Re: Expressing fractions

From: Don A. Gilmore (eromlignodNOSPM_at_kc.rr.com)
Date: 01/04/05 

Date: Tue, 4 Jan 2005 13:13:12 -0600

"Javi" <poziNOSPAMyo@hotmail.com> wrote in message
news:3402jcF44n2c5U1@individual.net...
> Harvey Van Sickle wrote:
> > On 03 Jan 2005, Don A. Gilmore wrote
> >>The problem with higher voltages is that they are more dangerous
> >>if you come in contact with them and, if large enough, can present
> >>arcing problems if conductors are not spaced and/or insulated from
> >>each other enough.
> >
> >
> > The use of 220V for standard appliances -- like table lamps --
> > frightened me a bit when I moved to the UK, as I'd only ever seen that
> > voltage used for things like electric cookers/stoves.
>
> The use of a higher voltage implies that less intensity is necessary for
> the same power (wattage). A high voltage is not necessarily more
> dangerous than a lower voltage, usually it is the opposite. What kills
> people is high intensity, not high voltage, and 220V needs half the
> intensity than 110V for the same wattage. When I was younger and had a
> small motorbike that needed frecuently spark-plug cleaning, I sometimes
> received a electric discharge from the spark-plug when testing it, and
> that was around 4000-5000 volts; it hurt a bit, but did not kill me.

Intensity?? That's a new one on me. Voltage is indeed what you must avoid.
There is an often-misunderstood statement that "it's the current that kills
you", citing that as little as 0.1 amperes can cause death. This is
technically true, but such a statement is very misleading to those who don't
understand the elementary principles of electricity. What does it take to
*produce* 0.1 amps of current through your body?

The electrical outlets in your home can essentially be treated as "ideal"
voltage sources. In other words, the outlet provides a constant 120 volts,
regardless of the load attached to it. The amount of current that results
from the connection is a result of what appliance is connected to it.

A 60-watt bulb (in the US) is designed to require 60 watts of power when
connected to a 120-volt circuit. That means that it will draw a current of
0.5 amps. If you replace this bulb with a 120-watt one, it will pull 1.0
amp. The 120-watt bulb has a filament that has a lower resistance to
electricity and can withstand more current and dissipate more energy in the
form of light. If you plug a 1200-watt space heater in, there will be 10
amps of current in the line.

So what happens when you plug your fingers into the socket? Well, that
depends on how much the part of your body that you plug into it resists the
electricity, just like any other appliance. It also depends on what the
electricity passes through. If it passes from the forefinger to the middle
finger of your right hand, there will be less resistance than through your
whole body, so more current will pass. But since there aren't really any
vital organs in your hand, it probably won't kill you. In fact you could
survive a pretty high voltage through just one hand. With really high
voltages, it might even vaporize your hand, but this is arguably a
survivable injury.

If you grab two wires, one in each hand, or grab a hot wire while standing
in a pool of water that is grounded, then you are passing current through
vital organs. The lethality still depends on your resistance and what
voltage we are talking about. Your body's resistance can vary a lot due to
how hydrated you are, your salt content, the humidity, etc. If it takes 0.1
amps through your heart to kill you and you grab onto 120 volts, then you
would need a minimum resistance of 1200 ohms, which is possible, but
extremely low for living tissue and thus unlikely. I have grabbed 120 and
240 many times in my career and have lived to tell the tale.

Incidentally, I just took my ohmmeter out of my desk drawer and I measure
about 700,000 ohms between my left and right hands. 

-- 
Don A. Gilmore
Mechanical Engineer
Kansas City

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