David Massey wrote:
> What you have is a high impedence connection in your
For our purposes here, impedance is just another engineering term for
resistance.
Both are measures of a circuit or device's ability to pass current.
The governing equation is known as Ohm's law, which states the
resistance is the applied voltage divided by the resultant current. Or
restated slightly :
Volts = Amps times Ohms. (Which also means Amps = Volts divided by
Ohms; and that Ohms = Volts divided by Amps.)
To put some specific numbers on Bruce's situation, a voltmeter typically
draws only a few microamps (1/1,000,000 of an amp) at 12v (even less if
it's a DMM). A backup light bulb draws one or two amps. So if, for
instance, Bruce's backup circuit had a resistance of 100 ohms due to a
corroded connection or bad contacts inside the switch, the voltmeter
alone would see battery voltage, less the voltage drop of (.000003 amps
times 100 ohms) or .0003 volts (which obviously is much too small to
notice). But, the voltage drop when the backup bulb is connected would
be (1 amps times 100 ohms) or 100 volts, which is clearly impossible.
Obviously the voltage across the bulb (and the current through it) will
be much smaller. Even with no voltage across the bulb, the current will
only be (12 volts divided by 100 ohms) or 0.12 amps, which is not enough
to light the bulb.
In theory, we could use Ohm's law to find just what the current is, and
then the voltage across the bulb. However in reality, the resistance of
a bulb filament varies greatly with the temperature of the filament (how
bright the bulb is glowing), so the above analysis will have to do.
BTW don't worry if the garden hose analogy doesn't make much sense to
you. IMO it's a lousy analogy, electricity can't run out in a puddle at
your feet. It's just the closest thing anyone has come up with.
Randall
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