=v/r applies.

So, how could a cattle fence be high voltage/low current? first, the cow itself has high resistance. Second, you can limit the current by putting a high value resistor between the power supply and the lines of the fence. This'll create an effective maximum on the amount of current the fence can produce.

But that's not explaining how the tazer itself is "high voltage low current".

Surely if you put a resistor in between (in series) you'll drop the voltage, so you won't actually get 4000v "at the cow"?

Jimbo07 · 19th July 2006, 03:27 PM

Originally Posted by pjh

But that's not explaining how the tazer itself is "high voltage low current".

I showed the output. Assuming your non-lethal weapon of choice has approx. 50000 V across the electrodes and the resistance of human skin is approx. 1 MOhm, then you'd only conduct approx. 50 mA across your skin. As others have pointed out, it's a different story going through your heart.

For the input side...

You'd step up from some voltage source to 50kV, using a transformer, say. Your power output is limited through the transformer so while your voltage climbs, the current delivered for a given power drops. This is what high voltage power lines are about. You're trying to deliver power to a load.

P = IV from the source and P = I²R in a load.

So if you had 100W = 10 A (10 V), then stepped up to 100 V, you'd get 100 W = 1 A (100 V). Now, because P = I²R in a load, that resistivity is going to give you some losses (due to heating). You are better to run your lines at a high voltage and thereby reduce losses due to a high current.

ETA: In case it's not clear, I'm referring to the resistivity of the transmission lines and apparatus themselves. At the 'useable load' end, you'd have to step back down through a transformer.

kevin · 19th July 2006, 04:17 PM

while wetting the skin does lower resistance, it's the resistenance at the skin, the resistance of the inside of the body will remain pretty constant. This can actually make the charge travel in the surface of the body rather than in the body (and therefore not across the heart and a bit more safe).

The reason water is dangerous is more as a providing a path to ground rather than lowering resistance causing a higher current. If you have thick rubber soled shoes on, but standing in a puddle of water you're negating that insulation value and can setup a path to ground through the water.

AC current is more dangerous than DC current (which is why Edison went around the country electrocuting cats.) The closer an AC current is to your heartbeat the more dangerous it is. That's generally why US 60Hz is considered more dangerous than the European 50Hz.

Current directly through the heart is the most dangerous. This is why you typically learn in electric labs to keep one hand in your pocket. If you use one hand to ground yourself, and the other hand gets shocked the path for the current is straight across the heart. With one hand in your pocket, a) you're less likely to be grounded (if you have good insulating shoes) anb b) if you do get zapped it's more likely to go through your leg than your heart.

kevin · 19th July 2006, 04:36 PM

and there is another meaning to the high voltage/low current statement. Electric power is the current times the voltage (P=IV). If you increase the voltage, you can decrease the current and still produce the same power. So the higher the voltage the lower the current can be to do the same work.

Electrical generation plants typically generate high voltages but can supply only low currents. This is done to increase the efficiency of the power distribution system. The power lost to the transmission system uses the same equation so filling in ohms law (V=IR) your power loss is:
P=(I^2)R

Where R is the resistance in your power lines. So if you needed to deliver 100 watts at the far end and decided to do it by providing 1 volt and 100 amps at the far end, and your distribution system had a resistance of 100 ohms, your distribution losses would be (100 amps^2)100.

But if you provided the same power by delivering 100volts and 1 amp your losses would be (1^2)100.

It's a little different calc for AC, but close enough.

Power distribution also uses the fact that if you reduce the voltage by way of a transformer you increase the current that can be provided at the same time. So stepping down a 48 kV line that can only provide a few amps to 100 Volts can provide a lot of amps.

phildonnia · 20th July 2006, 04:22 PM

Originally Posted by Jimbo07

Now, if you connected the terminals of a car battery with two ends of a metal rod that you were holding onto...

...You could get a nasty burn from Joule heating, but you wouldn't be electrocuted, since the current is not passing through your body.

On the general topic, the whole voltage/current thing is misleading, since Ohm's law generally applies. We've heard "volts tickle, amps kill", but it's actually current that both tickles and kills.

This begs an explanation of why you can survive the 50,000 volt taser just fine, but you can die from 120v household voltage. I think the lethal difference is that the taser zaps you for a few milliseconds at a time, so as not to disrupt your vital processes. The household current just keeps on coming.

joe87 · 20th July 2006, 06:17 PM

Originally Posted by kevin

AC current is more dangerous than DC current (which is why Edison went around the country electrocuting cats.) The closer an AC current is to your heartbeat the more dangerous it is. That's generally why US 60Hz is considered more dangerous than the European 50Hz.

That can't be right. A typical pulse rate of 72 beats per minute is only 1.2Hz. I've always heard that the European 50Hz is more dangerous than the US 60Hz because the brain's electrical rythms are closer to 50Hz than 60Hz. The principal rythms are:

Beta wave frequency = 13 to 30 Hz
Alpha wave frequency = 8 to 13 Hz
Theta wave frequency = 4 to 7 Hz
Delta wave frequency = 0.5 to 4 Hz

Also, I have heard that 13Hz is the most dangerous frequency in terms of not being able to let go of a hot wire, but I have not been able to find verification of that.

Freethinker · 20th July 2006, 06:34 PM

A cattle fence isn't AC or DC. It is pulsed. 7KV or so in pulses on the order of microseconds long.

American · 20th July 2006, 06:47 PM

Originally Posted by kevin

and there is another meaning to the high voltage/low current statement. Electric power is the current times the voltage (P=IV). If you increase the voltage, you can decrease the current and still produce the same power. So the higher the voltage the lower the current can be to do the same work.

Electrical generation plants typically generate high voltages but can supply only low currents. This is done to increase the efficiency of the power distribution system. The power lost to the transmission system uses the same equation so filling in ohms law (V=IR) your power loss is:
P=(I^2)R

Where R is the resistance in your power lines. So if you needed to deliver 100 watts at the far end and decided to do it by providing 1 volt and 100 amps at the far end, and your distribution system had a resistance of 100 ohms, your distribution losses would be (100 amps^2)100.

But if you provided the same power by delivering 100volts and 1 amp your losses would be (1^2)100.

It's a little different calc for AC, but close enough.

Power distribution also uses the fact that if you reduce the voltage by way of a transformer you increase the current that can be provided at the same time. So stepping down a 48 kV line that can only provide a few amps to 100 Volts can provide a lot of amps.

And that is why we do not judge brains by peoples' avatars.

kevin · 20th July 2006, 09:27 PM

Originally Posted by joe87

That can't be right. A typical pulse rate of 72 beats per minute is only 1.2Hz.

hmm, i was going by what I remember from my first electrical lab, but that was many years ago. Perhaps because 60hz is a harmonic of 1.2Hz and 50 isn't?

from wikipedia, although it lumps 50 and 60 hz together:

Quote:

Low frequency (50 - 60 Hz) AC currents can be more dangerous than similar levels of DC current since the alternating fluctuations can cause the heart to lose coordination, inducing ventricular fibrillation, which then rapidly leads to death.

Quote:

Also, I have heard that 13Hz is the most dangerous frequency in terms of not being able to let go of a hot wire, but I have not been able to find verification of that.

Sounds like a good number. I dug up electromyography (emg) which measures the electrical signal used by muscles:
http://en.wikipedia.org/wiki/Electromyography

then this page that talks about frequencies:
http://moon.ouhsc.edu/dthompso/pk/emg/emg.htm

that page indicates the typical muscle signals range from 20-200Hz but actual movement is at rate of 10Hz. Not sure 13 Hz would be a particularly dangerous frequency for causing grabbing.

I did find this too, apparently there is a 13Hz signal in the brain:
http://www.ncbi.nlm.nih.gov/entrez/q...&dopt=Abstract

kevin · 20th July 2006, 09:30 PM

Originally Posted by American

And that is why we do not judge brains by peoples' avatars. http://forums.randi.org/customavatars/avatar5789_2.gif

don't make fun of the Dom-kun, he'll eat your kittens!

19th July 2006, 01:36 PM	#1
pjh Thinker Join Date: Jan 2005 Posts: 218	High voltage - low current Randi used this phrase recently himself in one of his columns, and it's something that's never been clear to me. Given Ohms law (I=V/R), what exacty is meant by the phrase "High Voltage - low current" What are devices like tazers and cattle fences actually doing? Is it that the discharge is almost instantaneous or is something else happening? The same with static eletricity and van der graaff machines - high voltages but and instant discharge. Here's an example of a typical answer to this question (not posted by me) http://forum.allaboutcircuits.com/showthread.php?t=1052 But is this correct? I've used 12v car batteries all my life and never heard they were dangerous - I've regularly been in contact with both terminals and never felt a shock.
pjh Thinker Join Date: Jan 2005 Posts: 218

19th July 2006, 01:41 PM	#2
LTC8K6 Penultimate Amazing Join Date: Nov 2002 Location: Directly under a deadly chemtrail Posts: 12,657	A car battery can't knock you on your ass unless someone hits you with one. People probably get hit with the spark plug voltage, and just assume the battery was what shocked them.
	__________________ What a fool believes, no wise man has the power to reason away. What seems to be, is always better than nothing. 2 prints, same midtarsal crock..., I mean break?

19th July 2006, 02:24 PM	#3
Jimbo07 Illuminator Join Date: Jan 2006 Posts: 3,417	V = IR Car battery: 12 V = I (approx. 1000000 Ohms across the skin at low voltage) I = 12 V/1000000 Ohm I = 12 microamps Taser: 50000 V = I (1000000 Ohms) I = 50 milliamps In short, a Taser probably won't kill you. Now, if you connected the terminals of a car battery with two ends of a metal rod that you were holding onto...
Jimbo07 Illuminator Join Date: Jan 2006 Posts: 3,417	__________________ This post approved by your local jPac (Jimbo07 Political Action Committee), also registered with Jimbo07 as the Jimbo07 Equality Rights Knowledge Betterment Action Group. Atoms in supernova explosion get huge business -- Pixie of key

19th July 2006, 02:56 PM	#5
davefoc Philosopher Join Date: Jun 2002 Location: orange country, california Posts: 7,255	I didn't notice that anybody said anything wrong in any of the links or here, but I might say things a little differently. The issue of what is meant by voltage is routinely misrepresented in the popular media. Knowing the voltage does not provide adequate infromation to know about the potential of an electric source to do harm. One way to think about this is with an analogy to water. The voltage can be seen as the water pressure and the current can be seen as a measurement of water flow. A pump capable of delivering high water pressure but only through a very small outlet can't do much damage or deliver much energy . Conversely a low pressure pump with a big outlet might be capable of delivering much more energy. This is the situation with electricity. Lots of voltage but with sufficiently small current won't harm anything. Real voltage sources (like batteries) can be modeled as a combination of an ideal voltage source (a voltage source that regardless of the load maintains the same output voltage) and a series resistor. One side of the series resistor is connected to the output of the ideal source and the other side of the resistor is the output of the model for the real voltage source. The size of the resistor in ohms is referred to as the source impedance of the battery. The size of the series impedance limits the maximum amount of current available out of the battery. For example a AAA alkaline battery has a much higher source impedance than a D cell alkaline battery but they both have the smae output voltage when they are new. The lower source impedance means that the D cell can supply a much larger current than the AAA cell. The current transferred from a voltage source to a load is not only a function of the voltage and the source impedance of the source, it is also a function of the resistance of the load. The resistance of the human body is hign enough that even if the source impeadance were zero a low voltage source like a car battery under most circumstances can not push enough current through the body to do damage. So if you are trying to kill somebody with a 12 volt battery, your best bet is to him on the head with it as was mentioned above. There are a few other factors in all this that affect how lethal a particular source is. 1. High voltages can drive significant currents outside the body and thereby allow survival from contact that would otherwise be fatal if all the current flowed through the body. 2. The area of the human body that the current flows through. Current that bypasses the heart but destroys a limb can be survived. Current that goes through the heart and destroys it will generally result in death. 3. Interestingly, AC current can be fatal at far less current that DC current. According to several articles on electrocution that I have read AC current can disrupt the heart rhythm and cause death at much lower levels than DC currents. The sweet spot for this effect is between 40 and 400 hertz. One other thing I might mention here, in what I hope isn't too long a response, is that some voltage sources are best modeled as a capacitor with a series resistance. A capacitor is a device that stores a fixed charge . A capacitor is discharged when a current flows from it to the load. ESD (electrostatic discharge) is generally modelled as a capacitor. The capacitor has a small value so not a lot of energy can be stored unless the voltage is very high. So the reason a 10KV ESD discharge into a human won't be fatal and a 10KV discharge from a high tension wire will be is that even though a very high current flows during the ESD event it only flows for microseconds whereas the discharge from the 10KV high tension source can continue untill the victim has been converted to a hot gas.
	Last edited by davefoc; 19th July 2006 at 03:20 PM.

19th July 2006, 03:04 PM	#7
pjh Thinker Join Date: Jan 2005 Posts: 218	Originally Posted by roger Not entirely sure what you are asking but I=v/r applies. So, how could a cattle fence be high voltage/low current? first, the cow itself has high resistance. Second, you can limit the current by putting a high value resistor between the power supply and the lines of the fence. This'll create an effective maximum on the amount of current the fence can produce. But that's not explaining how the tazer itself is "high voltage low current". Surely if you put a resistor in between (in series) you'll drop the voltage, so you won't actually get 4000v "at the cow"?
pjh Thinker Join Date: Jan 2005 Posts: 218

19th July 2006, 02:39 PM	#4
roger Penultimate Amazing Join Date: May 2002 Location: Mountain View, CA Posts: 11,021	Not entirely sure what you are asking but I=v/r applies. Your skin resistance is high, so you grab onto the terminals of a car battery and you have 12 divided by a big number, so you get little current. Wet both your hands, and look out! Resistance drops, and current increases. Typically, it's current that kills. We spent some time studying this in a power class, and it's not quite that straightforward. We respond differently to AC and DC. In AC different frequencies have different effects at the same voltage and amperage. You would think that since current kills, that high voltage is more dangerous. Not necessarily. 500~750V is especially dangerous, because it makes your muscles seize up. If you grab a power line at that voltage, your muscles will clamp your hand to it, and you will stay there until dead, unable to open your fingers even though you try with all your might (and you die because the same thing is happening to your heart). On the other hand, your muscles spasm at 10,000V, so touch a line at that level and you'll be "blown" away from the line by the force of your convulsions, moving you to safety. Electric chairs, designed to kill, typically will cycle through different frequencies and voltages to maximize effects. I.e. some time is spent at 500-750V to make your heart seize, then to a higher frequency and voltage to confuse your heart's pacemaker, etc. So, how could a cattle fence be high voltage/low current? first, the cow itself has high resistance. Second, you can limit the current by putting a high value resistor between the power supply and the lines of the fence. This'll create an effective maximum on the amount of current the fence can produce.

19th July 2006, 03:01 PM	#6
Terry Zeitgeist-impaired Technical Moderator Join Date: Jun 2003 Location: logged in to the server Posts: 6,450	Output impedance may be a key factor here.

19th July 2006, 03:27 PM	#8
Jimbo07 Illuminator Join Date: Jan 2006 Posts: 3,417	Originally Posted by pjh But that's not explaining how the tazer itself is "high voltage low current". I showed the output. Assuming your non-lethal weapon of choice has approx. 50000 V across the electrodes and the resistance of human skin is approx. 1 MOhm, then you'd only conduct approx. 50 mA across your skin. As others have pointed out, it's a different story going through your heart. For the input side... You'd step up from some voltage source to 50kV, using a transformer, say. Your power output is limited through the transformer so while your voltage climbs, the current delivered for a given power drops. This is what high voltage power lines are about. You're trying to deliver power to a load. P = IV from the source and P = I²R in a load. So if you had 100W = 10 A (10 V), then stepped up to 100 V, you'd get 100 W = 1 A (100 V). Now, because P = I²R in a load, that resistivity is going to give you some losses (due to heating). You are better to run your lines at a high voltage and thereby reduce losses due to a high current. ETA: In case it's not clear, I'm referring to the resistivity of the transmission lines and apparatus themselves. At the 'useable load' end, you'd have to step back down through a transformer.
Jimbo07 Illuminator Join Date: Jan 2006 Posts: 3,417	__________________ This post approved by your local jPac (Jimbo07 Political Action Committee), also registered with Jimbo07 as the Jimbo07 Equality Rights Knowledge Betterment Action Group. Atoms in supernova explosion get huge business -- Pixie of key Last edited by Jimbo07; 19th July 2006 at 03:29 PM.

19th July 2006, 04:17 PM	#9
kevin Graduate Poster Join Date: Aug 2005 Posts: 1,666	while wetting the skin does lower resistance, it's the resistenance at the skin, the resistance of the inside of the body will remain pretty constant. This can actually make the charge travel in the surface of the body rather than in the body (and therefore not across the heart and a bit more safe). The reason water is dangerous is more as a providing a path to ground rather than lowering resistance causing a higher current. If you have thick rubber soled shoes on, but standing in a puddle of water you're negating that insulation value and can setup a path to ground through the water. AC current is more dangerous than DC current (which is why Edison went around the country electrocuting cats.) The closer an AC current is to your heartbeat the more dangerous it is. That's generally why US 60Hz is considered more dangerous than the European 50Hz. Current directly through the heart is the most dangerous. This is why you typically learn in electric labs to keep one hand in your pocket. If you use one hand to ground yourself, and the other hand gets shocked the path for the current is straight across the heart. With one hand in your pocket, a) you're less likely to be grounded (if you have good insulating shoes) anb b) if you do get zapped it's more likely to go through your leg than your heart.
kevin Graduate Poster Join Date: Aug 2005 Posts: 1,666	__________________ Long story short, if you wanna get famous, it helps if you're taking a dump. -- RealityBites

19th July 2006, 04:36 PM	#10
kevin Graduate Poster Join Date: Aug 2005 Posts: 1,666	and there is another meaning to the high voltage/low current statement. Electric power is the current times the voltage (P=IV). If you increase the voltage, you can decrease the current and still produce the same power. So the higher the voltage the lower the current can be to do the same work. Electrical generation plants typically generate high voltages but can supply only low currents. This is done to increase the efficiency of the power distribution system. The power lost to the transmission system uses the same equation so filling in ohms law (V=IR) your power loss is: P=(I^2)R Where R is the resistance in your power lines. So if you needed to deliver 100 watts at the far end and decided to do it by providing 1 volt and 100 amps at the far end, and your distribution system had a resistance of 100 ohms, your distribution losses would be (100 amps^2)100. But if you provided the same power by delivering 100volts and 1 amp your losses would be (1^2)100. It's a little different calc for AC, but close enough. Power distribution also uses the fact that if you reduce the voltage by way of a transformer you increase the current that can be provided at the same time. So stepping down a 48 kV line that can only provide a few amps to 100 Volts can provide a lot of amps.
kevin Graduate Poster Join Date: Aug 2005 Posts: 1,666	__________________ Long story short, if you wanna get famous, it helps if you're taking a dump. -- RealityBites

20th July 2006, 04:22 PM	#11
phildonnia Master Poster Join Date: Oct 2001 Location: Sac'to CA Posts: 2,339	Originally Posted by Jimbo07 Now, if you connected the terminals of a car battery with two ends of a metal rod that you were holding onto... ...You could get a nasty burn from Joule heating, but you wouldn't be electrocuted, since the current is not passing through your body. On the general topic, the whole voltage/current thing is misleading, since Ohm's law generally applies. We've heard "volts tickle, amps kill", but it's actually current that both tickles and kills. This begs an explanation of why you can survive the 50,000 volt taser just fine, but you can die from 120v household voltage. I think the lethal difference is that the taser zaps you for a few milliseconds at a time, so as not to disrupt your vital processes. The household current just keeps on coming.

20th July 2006, 06:17 PM	#12
joe87 Scholar Join Date: Mar 2006 Posts: 89	Originally Posted by kevin AC current is more dangerous than DC current (which is why Edison went around the country electrocuting cats.) The closer an AC current is to your heartbeat the more dangerous it is. That's generally why US 60Hz is considered more dangerous than the European 50Hz. That can't be right. A typical pulse rate of 72 beats per minute is only 1.2Hz. I've always heard that the European 50Hz is more dangerous than the US 60Hz because the brain's electrical rythms are closer to 50Hz than 60Hz. The principal rythms are: Beta wave frequency = 13 to 30 Hz Alpha wave frequency = 8 to 13 Hz Theta wave frequency = 4 to 7 Hz Delta wave frequency = 0.5 to 4 Hz Also, I have heard that 13Hz is the most dangerous frequency in terms of not being able to let go of a hot wire, but I have not been able to find verification of that.
joe87 Scholar Join Date: Mar 2006 Posts: 89

20th July 2006, 06:34 PM	#13
Freethinker Graduate Poster Join Date: Oct 2005 Location: The Uncanny Valley Posts: 1,358	A cattle fence isn't AC or DC. It is pulsed. 7KV or so in pulses on the order of microseconds long.
	__________________ "Did it indeed seem probable, as he had once overheard Dunbar ask, that the answers to the riddles of creation would be supplied by people too ignorant to understand the mechanics of rainfall? Had Almighty God, in all His infinite wisdom, really been afraid that men six thousand years ago would succeed in building a tower to heaven?" Thoughts of the Chaplain in Heller's Catch-22

20th July 2006, 06:47 PM	#14
American Illuminator Join Date: Jul 2001 Location: CA Posts: 3,842	Originally Posted by kevin and there is another meaning to the high voltage/low current statement. Electric power is the current times the voltage (P=IV). If you increase the voltage, you can decrease the current and still produce the same power. So the higher the voltage the lower the current can be to do the same work. Electrical generation plants typically generate high voltages but can supply only low currents. This is done to increase the efficiency of the power distribution system. The power lost to the transmission system uses the same equation so filling in ohms law (V=IR) your power loss is: P=(I^2)R Where R is the resistance in your power lines. So if you needed to deliver 100 watts at the far end and decided to do it by providing 1 volt and 100 amps at the far end, and your distribution system had a resistance of 100 ohms, your distribution losses would be (100 amps^2)100. But if you provided the same power by delivering 100volts and 1 amp your losses would be (1^2)100. It's a little different calc for AC, but close enough. Power distribution also uses the fact that if you reduce the voltage by way of a transformer you increase the current that can be provided at the same time. So stepping down a 48 kV line that can only provide a few amps to 100 Volts can provide a lot of amps. And that is why we do not judge brains by peoples' avatars.

20th July 2006, 09:27 PM	#15
kevin Graduate Poster Join Date: Aug 2005 Posts: 1,666	Originally Posted by joe87 That can't be right. A typical pulse rate of 72 beats per minute is only 1.2Hz. hmm, i was going by what I remember from my first electrical lab, but that was many years ago. Perhaps because 60hz is a harmonic of 1.2Hz and 50 isn't? from wikipedia, although it lumps 50 and 60 hz together: Quote: Low frequency (50 - 60 Hz) AC currents can be more dangerous than similar levels of DC current since the alternating fluctuations can cause the heart to lose coordination, inducing ventricular fibrillation, which then rapidly leads to death. Quote: Also, I have heard that 13Hz is the most dangerous frequency in terms of not being able to let go of a hot wire, but I have not been able to find verification of that. Sounds like a good number. I dug up electromyography (emg) which measures the electrical signal used by muscles: http://en.wikipedia.org/wiki/Electromyography then this page that talks about frequencies: http://moon.ouhsc.edu/dthompso/pk/emg/emg.htm that page indicates the typical muscle signals range from 20-200Hz but actual movement is at rate of 10Hz. Not sure 13 Hz would be a particularly dangerous frequency for causing grabbing. I did find this too, apparently there is a 13Hz signal in the brain: http://www.ncbi.nlm.nih.gov/entrez/q...&dopt=Abstract
kevin Graduate Poster Join Date: Aug 2005 Posts: 1,666	__________________ Long story short, if you wanna get famous, it helps if you're taking a dump. -- RealityBites Last edited by kevin; 20th July 2006 at 09:30 PM.

20th July 2006, 09:30 PM	#16
kevin Graduate Poster Join Date: Aug 2005 Posts: 1,666	Originally Posted by American And that is why we do not judge brains by peoples' avatars. http://forums.randi.org/customavatars/avatar5789_2.gif don't make fun of the Dom-kun, he'll eat your kittens!
kevin Graduate Poster Join Date: Aug 2005 Posts: 1,666	__________________ Long story short, if you wanna get famous, it helps if you're taking a dump. -- RealityBites