# Car capacitance

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For those who are interested in the numbers, the
capacity between two cars is somewhere around 1000 pf
car-to-car.
Which means that any one car to ground is probably
something like 2000 pf or thereabouts.
Jim
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"jim rozen" wrote: (clip) capacity between two cars is somewhere around 1000 pf car-to-car.(clip) ^^^^^^^^^^^^^ Interesting fact, Jim, but where will I get a chance to use it? :-) Also, since capacitance is dependent on the distance between the "plates" (cars), what is the distance at which this figure is correct? Please respond soon, in case I have to know.
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Jim
Are the cars far enough apart that their car to car capacitance is negligable?
Jerry
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They were 10 feet apart.
Jim
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Jim,
Does this mean 1000 pf is the capacitance of two cars very close together with each forming one plate of a capacitor?
Bob (tired of the antenna thing) Swinney
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With the earth in between. The implication is that one car to ground is about 2000 pf. This is within a factor of two from a simply calculated value, with 10 cm being the separation, and 10 sq M of plate area.
What this says is that at 1 MHz, the car body is coupled to the ground via an approximately 80 ohm resistor. This is large enough that there could be significant rf voltages developed on the exterior of the car body at that frequency.
It implies that Jerry's suggestion (at am frequencies the whip may be an electrostatic probe to measure the body voltage) is indeed plausible.
Jim
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What was the relative humidity?
Gunner
It's better to be a red person in a blue state than a blue person in a red state. As a red person, if your blue neighbors turn into a mob at least you have a gun to protect yourself. As a blue person, your only hope is to appease the red mob with herbal tea and marinated tofu.
(Phil Garding)
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Have you tried the new 'green' electrolytic cars?
Mark Rand RTFM
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Jim, I have found the capacitance between cars to increase with the distance. In fact, with the cars 6 inches the capacitance is zero. Unless you use real skinny people. Then the capacitance rises with the length of the cars. ERS
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Back in the good old days we used to charge our cars up with a model T spark coil and a groundwire. Until the charge wore off, anyone touching a door handle would get quite a jolt. Engineman1
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Hmm. It was last night when I did it - my guess was it was around 20 percent. But only a guess there. Clear night, cold, around 25F.
Jim
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Is that free air capaciatance or using an oil dielectric?
If using an oil dielectric have you noticed any variation in changing from 10W-40 to 5W-50??
Enquiring minds want to know-
H.
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>jim rozen wrote: > > >>For those who are interested in the numbers, the >>capacity between two cars is somewhere around 1000 pf >>car-to-car. >> >>Which means that any one car to ground is probably >>something like 2000 pf or thereabouts. >> >>Jim >> >> >>
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I sure blew that estimate! There's no substitute for real data, thanks for doing the measurement.
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Reactance rather than resistance. You knew that!
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For this particular forum, "resistance" conveys the best concept. Those who understand the difference, undestand why I used that term.
Jim

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