[davidhart890]

hi, I need better (and simpler) answers..
I need to get rid of this headache (lol)

if there is a relationship between voltage and current
i.e. the formula Voltage = current * resistance
then..
how come people say.. its not the voltage... but the current that kills..
in the same vein.

how come 15,000 volts won't kill (ie. A car battery)

thanks.
HELP!

David

[medic-dan]

Yes, I've heard that many times. I'll try to explain it as I understand it. I'm sure we'll have a lot of good explanations later.

Most car batteries are 12 volt, not 15,000. You can also find 6 and 24 volts systems. There are even some at higher voltages.

You will find 20-30,000+ volts in the ignition system.

You can put your dry hands across a 12 volt battery, which can put out 500-600 amps and nothing will happen. Your internal resistance is too high. I=E/R . Dry human skin has a resistance of 1000 - 100000 ohms.

Car battery: 12v/1000ohms= .012a

Now try grabbing the spark plug cable. Even on your lawnmower you are going to jump. The mower might have 15,000 volts.

Ignition system: 15000v/1000ohms = 15amps. In the lawnmower it only can produce a few milliamps, just enough to jump the gap. You're actually current limited.

Let use water to make the connection. Think of voltage as the height of the water. Think of current as to how big the river is.

The high voltage has more "push" behind it.
A higher current (amps) means more "volume".

Too much current overrides the hearts pacemaker and affects the rythym. A defibrillator, which can burn the skin, which is why conductive pastes are used, uses electricity to stop the heart so that the natural pacemaker can "reboot" when it is not beating in the correct rythym. High voltage, low current.

So, high voltage, low current is like a raindrop. It might hurt but that's it.

Low voltage, high current. Try to swim across a very wide slow river. You're going to move in relation to the current or fight it every step of the way.

FYI, 12 volts can easily kill. It just has to be delivered in the right place. So can 15,000.

Make sense?

EMT/Volunteer Firefighter/ARFF Firefighter

[KISS]

Here is a stupid example:

Voltage is like pressure. Current is as medic-dan said is volume and resistance is related to the diameter of the pipe.

Lets put 10,000 gallons of water over your head through a 4 foot piece of pipe with a valve on it. (high voltage, low resistance)

Open the valve - Might you be alive? Doubt it.

Now let's take that same 10,000 gal of water, but this time lets put a valve on a pipe about the size of a drinking straw.

High voltage, high resistance

Open the valve - Might you be alive? Yep.

To kill you need to get about 10 mA (a little less) of current across the heart muscle. That's enough to disrupt the body's electrical impulses. So, instead of the heart beating periodically, it stops dead in it's tracks.

Now we can apply it directly across the heart muscle, like say a pacemaker does but at the wrong frequency we dirupt the hearts normal rythym from a stopping to beeting slower ot beatling faster.

Now suppose we put a large are contact along your side and another on your finger on the opposite hand. Electricity will find the path of least resistance.

We can vary the resistance by sweating, or being wet to being totally dry. Those contact points even though they are the same, don't have the same resistance when they are dry or wet.
And thus different voltages would be required to get say 10 mA of current.

I picked 10 mA, I know I'm high. A GFCI or Ground Fault Circuit Interrupter looks for about 7 mA of stray current before it turns off power. Stray current is a 7 mA imbalance of what's flowing through hot and neutral. It may have gone through you to reach ground, so power is turned off.

Did I do better or worse?

[davidhart890]

You can put your dry hands across a 12 volt battery, which can put out 500-600 amps and nothing will happen. Your internal resistance is too high. I=E/R . Dry human skin has a resistance of 1000 - 100000 ohms.


Please indulge me a little.. I woke up this morning thinking that I understood
because I dreamt about the formulas (lol.. get a life David)

since there is an intimate relationship between current and voltage..
are you saying that since I= e/r then.. if I = 12 v and R = 1 then E = must be 12
and if so, would R = 1 ohm?

I thought the math solved my questions

[KISS]

True.

Just don't expect the relationship to be linear for all voltages. I'm talking from microvolts to thousands of volts. For example, at some point breakdown occurs or in semiconductors there could be diode behavior.

And don't expect this relationship to work for AC voltages 100% of the time. A neon lamp is a perfect example. There is a different turn on voltage and turn off voltage and a different sustaining voltage.

Early monochromatic plasma screens like in the 70's used this principle. A fixed voltage was always applied to the cells. When it went below certain value for a short time the pixel would permnently turn off and when the voltage went above a certain threshold the pixel would permanently turn on.

[medic-dan]

"since there is an intimate relationship between current and voltage..
are you saying that since I= e/r then.. if I = 12 v and R = 1 then E = must be 12
and if so, would R = 1 ohm?"

The math is right but watch the labels.

I=12 amps, E= 12 volts, R= 1 ohm.

As KISS says, A/C can be a different animal. We're dealing with DC here.

[elscarta]

The formula I = E/R works the same with AC as it does with DC if you are dealing with normal resistance devices. But AC is deadlier than DC and can kill even if the voltage is high and the current is low.

Normal household electricity supply is either 50 Hz or 60 Hz depending where in the world you live, if this current passed through your heart it would try to make the heart beat either 50 or 60 times a second. Your heart enters a state called fibrillation, where it quivers rapidly with little pumping action. This leads to death unless treated by a defibrillator.

The defibrillator passes a DC current through your heart which causes your heart to contract for as long as the current is on. When the current switches off, your heart has a chance to start its natural rhythm again (doesn't always work the first time!).

[davidhart890]

Here is a stupid example:

Voltage is like pressure. Current is as medic-dan said is volume and resistance is related to the diameter of the pipe.

Lets put 10,000 gallons of water over your head through a 4 foot piece of pipe with a valve on it. (high voltage, low resistance)

Open the valve - Might you be alive? Doubt it.

Now let's take that same 10,000 gal of water, but this time lets put a valve on a pipe about the size of a drinking straw.

High voltage, high resistance

Open the valve - Might you be alive? Yep.

To kill you need to get about 10 mA (a little less) of current across the heart muscle. That's enough to disrupt the body's electrical impulses. So, instead of the heart beating periodically, it stops dead in it's tracks.

Now we can apply it directly across the heart muscle, like say a pacemaker does but at the wrong frequency we dirupt the hearts normal rythym from a stopping to beeting slower ot beatling faster.

Now suppose we put a large are contact along your side and another on your finger on the opposite hand. Electricity will find the path of least resistance.

We can vary the resistance by sweating, or being wet to being totally dry. Those contact points even though they are the same, don't have the same resistance when they are dry or wet.
and thus different voltages would be required to get say 10 mA of current.

I picked 10 mA, I know I'm high. A GFCI or Ground Fault Circuit Interrupter looks for about 7 mA of stray current before it turns off power. Stray current is a 7 mA imbalance of what's flowing through hot and neutral. It may have gone through you to reach ground, so power is turned off.

Did I do better or worse?

you did good lad..
I believe the penny finally dropped..

A voltage of 10000 may be generated because of either high current x low resistance OR even high resistance. So the voltage may be packed with amps or it might be packed with resistance? Sorry... I am a visual person

[KISS]

Voltage is defined as potential difference, so in my example even though they are at the same height, the volume (current or charge/unit time or coulombs/sec) of water that they can deliver are way different.

[medic-dan]

you did good lad..
i believe the penny finally dropped..

A voltage of 10000 may be generated because of either high current x low resistance OR even high resistance. So the voltage may be packed with amps or it might be packed with resistance? sorry...i am a visual person

Umm, the problem is you really can't set the current without the volts and resistance. It is easier to use the formula I=e/r.

I can adjust volts and I can adjust resistance. I can only change current by adjusting either volts or resistance. To increase current I must increase volts or decrease resistance.

Take the car battery example. With you at 1000 ohms resistance and 12 volts = .012 amps Now, drop a wrench across the terminals, say .01 ohm resistance and now you have 1200 amps.

The key is determining the current flow. In the end it is the current that kills.

[davidhart890]

Umm, the problem is you really can't set the current without the volts and resistance. It is easier to use the formula I=e/r.

I can adjust volts and I can adjust resistance. I can only change current by adjusting either volts or resistance. To increase current I must increase volts or decrease resistance.

Take the car battery example. With you at 1000 ohms resistance and 12 volts = .012 amps Now, drop a wrench across the terminals, say .01 ohm resistance and now you have 1200 amps.

The key is determining the current flow. In the end it is the current that kills.

You are correct.. and more to the point. Precise..
I was trying to say the same thing..
That the voltage or resistance will generate more or less amps... depending.
Thanks.. I have it.. I think for me it was always a math issue.. I needed to get the
Terms right.. and the scale. Many thanks
David

[KISS]

Well batteries are voltage sources, however current sources exist too. e.g. http://www.keithley.com/data?asset=15911 with voltage compliance.

Voltage sources exist with current compliance. Current limit, fold back current limiting, 4 quadrant (able to source and sink current)

AND

Electronic loads exit where P is controlled or R is controlled.

Pick one.

[medic-dan]

Yes, current sources exist. The only thing is, no voltage, no current. You still need voltage. Of course, I can also say infinite resistance (open) and no current too. It looks like most of those current devices manipulate voltage and resistance to maintain the selected current.

I agree, in many cases a device can be current limited. I'm not going to get 1200 amps across that same wrench from a 12 volt lighter battery.

Yet, at a given voltage, I can't force more amps through a device than the resistance allows.

I can run a tiny 12 volt "grain of wheat" lamp off a car battery which uses much less current than a 65 watt headlight. Same voltage, different resistances, different current.

If I want to increase the current I up the voltage, yet the resistance stays the same (all right it actually increases slightly as the temperature goes up). The current increases, the lamp gets really bright and then burns out because it couldn't handle the current.

Thanks KISS and David I love this stuff!