Re: Expressing fractions
From: Don A. Gilmore (eromlignod_at_kc.rr.com)
Date: 01/04/05
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Date: Tue, 4 Jan 2005 17:54:01 -0600
"Javi" <poziNOSPAMyo@hotmail.com> wrote in message
news:340gb8F42kh1bU1@individual.net...
> Don A. Gilmore wrote:
>> Intensity?? That's a new one on me.
>
> Sorry. I meant "current intensity". Maybe "amperage" is the word that I
> should have used. English is not my first language and I am not an
> engineer, so I am a bit lost when using technical terminology.
There's still no such thing. "Current" is measured in amperes. Sometimes
it is referred to as "amperage", but that's strictly bush league. Current
is the flow of charge (coulombs per second).
>> Voltage is indeed what you must avoid.
>
> I don't think so.
>
>> 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.
>
> No, my outlets provide 220-240V.
>> 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.
>
> That is exactly what I am using now to heat this room: a 1200 watt air
> heater, but as my voltage is 240V, I guess that I am using 5 amps.
That is correct. The current drawn by an appliance is the power (in watts)
divided by the voltage. Higher voltages cause proportionately larger
currents in the same object.
>> 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.
>
> So did I. The conclusion, to me, seems to be that the voltage is not what
> kills you, but the rate between the max amperage allowed by the main line
> (25 amp in my home at 240V) and the resistivity of human tissues, that
> varies according to circunstances (wet or dry skin, salinity, etc.)
> Anyway, I am not specialist in the matter, but I am interested in
> understanding it.
This is foolishness. If you don't understand electricity, please don't post
misleading drivel based on a voodoo-like understanding of science.
The current limiting device (fuse or circuit breaker) is sized based on how
much current would harm the connecting wires and insulation from the breaker
to the socket, due to heat. The voltage remains constant. The amount of
current that flows through the wires is entirely determined by what load you
put on it. Even if the fuse is sized for 1,000,000 amps, a 120 watt bulb on
a 120 volt line will *always* pull 1 amp. The socket can't "supply" it more
current than it physically draws. All the fuse does is disconnect the
circuit if you try to pull more than its rating, which is usually caused by
either plugging too many appliances into it, or by short-circuiting, which
tries to demand infinite current. There are successive current limiting
devices all the way from that outlet to the power plant. If there weren't,
a short-circuit could literally burn up every wire all the way back
(providing they lasted long enough not to be a "fuse" in themselves).
Think of an electrical outlet like a fire hydrant and voltage like pressure.
If nothing is connected to it, no water will flow, no matter what the
pressure inside. Think of appliances as different sized firehoses. Bigger
hoses can allow more water to flow than smaller ones at the same pressure.
The hydrant has no way of affecting how much water will flow through a
particular hose at a fixed water pressure, it just provides water. The only
way to force more water through the hose is to increase the pressure
(voltage), which doesn't happen in an electrical outlet.
>> Incidentally, I just took my ohmmeter out of my desk drawer and I measure
>> about 700,000 ohms between my left and right hands.
>
> Try again in the summer, when your skin is probably most moist than now,
> and tell me again.
Sure, there is no way to accurately know what your body's resistance is at
any moment without measuring it. But make no mistake, Javi: If 120 volts
causes 0.1 amps to flow through your body on a given day, then 240 V will
cause 0.2 A that day, 480 V will cause 0.4 A and 6000 volts will cause 5.0
A. That's why voltage is the name of the game.
-- Don A. Gilmore Mechanical Engineer Kansas City
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